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Minimum requirements to use Sky View Café: A Java-capable web browser (JDK 1.1 or later). To be able to save and load settings, JavaScript capability must be switched on and cookies must be enabled. The calculations performed by this software are fairly demanding, so a computer running at 300 MHz or better is recommended. 800 x 600 displays are supported, but can be a bit cramped for this software -- 1024 x 768 or better is also recommended. Click here for more information.


Getting Started

Sky View Café 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 Sky View Café 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 out to refer to the Sun and the Moon, as well as any of the nine major planets.  [back]

Only Time Will Tell

It seems like time should be a simple matter, but in a world where millions of VCRs are still blinking 12:00, 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, Sky View Café 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. (I've already had one person e-mail me wondering why his sky display looked correct, but three hours off. 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.

Sky
View Clock 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, choosing whichever digit you wish to change. Simply type in the digits of the date and time desired. The left and right arrows of 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 next to the clock to make adjustments of the selected digit.

Sky View Café 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 Sky View Café synchronized with the current time. Note that the clock can't be edited to another time until you switch Track current time back off.  [back]

In the Zone

Sky View Café 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, Sky View Café 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 Standard Time no DST adjustments will be made for any date or time.

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

Location, Location, Location

Sky View 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. If you don't know your latitude and longitude, a link is provided on the main Sky View Café page where you can look up your coordinates online. If you're lucky, Sky View Café will start up with coordinates already filled in for your city, or a major city near you. 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 Sky View Café 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.

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 Sky View Café. You may even want to save more than one location and quickly switch between them. 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 this information is saved in a "cookie", which 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, click the Delete... button.  [back]

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 stars2.

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.)  [back]

The Moon Also Rises

Sky View Events Suppose you want to know when the Moon will rise next, or for that matter, the rising or setting time of any of the planets, or perhaps some other kind of astronomical event. Simply select a type of event from the 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:

Event TypeComments
RisingIncludes a standard adjustment for atmospheric refraction.
TransitTechnically, this is when a planet "crosses the local meridian". At transit a planet reaches the highest point in the sky in its daily path, and, unless you live in or near the tropics, a planet is typically due south at transit for northern hemisphere observers, and due north for those in the southern hemisphere.
Set - 1 min.The time one minute before the time a planet sets, useful for seeing where on the horizon a planet will be when it sets.
SettingIncludes a standard adjustment for atmospheric refraction.
New Moon
First Quarter
Full Moon
Last Quarter
The moments when the difference in the apparent longitudes of the Sun and the Moon are 0°, 90°, 180°, and 270°, respectively.
Spring Equ.
Summer Sol.
Fall Equ.
Winter Sol.
The times of equinoxes and solstices, marking the beginnings of the seasons, when the apparent longitude of the Sun is 0°, 90°, 180°, and 270°, respectively.
Lunar Eclipse The moment of maximum lunar eclipse. The eclipse may be penumbral, partial, or total, and won't necessarily be visible from the location that you have set. If you switch on the Show event messages checkbox in the Tables panel before skipping to a lunar eclipse event, you will provided you with a text message about the eclipse.
Solar Eclipse The moment of maximum solar eclipse, for the planet as a whole. The eclipse may be partial, annular, total, or hybrid (annular/total) and won't necessarily be visible from the location that you have set. The Show event messages option described above can also be used with solar eclipses.
Opposition The moment when the difference in the apparent longitudes of the Sun and a planet is 180°. For Mars especially, the time of opposition provides the best viewing of the planet.
Sup. Conj. Superior conjunction, the moment when the difference in the apparent longitudes of the Sun and a planet is 0° and the planet is farther from the Earth than the Sun is from the Earth. A planet is difficult or impossible to see at this time due to the glare of the Sun. For an outer planet (Mars out to Pluto), superior conjunction is generally referred to simply as conjunction.
Inf. Conj. Inferior conjunction, the moment when the difference in the apparent longitudes of the Sun and a planet is 0° and the planet is closer to the Earth than the Sun is to the Earth. A planet is typically difficult or impossible to see at this time due to the glare of the Sun, but may sometimes be seen, using the appropriate safe techniques, crossing the face of the Sun. Only the inner planets Mercury and Venus can be in inferior conjunction.
Gr. Elong. Greatest elongation, the moment when the apparent angular separation of the Sun and a planet reaches a maximum. This applies only to the inner planets Mercury and Venus, and typically marks some of the best viewing conditions for these planets. The Show event messages option described above provides more information about greatest elongation events.
Perihelion The moment when a planet is closest to the Sun in its orbit. This moment is rounded (not truncated) to the nearest hour for Mercury, Venus, and Mars, and rounded to 00:00 UT of the nearest day for the other planets. (Rather than rounding, many listings of perihelion time record any moment between, say, 3:00:00 and 3:59:59 as 3h, or between 2046/11/02 00:00:00 and 2046/11/02 23:59:59 as 2046/11/02.) For Neptune and Pluto, only mean orbital elements are considered here in computing the time of perihelion.
Aphelion Calculated and rounded with the same considerations as perihelion above, this is the moment when a planet is furthest from the Sun in its orbit.
Galilean Moon A Galilean moon event, meaning an event involving one of the four major moons of Jupiter -- Io, Europa, Ganymede, and Callisto. These events include beginnings and endings of the transits of these moons and their shadows across the face of Jupiter, and the appearance and disappearance of these moons due to occultation behind Jupiter's disc, or due to the shadow of Jupiter cast by the Sun upon these moons. The Show event messages option described above provides more information about each Galilean moon event.

