Getting Started — When and Where

Sky View Café (SVC) presents a number of different ways to look at the movements of the Sun, Moon, stars, and planets through the sky. What you see in the heavens or on your screen depends very much on two things: when and where. In order to proceed, you must tell SVC the moment in time you're interested in and, for many things, the software also needs to know from where on this planet you'd like to be taking in the view.

Note: For brevity, the term planet will be used from here on out to refer to the Sun, the Moon, and even to the now demoted "dwarf planet" of Pluto, as well as any of the eight major planets.

Only Time Will Tell

One might hope that specifying a moment in time would be a simple matter, but issues involving time zones and Daylight Saving Time can get confusing. How many of you have gotten (or placed) a phone call way too early in the morning, or too late at night, because someone added the difference between your time zone and theirs when they should have subtracted, or vice versa? Taking a few paragraphs to discuss the issues of time format, time zones, and Daylight Saving Time seems in order.

If your computer is set up correctly, SVC will start up showing you the current time in your time zone. Most people manage to get the time on their computers set more or less correctly, but it's not too unusual that settings for the local time zone and Daylight Saving Time are a bit off. (A while back, I had one person e-mail me wondering why his sky display looked correct on SVC, but with the SVC clock three hours off. It turns out that he lived in Oregon, but his computer came out of the box set for Eastern time, and he'd never changed it.) If you're uncertain about your time zone settings, check your computer's Control Panels — and while you're at it, the more accurately you set your computer's time, the better.

The clock you see here shows the time starting with the year, then the month, day, hour, and minute. The time is shown in 24-hour format. To change the time, click on the clock, clicking on whichever digit you wish to change. Simply type in the digits of the date and time desired. The left and right arrows on your keyboard can also be used to select a digit, while the up and down arrows will increase or decrease the selected digit. For keyboard-free time entry, click on the arrows at the right-hand side of the clock after clicking on a digit to adjust the value of the selected digit.

SVC uses the convention of marking times during Daylight Saving Time with the § symbol. 21:40§ means the same thing as 9:40 PM Daylight Saving Time, which is the same as 8:40 PM Standard Time. The notation ^ signifies Daylight Saving Time where only a half hour, rather than a full hour, clock change has been made.

Clicking on the Now button resets the clock to the current time. The Track current time option keeps SVC synchronized with the current time. Note that the clock can't be edited to another time until you switch Track current time off.

For years before 1 AD, SVC uses the system of astronomical dates by which the year 1 BCE is represented as 0, the year 2 BCE as -1, 3 BCE as -2, etc. The earliest year you can enter in this manner is -6000 (6001 BCE). After clicking on the clock, press the "-" key to change the year to a negative value. Press "+" to switch back a positive year.

In the Zone

SVC defaults to using the same time zone setting that your computer is set up with. If you'd like to use a different time zone, however, you can select the zone from the TIME ZONE menu (see the Location Settings panel below). Time zones are in hours +/- Universal Time (a newer standard which is very nearly the same thing as the older Greenwich Mean Time, or GMT).

Directly below the TIME ZONE menu is a menu for selecting a system of Daylight Saving Time (DST, also called Summer Time). The Your OS option uses your computer's operating system to decide when DST is in effect — a choice that will generally give good results for your own location and time zone. When viewing other locations around the world, or if the Your OS option gives incorrect results, you can explicitly choose another DST system. In most cases, SVC starts and ends DST at the exact hour recommended, but in any event will make clock changes sometime between midnight and 3:00 AM on the appropriate dates.

Note that when you choose a DST system you are not forcing the clock to turn ahead one hour at all times, but only on those dates and during those times when the clock change would normally be in effect according to the selected system. If you choose Don't Observe DST no DST adjustments will be made for any date or time.

If you're interested in historical dates, click here for more information.

Location, Location, Location

The latitude and longitude settings works very much the same way as the clock, with all the same options for typing or using arrow keys and buttons. You can also type the letters N, S, E or W for North, South, East and West where appropriate.

Rather than enter coordinates manually, however, you can click on the Find button and simply select a location from a list. When you see the Find dialog, type in the name of your city or town (often the first few letters will do), click the Search button, and then select from the search results. If the name you try isn't found, try something nearby. If you get too many matching results, you can refine your search by typing more of the name, or by specifying a state or country.

In SVC 4.1, the Find dialog has one blank into which to type the name of a city, and a second blank for an optional US state, Canadian province, or country. (I gave up on the single blank used in 3.0 because it was unbelievably hard to convince some people to type a comma!) SVC 4.1 provides a convenient pop-up list of states, provinces, and countries for when spellings or standard abbreviations escape you.

For SVC 3.0, here are some examples of what you can type into the Find dialog:

For SVC 4.1 the above examples also apply, but instead of typing the commas shown, put anything shown after a comma into the Find dialog's state/province/country blank.

Note: The shareware version of SVC has a small built-in database of around 2000 major cities from around the world which you can search while off-line. Otherwise, you must have an active connection to the Internet to perform a search. Not all search results will provide time zone information, and for some areas of the world the time zone information provided may not be accurate. If you select a search result item with no displayed time zone, your current time zone settings will be retained. The symbol § marks locations which observe Daylight Saving Time during some portion of the year.

When SVC starts up, the coordinates for your city, or a major city nearby, may have already have set up for you automatically. If you're curious about how this works (or why it sometimes doesn't work), you can read more about the NetGeo Server below.

Even though the SVC display shows latitude and longitude in degrees and minutes, you can enter a location in decimal form if you like. Just double-click on either the latitude or longitude fields and you'll get a dialog into which you can enter values in decimal form.

As a quick way to set your approximate location, you can use the Map View (described in more detail later). Just click on the Map tab, then click on the map of the world as close to your desired location as you can. You'll be able to use the national borders and physical features of the world displayed in this map to guide you.

Once you set up all the information for a location (latitude, longitude, time zone, and DST system), it's very convenient to save that information for the next time you use SVC. You may even want to save more than one location and be able to switch between them quickly. That's where the Save button comes in. Just click the Save button, fill in a name for your location, and click OK. Note that when you're using SVC through your web browser that this information is saved in a "cookie". This means you'll only be able to recall your settings from the same web browser on the same computer.

To remove an old location setting that you no longer wish to use, click the Delete button. To modify a saved location, simply set the new location/time zone values you want to use, and then save those settings using the location name you wish to update. (There's a pop-up menu of currently saved location names is the Save dialog for just this purpose.)

SVC 4.1 only: If you have saved multiple locations, you can choose a preferred default location which will automatically be selected whenever you start up SVC. Just go to the More menu in the upper right corner of the applet and select the Preferences item. (For SVC 3.0, you can use the trick of putting a leading space character in front of the name of your favorite location, which will make that location sort to the top of the location list.)

E pur si muove! ¹

The nice thing about using the arrow keys or arrow buttons to adjust the clock is that you can adjust the time quickly. Quickly enough (presuming your computer's CPU is up to the challenge) that the stars and planets begin to wheel around at high speeds, giving the effect of animation. Click on the tens-of-minutes digit, press and hold the up-arrow key, and watch how the Sun rises and sets, brightening and darkening the sky as the stars and planets spin by. Change the days quickly to see how the sky changes for the same time of day throughout the course of a year. Change the decades and centuries and watch the effects of precession on the positions of the stars ².

You can also animate changes in latitude and longitude, and animate through a sequence of events by clicking and holding down the event buttons described below. (Buttons or arrow keys not auto-repeating? Click here for browser-related issues.) For best performance animating the Sky viewing mode, use the Sky Color: Basic or Sky Color: Black options, rather than Sky Color: Multicolor.

The Moon Also Rises

Suppose you want to know when the next sunrise or moonrise will occur, or for that matter, the rising or setting time of any of the planets, or even perhaps some other kind of astronomical event. Simply select a type of event from the event menu on the left, and if necessary, a planet from the menu on the right. The left and right arrow buttons will then take you to the moment of the previous or next selected event. Event times which are given to one-minute precision are rounded, rather than truncated, to the nearest minute.

You can search for the following types of events:

Sometimes It Is Nice to Point

Beneath the Sky, Ecliptic, Orbits, Moons, and Insolation views, you'll see a small black display area which is referred to as the marquee. The marquee is used to provide additional information, if available, about items in the main display area. For instance, you can point at a planet in the Sky view and get its right ascension and declination, azimuth and altitude, visual magnitude, and more. Often the information is lengthy enough that it automatically scrolls by so it can all be seen. If you click the mouse button down and hold it, scrolling will pause until you release the mouse button.

Signs in the Heavens

You can ask SVC to label the planets you see on the screen, but for a less cluttered display, you can also rely on the color codes used to represent the various planets, shown here. The symbol for the Moon shows the phase of the Moon (roughly, given the small scale), lightening and darkening as the Moon wanes. (In SVC 3.0, the shading should not be interpreted as the actual orientation of the Sun's shadow on the face of the Moon. SVC 4.1 shows an approximately correct orientation for the bright limb of the Moon.) The gray and white symbols used for Mercury and Venus might be confused with the stars you see on your screen, but to help differentiate, these symbols for the planets are always drawn larger than those for the stars.

When you have labeling turned on (using the Show Names menu or Show Planet Names button) the screen can sometimes become very crowded with labels. As you pass the cursor over the display, SVC tries to reduce the clutter by hiding all of the labels in a wide swath around the cursor — except the one label nearest to the cursor. If the cursor is closer to an unlabeled object than the labeled one, the nearest labeled object will still be labeled, but it will appear in dark gray to indicate that the information in the marquee pertains to a different object.

The Big Picture

SVC doesn't need to stay at a fixed size bound to the confines of your web browser's window. Choose the New Window item in the More menu (in SVC 3.0, use the New Window button in the upper right-hand corner of the applet, to the left of the Print button) to get an independent, resizable window. These independent windows will be especially helpful if you're using the SVC applet on a small display. You can also use this feature to have more than one window open at a time, showing different views.

Of course, if you're running the stand-alone version of SVC, you'll automatically have a roomier, resizable window, and can open many windows at once.

Start the Presses!

If you're using the stand-alone version of SVC, or SVC 4.1 on the web, printing is available in a straight-forward manner by selecting the Print option in the More menu (SVC 4.1). For printing with SVC 3.0, use the Print button in the upper right corner of the SVC display.

The web version of SVC 3.0 faces some printing limitations. Java applets like SVC are typically restricted from printing by some older versions of Java. The usual solutions to this printing limitation can be off-putting to users, who must twiddle with lowering their security settings, or click "OK" when a security dialog pops up in their faces, asking something like "Are you certain you want this suspicious looking applet over here to pillage your hard drive?"