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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.  [back]

Signs in the Heavens

Sky View Symbols You can ask Sky View Café 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 from the right as the Moon waxes, and darkening from the right as the Moon wanes -- the shading should not be interpreted as the actual orientation of the Sun's shadow on the face of the Moon, however. 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 for... menu or Show Planet Names button) the screen can sometimes become very crowded with labels. As you pass the cursor over the display, Sky View Café 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.  [back]

The Big Picture

Sky View Café doesn't need to stay at a fixed size bound to the confines of your web browser's window. Just click on the New Window button in the upper right-hand corner of the applet to get an independent, resizable window -- especially helpful if you're using this applet on an 800 x 600 display. You can also use this feature to have more than one window open at a time, showing different views.  [back]


More Than One Way of Looking at Things

Sky View Tabs

Sky View Café has a number of different viewing modes which you can access by clicking on the various tabs you see above.  [back]

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". 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 to blue 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. Click off the Show Daylight/Twilight check box if you want to see the sky fully darkened at all times.

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, but 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 the same way flat maps of the Earth distort the shapes of land masses, the flattened view of the sky distorts 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 at a glance -- the perspective can't help but be somewhat unrealistic.  [back]

There are two ways of seeing the full sky at a glance, Full Sky - Flat and Full Sky - Dome. These represent two different ways of projecting a map of the sky onto a flat screen. You can experiment by 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.  [back]

When you're interested in how the sky appears looking toward the horizon, you can change the view presented by this software 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 from the horizon at 0° on 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 near the bottom of the display. The vertical altitude markings are only accurate near the middle of the display.  [back]

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 Sky View Café will automatically shift to a 45°-wide horizon view centered on that object. If the place where you click is higher in the sky than 42.5°, the software shifts instead to a 100°-wide view centered around the zenith3. This same view can be selected from the pop-up menu with 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 even in an otherwise clear sky. Atmospheric refraction also comes into play as you look closer the horizon, slightly shifting the positions of the celestial objects you see. By default, Sky View Café uses a standard formula to approximate the refraction effect, but variable factors such as temperature, humidity and local elevation make this an inexact calculation.  [back]

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 show the Moon even when the Moon is below the local horizon. A green background, which ends at the local horizon, indicates that the Moon is not present in the sky at a particular location.  [back]

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 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 it 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, 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 so that information about whatever you point at will be displayed rather than information about your specific selection.  [back]

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 planets except Pluto 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 the unusual circumstance which occasionally occurs 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 is therefore much more of an abstraction than a representation of the real sky.

Ecliptic View formation diagram
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 Sky View Café shows in the Ecliptic View, removed from the whole of the celestial sphere and flattened out as shown in the illustration above.

Sky View Café 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 CSULB's Starcharts and 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.  [back]

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 AU4 from the Sun, all of the "naked eye"5 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 to include entirely within the view.

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 6 understanding of the solar system expressed in the Orbits View from the motions of the planets among the stars that you see in the Sky View.  [back]

Moons View

The Moons View shows the four major moons of Jupiter (often referred to as the Galilean moons after Galileo, who discovered them) 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 planetary east to the left. Checking and unchecking the North on top and 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. 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.  [back]

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 Sky View Café'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.
 