If you're using an old version of Java which doesn't prompt you for permission to let the applet print, SVC gets around the problem by not trying to actually print directly to your printer, but by instead creating a printable web page for you that you can print using your web browser's Print command. The main drawback to this printing solution is speed, especially if you're on a slow connection. Basically, you have to wait for your image (typically 25K-200K in size) to be sent SVC's web server, then be bounced back to you again for displaying the printable page. (If you're using a dial-up connection, or your internet connection is otherwise bogged down, please be patient!

Since many of the views in SVC feature dark backgrounds, you will be given the option of creating printouts which use ink-saving color schemes, such as black stars on a white background. Of course, if you're printing at the office instead of at home, go ahead and print white stars on a black background — I'm certain your company will gladly provide all of the ink, toner, and paper you need. :-)

SVC 4.1 only: You can use the Preferences dialog from the More to select and save your preferred Ink Saver policy.

Please note: The Tables view cannot be printed using the web version of SVC.

More Than One Way of Looking at Things

SVC has a number of different viewing modes which you can access by clicking on the various tabs you see above.

Sky View

What you see in the Sky View is a view of the sky showing the planets and over 2000 of the brightest stars, as well as the locations of a number of "deep sky objects" (star cluster, galaxies, nebulae, etc.). When the Sun is well below the horizon, the view of the sky will be solid black. To indicate twilight and daylight conditions, the background of the sky brightens the higher the Sun goes, washing out some of the dimmer stars, but still allowing you to see much more than you would under real daylight conditions.

This brightening effect can be switched off. Using SVC 4.1, you have three sky color modes: Sky Color: Basic, Sky Color: Black, and Sky Color: Multicolor. Black paints a solidly black sky for all times, day or night. Basic uses a simple, single color for the whole sky, varying from black through different shades of blue to indicate daylight levels. The third mode, Multicolor, provides a rough approximation of clear-weather sky coloration for daytime sun glare, sunrise, sunset, and twilight. The Multicolor option can be a little slow to draw, and is not ideal for animation.

SVC 3.0 is limited to equivalents of the Basic and Black options. Having the Show Daylight/Twilight check box checked on is the equivalent of the Basic option.

Some people might wonder why the E for East is on the left of the display while the N for north is on the top. This takes a little getting use to for some people, but you have to realize that you're seeing a representation of a view that you'd see looking upward into the sky. In fact, it's useful to imagine yourself lying flat on your back with your head pointing North, gazing up into the sky.

Out in the real world, the sky appears to be like a giant dome surrounding us, a hemispherical shape. Your computer screen, however, is basically flat. It's useful to keep in mind that in the same way flat maps of the Earth distort the shapes of land masses, flattened views of the sky distort the shapes of the constellations and the angular distances between the stars. Imagining yourself lying on your back again, you'd have to turn your head quite a bit to see all the way from the eastern horizon to the western horizon. Looking at your computer screen you can see the whole 180° span of the sky at a glance — such a visual perspective can't help but be being somewhat unrealistic.

There are two ways of seeing the full sky at a glance in SVC, Full Sky - Flat and Full Sky - Dome. These represent two different ways of projecting a map of the sky onto a flat display. You can experiment with making rapid time or location changes to get a sense of the different perspectives that each choice provides. Switching on the Constellations checkbox will show even better the differences between these two viewing submodes.

When you're interested in how the sky appears looking toward the horizon, you can change the view presented by SVC to display a span of the horizon that's either 45°, 90° or 120° wide. (Just click where it says Full Sky in the Options panel and you'll see the other viewing options available.) All of these horizon views show the sky starting from the horizon at 0°, going up to 45° above the horizon.

There is an additional horizon view called Horizon to Zenith. At the expense of a bit more distortion than some of the other views, this view lets you see a 90°-wide view of the horizon all of the way up to the zenith, 90° above the horizon. In this view, the compass headings along the bottom of the display are only accurate for objects in the sky shown close to the bottom of the display. The vertical altitude markings are only accurate near the middle of the display.

If you're looking at a full-sky view and see something near the horizon that you'd like to see from a horizon perspective, just double-click on the object and SVC will automatically shift to an horizon view centered on that object. If the place where you click is higher in the sky than 42.5°, SVC shifts instead to a 100°-wide view centered around the zenith ³. This same view can be selected using the pop-up menu option Zenith - 100° Span.

In real life, objects near the horizon are often blocked by trees, buildings, mountains, and distant clouds, so you might have a hard time seeing everything shown by SVC's display, even in an otherwise clear sky. Atmospheric refraction also comes into play as you look closer to the horizon, slightly shifting the positions of the celestial objects you see. By default, SVC uses a standard formula to approximate the refraction effect, but variable factors such as temperature, humidity and local elevation make this an inexact calculation.

For a closer look at the Moon, especially for viewing lunar and solar eclipses, as well as lunar occultations, you can choose Moon - 4° Span, Moon - 8° Span, or Moon - 16° Span. In these views, you can see an image of the Moon shaded to indicate the Moon's phase, and see the penumbral and umbral shadows cast by the Earth upon the Moon during a lunar eclipse. You can also watch the Moon pass in front of the Sun during a solar eclipse, or in front of stars and planets during occultations. These various views always display the Moon even when the Moon is below the local horizon. A green background, which ends at the local horizon, indicates where the body of the Earth blocks the view of the Moon from a particular location.

The Parallel to horizon option presents the Moon as it appears oriented towards the nearest point on the horizon. Except when the Moon is very high in the sky, this view most closely matches the way the Moon will tend to appear an observer in real life. The higher the Moon is, however, the less relevant horizon orientation becomes, and, of course, if the Moon is exactly at the zenith, there is no closest point on the horizon.

The Parallel to ecliptic option holds the orientation of the face of the Moon steady (SVC doesn't currently show libration effects), with ecliptic North always at the top of the display.

SVC 4.1 also offers the options Sun - 4° Span, Sun - 8° Span, and Sun - 16° Span. As you might expect, these viewing options work in pretty much the same way as the Moon options above, but with the Sun being at the center of attention. You might find a Sun-centered view the best representation for observing the progression of solar eclipses.

If you want to find out where a specific star, planet, or deep sky object is located, use the following procedure:

1. Click on the Show Names menu in the Options panel.
2. Select Mark One Specific Object
3. Choose from the list that appears the object that you're interested in finding.
4. Make sure your mouse is not pointing at the sky display.

If the object you chose is within the current sky display, you will see its name appear beside the object, and information about that object will be presented in the marquee below. If the object is not within the current field of view, you won't see its name in the image of the sky, but information about the object will still appear in the marquee — with orange lettering to indicate the off-screen status of the object. If you point at something within the sky display your pointing action will have precedence — information about whatever you point at will be displayed rather than information about your Mark One Specific Object selection.

Ecliptic View

The ecliptic is the plane of the Earth's orbit around the Sun, or the intersection of that plane with the imaginary sphere of the stars. Since all of the major planets⁴ orbit in nearly the same plane around the Sun, most of the action involving the planets takes place within a few degrees of the ecliptic. Barring unusual circumstances which can occasionally occur within the Arctic or Antarctic circles (where the ecliptic can run along the horizon) you'll never see more than half of the ecliptic at the same time. The Ecliptic view, where you are always shown all of the ecliptic at once, is therefore much more of an abstraction than a representation of the real sky.

Imagine the Earth at the center of a sphere surrounding the Earth at a great distance, with the stars and planets projected upon the sphere's surface. The green line represents the ecliptic, and the blue and red lines represent circles some distance north and south of the ecliptic. This band of the celestial sphere is what SVC shows in the Ecliptic view, removed from the whole of the celestial sphere and flattened out as shown in the illustration above.

SVC draws a dark green line to represent where your local horizon intersects the ecliptic band, along with a dotted green line showing the portion of the band which is above the horizon. The dark blue grid lines show geocentric latitude in steps of 5°, and geocentric longitude in steps of 15°. (For a more complete explanation of celestial and ecliptic coordinates, see Sky and Telescopes's Understanding Celestial Coordinates.)

Using animation with the Ecliptic view is a great way to see the motions of the planets against the background of the stars, and to see events like retrograde motion, maximum elongations, conjunctions and oppositions of the planets. Switch off Show Local Horizon for faster animation, and when the background of the stars is not desired, switching off Show Stars will give an even greater boost to animation performance.

Orbits View

The Orbits view lets you see the solar system at a glance, showing the planets in their orbits around the Sun. The scales of distance of the orbits of the planets vary widely, with four of the planets within under two AU ⁵ from the Sun, all of the "naked eye" ⁶ planets within under 12 AU, and Pluto sometimes orbiting as far away as nearly 50 AU. Because of this variety of scale, the top menu in the Options panel lets you choose which orbits you wish to keep entirely within the scope of the display.

If you want to get a better understanding of the movements of the planets through the sky, switch back and forth among the Sky, Ecliptic, and Orbits views, compare what you see, and try to mentally account for the shifts in perspective. Also, try using animation with the Orbits view option Center on Earth selected. These exercises may give you an appreciation of why it took humankind a long time derive the Copernican ⁷ understanding of the solar system, expressed in the Orbits view, based solely on watching the motions of the planets among the stars, as expressed in the Sky View.

The stand-alone version of SVC adds an extra option to the Orbits view, Gray orbits below horizon. This option helps you see which planets are in the sky for your chosen location (even when hidden by the glare of the Sun), and how the local horizon intersects the orbits of the planets and the ecliptic.

Moons/GRS View

The Moons/GRS view shows the four major moons of Jupiter (often referred to as the Galilean moons, after Galileo Galilei, who discovered them), Jupiter's Great Red Spot (GRS), and eight of the moons of Saturn. The image of Saturn also displays the orientation of the rings of Saturn as seen from Earth.

The default view shows planetary north at the top of the view, and celestial east to the left. Checking and unchecking the North on top and Celestial east on left checkboxes in the Options panel can help you match the displayed view to what you might see using binoculars or various kinds of telescopes. When only one of the two checkboxes is checked, the image shown is a mirror image (as seen in many telescopes). When both checkboxes are on, or both are off, the image is a direct image. The Zoom In and Zoom Out buttons allow you to select differing levels of magnification.