       
             
Night
Sun below 18°
Astronomical twilight
Sun -18° to -12°
Nautical twilight
Sun -12° to -6°
Civil twilight
Sun -6° to -3°
Civil twilight
Sun -3° to -0.833°
Early sunrise
Sun -0.833° to 4°
Late sunrise
Sun 4° to 8°
Daylight
Sun above 8°
               
    No moonlight
(or < 17% full)
Moon above 0°
17%-50% full
Moon above 0°
50%-84% full
Moon above 0°
84%-100% full
   

As an experiment, try changing the LATITUDE setting and see what this does to the patterns of daylight and twilight, especially as you approach the poles.  [back]

Calendar

The Sky View Café 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 transit7 time), or the start and end times for varying degrees of twilight.  [back]

Tables

The Tables View of Sky View Café 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 Sky View Café 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: In this table the times of various Galilean moon events are provided. The traditional notation for these events is used, 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.

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.  [back]

Accuracy and Systems of Time

Numerically speaking, most of what you see presented by Sky View Café 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.

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 of time.

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 Sky View Café 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.  [back]

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 always 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 Sky View Café's clock is UT, so there are no leap seconds, but unless the difference of a second is important, you can consider this to be essentially the same thing as the UTC (adjusted locally for your time zone) that you set your watches and clocks to match. Enough astronomical data has been gathered historically to create a tabular conversion of UT to TDB that covers the years 1620-2000, accurate within a few seconds. Earlier than 1620 may never be known with great accuracy, and accuracy beyond 2000 depends on measurements that won't be taken until each new year is upon us, so outside of the range 1620-2000 approximation formulae are used that become more and more inaccurate the further outside of this range you go. When setting the year of Sky View Café's clock to its past and future limits, 1 AD and 9999 AD, the inaccuracy of UT may amount to several minutes.  [back]

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 that 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 at other times retroactively adjusted to February 22, 1732.

Sky View Café 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 and 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 -- so has the clock. 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 Sky View Café, 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. Sky View Café 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.  [back]

Stars and Deep Sky Objects

To display stars and deep sky objects, Sky View Café 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.  [back]

Web Browser Compatibility Issues

Sky View Café is an applet written in the Java language. While cross-platform compatibility is one of the high points of Java, it's a high point that isn't always achieved, making the performance of Java applets difficult to predict at times.

From my own testing and from user feedback so far, the best web browser for Sky View Café for Windows users is Internet Explorer 5.5. For Macintosh users, the earlier version 4.5 of Internet Explorer is preferable to the current 5.0 version because of a couple of Java glitches that were introduced in 5.0 (which will hopefully be fixed soon.) The only feedback I have from Unix and Linux users is with Netscape -- this applet runs in the later versions of Netscape (4.06 and later), albeit somewhat sluggishly compared to Windows and Macintosh performance, where the applet benefits from JIT technology not deployed in Unix/Linux versions of Netscape.

Windows users can use Internet Explorer 4.01 or later (5.5 preferred), or Netscape 4.06 or later (4.75 preferred). I have one report of success using Opera 3.62 for Windows. Though Netscape generally runs quite well, I have seen Netscape freeze on rare occasions after closing the window Sky View Café is running in.

Macintosh users can run this applet on Internet Explorer 4.5 or 5.0 (4.5 preferred until 5.0 bug fixes are issued). I've had several reports of success with the iCab browser. But even the latest officially released versions of Netscape for Mac, however, unfortunately use an antiquated version of Java insufficient to run Sky View Café. I provide the option of running on Netscape using the Mozilla MRJ plug-in, but this is beta software and not completely stable. When Netscape 6 is finally released it should run this applet quite well, but for now the preview versions of Netscape 6 are unstable with any Java applets. Mac users should make sure they keep up to date with the latest releases of MRJ from Apple, available at http://www.apple.com/java/.

Unix and Linux users can use Netscape 4.06 or later, and perhaps other web browsers, but I've had little feedback in this area and not much chance to test in these environments myself.  [back]

The following are known issues:

Internet Explorer 5.0 for Mac 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.

Netscape's Java doesn't process mouse release events reliably, so auto-repeat on the graphical arrows is disabled for 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.  [back]

The NetGeo Server

Using the NetGeo server, Sky View Café 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 Sky View Café 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)  [back]

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 Ephemeris Generator, and USNO's Data Services.

John Walker's Your Sky web site was an inspiration and a touchstone for Sky View Café. Your Sky is less interactive than Sky View Café, but by keeping its computing resources on a web server it is able to hold a far 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.

I'm far from a fan of astrology -- considering it not only superstitious nonsense but routinely unimpressive superstitious nonsense -- but I must nonetheless offer thanks to the Astrodienst web site for providing free public access to one of the best and easiest to use online databases for looking up the locations of cities and towns around the world.  [back]



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 Sky View Café. [back]

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

4. 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 8 1/3 minutes to travel this distance. [back]

5. 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]

6. 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]

7. 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]

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