The Photographic planets option is mostly decorative — the images shown shouldn't be taken too literally as accurate depictions of Jupiter and Saturn at any particular moment in time. Both planets are essentially gigantic balls of swirling gas, and Jupiter in particular undergoes a great deal of change in appearance as various atmospheric phenomena change dynamically over time. Both planets also undergo differential rotation, meaning that different parts rotate at different speeds. SVC makes no attempt to model or display any of this behavior.

An effort is made to show Jupiter's Great Red Spot in an approximately correct position. It should be noted, however, that the GRS is simply a great big storm — it is weather. While much more stable and much longer lasting than any storm on Earth (the GRS has been raging for close to 200 years at a minimum, perhaps even for 300 years or more), the behavior of the GRS is erratic and ultimately unpredictable. It's possible, perhaps even likely, that one these days the Great Red Spot will simply disappear.

Derived from data which can be found at jupos.org, SVC contains a table of GRS longitudes covering the time span (as of the moment that I'm writing this) from April 1969 to April 2007. These longitudes are based on curve fitting numerous individual telescopic observations of the GRS. During the time span covered by this table, and for a short period of time before and after the tabular data, SVC's positioning of the GRS should be (I must emphasize that I'm not totally certain of the degree of accuracy I've attained, since my work here is based on very "noisy" observational data) within 2-3 degrees of the correct real position, and SVC's GRS transit times should be correct within 3-5 minutes.

The display of the GRS's System II longitude seen in the Moons/GRS view will be green for times within the coverage of the longitude table, as well as for times within one year of that coverage, yellow for times from one to two years outside of the table, and red all other times, as an indication of the likely validity and accuracy of the data.

Insolation View

The Insolation view shows daylight (and optionally, moonlight) levels for each day of a selected year, (going vertically down the view) for all times throughout each day in five-minute intervals (going horizontally across the view). The overall shape of the displayed image shows how the length of the daylight hours at a particular location varies over the course of a year. A small open-centered cross hair marks where the moment in time entered in SVC's clock is plotted in the view. (For the purposes of this view, any time expressed as Daylight Saving Time is adjusted to Standard Time.)

As you pass your computer's cursor over the Insolation view, the marquee will give you information about the moment in time plotted underneath the cursor. The legend below describes the color coding used in the Insolation view for various altitudes of the Sun relative to the local horizon, and for the phases of the Moon. Barring unpredictable weather, the black areas of the view represent the times when the sky is darkest and best suited for viewing faint objects like galaxies and other deep sky objects.

As an experiment, try changing the LATITUDE setting and see what this does to the patterns of daylight and twilight, especially as the latitude approaches the north or south poles.

Map View

The Map view displays a cylindrical-projection map of the surface of the Earth. The map serves a number of functions:

• Showing the location you've set in the Location Settings panel.
• Showing where on Earth it's night, and where it's day.
• Showing the location of the subsolar point. The subsolar point is the place where an observer would see the Sun directly overhead, at the local zenith.
• Showing, when appropriate, the location of maximum solar eclipse, along with the shadow of the Moon where the Sun is 80% or more eclipsed. You can adjust the clock back and forth by minutes or tens of minutes to see the path the Moon's shadow makes during the course of an eclipse. Note that for many partial eclipses, no shadow will be shown at all.
• Providing a way to set a new observer location. By clicking on the map you can set a new location, at a resolution of one degree.

Calendar

The SVC Calendar lets you see a month's worth of astronomical events at a glance. The phases of the Moon are always shown, and can also choose from the Options panel one more type of event to display. By default, the times of the equinoxes and solstices are shown, marking the beginnings of each season. You can instead choose to show the rise and set times of a particular planet (optionally including transit ⁸ time), or the start and end times for varying degrees of twilight.

SVC 4.1 adds the option of daily images of the phase of the Moon. Each image represents the phase of the Moon at noon for the currently selected time zone.

Tables

The Tables view of SVC gives you access to raw numerical information, presented as plain text that can be copied from the display and pasted into other documents. Use the pop-up menu on the left to of the Tables view to choose a particular kind of table. After choosing any relevant options related to the chosen table, click the Generate Table button to have the table you have selected created for you.

Ephemeris: This table gives you information about all of the planets for a given moment in time, or about one planet over a range of times. Look in the Ephemeris Options menu in the Options panel for the choices you have for different kinds of information. Note that SVC normally deals with Universal Time, or UT. If you choose the table option Ephemeris - TDB, the time you set for your table will be treated as Barycentric Dynamic Time, a more uniform time standard than UT, often used for ephemeris generation. (You can read more about time systems below.)

When you choose All Planets, the table produced will show information for all of the planets for a single moment in time. If you chose a single planet, however, you have the option of a number of different time spans and time intervals for the data.

Rise/Set Times: This table presents the rise, transit, and set times for a selected planet, starting at the date entered on the clock, for a period of one week, one month, or one year. When the chosen planet is the Sun, twilight times are also included in the table, and you may select from civil, nautical, or astronomical twilight.

Lunar Phases: This table presents the times of the phases of the Moon for one, five, ten, or twenty years.

Equinox/Solstice: This table will display the times of the equinoxes and solstices for ten years, starting at the year entered into the clock panel.

Galilean Moons/GRS: In this table the times of various Galilean moon events are provided, along with Jupiter's Great Red Spot transit times. For the moon events, the traditional notation is used, as described below:

• Roman numerals I, II, III, and IV are used for Io, Europa, Ganymede, and Callisto, respectively.
• Ec.D.: "Eclipse Disappearance", the moment when a moon disappears into the shadow Jupiter.
• Ec.R.: "Eclipse Reappearance", the moment when a moon reappears from the shadow Jupiter.
• Oc.D.: "Occultation Disappearance", the moment when a moon disappears behind Jupiter.
• Oc.R.: "Occultation Reappearance", the moment when a moon reappears from behind Jupiter.
• Sh.I.: "Shadow Ingress", the moment when a moon's shadow begins to cross the face of Jupiter.
• Sh.E.: "Shadow Egress", the moment when a moon's shadow finishes crossing the face of Jupiter.
• Tr.I.: "Transit Ingress", the moment when a moon begins to cross the face of Jupiter.
• Tr.E.: "Transit Egress", the moment when a moon finishes crossing the face of Jupiter.

Example: 2001-01-30 01:04   II. Sh.I. — The shadow of Europa begins to cross the face of Jupiter at 01:04 (UT) on January 30, 2001.

If you'd like to see GRS transit times based on a particular observed GRS System II longitude, you can enter that longitude via the Moons/GRS view.

The Tables view is also where extra information about some types of events found using the PREVIOUS/NEXT EVENT controls will be displayed. Switch on the Show event messages checkbox to enable this feature.

Accuracy and Systems of Time

Numerically speaking, most of what you see presented by SVC derives from the book Astronomical Algorithms, by Jean Meeus, particularly his simplified versions of the VSOP87 series (P. Bretagnon and G. Francou) and the Chapront ELP 2000/82 lunar theory. The accuracy of these calculations is stated to be much better than one minute of arc, and typically within just a few arc seconds. No specific range of time validity is mentioned, but context leads me to believe that high accuracy is retained for several centuries before and after the year 2000. The accuracy is easily better than the pixel resolution of your computer screen.

If you're using the stand-alone version of SVC, you can get the full precision of VSOP87 and ELP 2000/82 when you generate an ephemeris in the Tables view.

Pluto is covered by Meeus' adaptation of the work of Aldo Vitagliano. The accuracy of the given method is said to be quite high, better than one-tenth of an arc second, but only for the years 1885-2099. I have no idea how much error creeps in outside of that time span, but I have made no effort to limit viewing Pluto to this range of years, so keep in mind that, beyond that span of time, I'm using the formula outside of its recommended range.

When you set a particular time on this applet's clock, most of what you see is computed for that time plus 30 seconds. This is for consistency with the way most event times are rounded to the nearest minute. When SVC says that Mars rises at 07:27, the calculated rise time can be anywhere from 07:26:30 to 07:27:30. When you set the clock to 07:27, the extra 30 seconds assures that the event of Mars rising will have taken place. The principal exception to this practice of offsetting the clock is the generation of ephemeris tables, which use the exact beginning of the minute of the time you set.

The accuracy of the methods mentioned above depends on a very accurate time scale, even more accurate than the Earth itself keeps as it spins about its axis. This kind of time used by astronomers is known as Dynamical Time, and the particular form of Dynamical Time used here is Barycentric Dynamic Time, or TDB. TDB's uniform scale means that it is not in perfect sync with Universal Time (UT), which is periodically adjusted to keep pace with the uneven rotation of the Earth. Our civil time is based on Coordinated Universal Time (UTC), which is maintained within 0.9 seconds of UT, and gets adjusted to keep pace with UT a full second at a time — these are the "leap seconds" you often hear about, especially on New Year's Eve. (For more information, see the Systems of Time web page provided by the U.S. Naval Observatory.)

What you enter into SVC's clock is UT, so there are no leap seconds. Except when the difference of a single second is important, you can consider UT to be essentially the same thing as the UTC (adjusted locally for your time zone) that we set our watches and clocks by. Enough astronomical data has been gathered historically to create a tabular conversion of UT to TDB which covers the years 1620 to the present, accurate within a few seconds — fractions of a second for the past century. From -500 to 1600, historical records of events such as solar eclipses have provided enough information for rough estimates of UT over that period. Earlier than -500 may never be known with great accuracy, and accuracy beyond the current year depends on measurements which won't be taken until each new year is upon us. For times before the year -500, or after the current year, approximation formulae are used which become more and more inaccurate the further outside of this range you go. When setting the year of SVC's clock to its past and future limits, the years -6000 and 9999, the inaccuracy of UT may amount to several minutes, or even a fews hours.

Having brought up the subject of setting the year, it should be mentioned that back in 1582 a curious thing happened to our calendar. The old calendar system, called the Julian calendar (after Julius Caesar), had drifted ten days off from the seasons — the first day of Spring was occurring around March 11 instead of March 21. A commission was set up by Pope Gregory XIII to amend the calendar, and they came up with an ingenious modification to the rules about leap years, developing a system which would be accurate to within one day every 3000 years. Under the new calendar there would be a leap year every four years, just like in the old system, but with one exception: If a year were divisible by 100, but not divisible by 400 — such as 1700, 1800, or 1900 — that year would not be a leap year. 1600 and 2000, being divisible by 400, are leap years.

The new leap year rule fixed the future course of the calendar, but in 1582 a whole lot of fixing had to be done all at once. So the ten days from October 5-14 were simply dropped: October 4 was followed immediately by October 15.

At least that's what happened in Rome. The rest of the world adopted the new calendar with varying degrees of enthusiasm and rapidity. Russia was one of the last holdouts — the so-called "October Revolution" occurred in November on nearly everyone else's calendar. The switch-over happened in the not-yet United States during the colonial era, when Great Britain adopted the Gregorian system in 1752. By this time the old Julian calendar was off eleven days. Between the shift of eleven days going from one calendar to the other, and an additional complication — New Year's Day having been observed on March 24 instead of January 1 — you'll sometimes see George Washington's birthday given as February 11, 1731, and in other references retroactively adjusted to the Gregorian date, February 22, 1732.

SVC uses the Gregorian system from October 15, 1582 onward, and always treats January 1 as the first day of the year. You'll need to keep this in mind if you're using this software to examine astronomical conditions on historical dates, to make sure you're using the right calendar system.

The calendar isn't the only aspect of time keeping that has been tinkered with through history — the way we set our clocks has undergone many changes as well. Before the era of the steam train, every town and city kept its own locally-determined time. It wasn't until the late nineteenth century that uniform time zones were established. The idea of Daylight Saving Time (DST), first considered by Benjamin Franklin in the eighteenth century, was not implemented anywhere until the twentieth century. It should be noted that when you use DST in SVC, no attempt is made to follow the long and complicated history of DST, which as been enacted, repealed, re-enacted, and legislatively tweaked many times over the years, with variations even at the city and town level as well as differences in DST between various countries. SVC simply takes the current rules for DST in various countries and projects them into the past, back to the year 1900, and indefinitely into the future.

Stars and Deep Sky Objects

To display stars and deep sky objects, SVC starts with the Basic Fifth Fundamental Catalogue (FK5), then supplements with extra stars and data from the Yale Bright Star Catalogue (BSC) and deep sky objects from the New General Catalogue (NGC) and the Index Catalogue (IC). Where stars do not have Bayer-Flamsteed designations (such as 66 Alp Gem), the applet identifies those stars by FK5 or BSC number. Deep sky objects are identified by their Messier Catalogue number (M), or by an NGC or IC number. The magnitude values shown are fixed values — integrated visual or photographic magnitudes which do not reflect changes in magnitude over time for variable stars.

The web version of SVC has over 2600 stars and deep sky object, while the stand-alone version has more than 9300 entries.

Web Browser Compatibility Issues (Last updated: 2007-03-29)

How well SVC runs on your your web browser depends on a number of things: which browser you're using (Internet Explorer, Safari, Firefox, etc.), which version of the browser you have installed, which version, if any, of Java that you have installed, and what OS you're using (Windows, Mac OS X, Linux, etc.). You might be able to run SVC 3.0, but not SVC 4.1, without updating your Java installation. If you're running Mac OS 9, the only way to run SVC 4.1 is to upgrade to OS X, because OS 9's Java is frozen at version 1.1.8, and will go no further.

Windows (98/ME/NT/2000/XP): Internet Explorer (5.0 or later) and Firefox (1.5 or later) will work well as long as you've got Java 1.3.0 or later installed (click here to download the latest version). Some Windows installations come only Microsoft's old 1.1-level Java, which can only run SVC 3.0 on Internet Explorer, or come with no Java at all.

I haven't personally tested SVC with Windows Vista yet, but I anticipate users will have no problems.

Mac OS X: Safari (1.3 or later) is probably your best bet for using SVC with Mac OS X. Firefox (1.5 or later) works well too, but can act a little funny when resizing your browser window, and doesn't save cookies for your SVC settings as well as Safari does. In case you prefer one look over another, SVC on Safari has the "brushed metal" look of iTunes, while Firefox presents the SVC applet with the lightly pinstriped version of OS X's Aqua interface. If you have an older version of OS X, which only has Java 1.3, or if you're using the no-longer-supported Internet Explorer 5.2 for Mac, you'll see a generic Java interface "look and feel" called "Metal".

Mac OS 9: You will only be able to run SVC 3.0, since Java development for OS 9 stopped at Java 1.1.8, and your only safe bet for running SVC in OS 9 is Internet Explorer (4.5 or later).

Linux: My personal testing on Linux has been limited, and I haven't had much user feedback one way or another concerning Linux performance. I do know that SVC 4.1 usually works better than SVC 3.0, because SVC 3.0 uses an AWT interface, and I've seen a lot of AWT bugs in Java for Linux.

The following are known compatibility issues:

• Under Windows 2000 and Windows XP, when using SVC 3.0 together with Sun's JRE 1.3 or 1.4, scrolling lists sometimes exhibit jagged and jumpy scrolling combined with the appearance of stray black lines. I've added a workaround for this problem that cleans up the bad scrolling by periodic repainting of these lists, but this workaround comes at the expense of some occasional screen flicker. You'll be better off using SVC 4.1.
• Firefox and other Mozilla-family browsers can sometimes be cranky about the browser's Reload function being used on an applet's web page. If you see a message like "Applet failed to initialize" after trying to refresh your browser, simply try to refresh again, or go to another web page and come back again.
• Some older versions of Opera on Windows may not successfully save your location settings.
• Internet Explorer 5.0 for Mac OS 9 (but not the older 4.5 version) has a problem sending auto-repeat of keystrokes to an applet, and the Tab key won't tab between input fields within an applet. The auto-repeat problem makes performing animation via holding down the arrow keys not function. There are two work-arounds for this: (1) Use the graphical arrows next to the clock — they will auto-repeat properly. (2) Click on the New Window button — the independent window is free from these keyboard problems.
• Pre-6.0 Netscape Java doesn't process mouse release events reliably, so auto-repeat on the graphical arrows is disabled for these earlier versions of Netscape. You can work around this by using the keyboard arrows instead. To make the event buttons (the blue left and right arrows at the top of the display) auto-repeat using Netscape, first click one of the buttons, then press and hold the Enter key.
• Many versions of pre-6.0 Netscape will not print in color when you use the Print button. Some early versions of Internet Explorer on Windows may print with incorrect colors.

The NetGeo Server

Using the NetGeo server, SVC will automatically default to performing an IP-based look-up of your longitude and latitude, so that without any special effort on your part there's a good chance that the sky charts you see will automatically match your real sky fairly closely, and that the rise and set times of the planets will be within a few minutes of your local times. Your IP address is not related in-and-of-itself to any physical location on the planet, but since these addresses do have to be reserved by people, businesses, and government agencies, there are databases that NetGeo can access with information about the IP address owners — including these owner's geographical addresses. Those addresses can in turn be turned into latitude and longitude through still other databases. Don't worry too much that "they" know where you live, however. "They" only know the business addresses of the Internet Service Providers that you use.

If you're a current user of SVC and have already entered and saved your location, your saved location will be used instead of a NetGeo look-up. New users may also want to refine the location that's automatically generated by NetGeo — occasionally it can be fairly far off: for example, AOL users tend to get various locations in Virginia no matter where in the United States they are really connecting from. Sometimes NetGeo will find no match for you at all, or will be offline and unavailable, making manual entry of your location necessary.

You can access NetGeo yourself with these links:

     http://www.caida.org/tools/utilities/netgeo/ (general site)
     http://netgeo.caida.org/perl/netgeo.cgi (direct link to IP look-up)

Where Credit is Due

A great deal of credit must go to Jean Meeus and his excellent book Astronomical Algorithms (Second English Edition, 1998, Willmann-Bell). Through his book, credit is further due to P. Bretagnon and G. Francou for their work on VSOP87, to Michelle Chapront-Touze and Jean Chapront for their ELP-2000 Lunar Theory, to Aldo Vitagliano for his work on Pluto, to Jay Lieske for his E5 theory of the Jovian moons, to Gérard Dourneau for his methods for the moons of Saturn, and to many others.

Paul Schlyter's web page Computing Planetary Positions was very helpful in my work, as well as some direct personal help from Paul himself provided through the sci.astro.amateur newsgroup. NASA and the United States Naval Observatory provide online data sources that I found quite useful in both generating and confirming my calculations, such as NASA's Astronomical Data Center and USNO's Data Services.

John Walker's Your Sky web site was an inspiration and a touchstone for SVC. Your Sky is less interactive than SVC, but by keeping its computing resources on a web server it is able to hold a richer data set of stars and deep sky objects — enough data to provide a useful "virtual telescope" feature. Your Sky is also more compatible with a wider range of web browsers.

The Map view uses a world map created from satellite photography, obtained from National Geographic's Map Machine.

1. E pur si muove! But it does move! — According to legend, Galileo muttered this under his breath at the trial where he was forced by church authorities to declare that the Earth always remains still. [back]

2. The north pole of the Earth is currently pointed fairly close to Polaris, otherwise known as Ursae Minoris or the North Star. But this is merely a temporary condition — the celestial north pole wanders over a period of about 26,000 years through an uneven circle of the sky about 47° wide. This change in the orientation of the axis of the Earth is due to precession, an effect of tidal forces from the Sun and the Moon that makes the Earth wobble like a spinning top. Over the centuries the effect is quite noticeable, and you can see the effect using SVC. [back]

3. The zenith is the point in the sky directly overhead, 90° above the horizon. [back]

4. Pluto's relatively very slanted and eccentric orbit is among the reasons, along with its small size, for its demotion from full-fledged planethood to dwarf planet status. [back]

5. The AU, or Astronomical Unit, is a unit of distance based on the average distance of the Earth from the Sun, approximately 150,000,000 kilometers (93,000,000 miles). It takes light about 8¹/₃ minutes to travel this distance. [back]

6. The "naked eye" planets are Mercury, Venus, Mars, Jupiter and Saturn. These planets can all be seen with the unaided eye, and have been known since ancient times. Although it would be difficult, it is conceivable to see Uranus directly as a very faint object, under conditions of very clear, dark skies. Uranus can easily be seen with good binoculars. [back]

7. Nicolaus Copernicus (1473-1543) is credited with first promoting the notion that the Sun, not the Earth, is at the center of what we now call (aptly enough) the solar system. Johannes Kepler (1571-1630) first worked out that the orbits of the planets were elliptical rather than circular. [back]

8. Imagine a line running along the sky from north to south, directly overhead. A celestial object is said to be at transit at your location when, as it moves from east to west across the sky, it crosses this line. This is when the object is highest in the sky during the course of a day. The planets are typically directly south at transit for Northern Hemisphere observers, and directly north for Southern Hemisphere observers — as you move closer to the equator, however, and the planet strays further from the celestial equator, the latter is not always true. [back]