Fun with S&T’s Interactive Sky Chart

Sky with Constellation Lines and Labels
S&T photoillustration by Steven Simpson and Akira Fujii.
One of the challenges of publishing an astronomy magazine for a global readership is that we don't all see the same sky. Most obvious are the different constellations visible from the middle latitudes of the Northern and Southern Hemispheres, where most people live. This is a consequence of our living on a giant ball of rock. Relative to the stars, "up" for me in Massachusetts is "down" for a fellow astronomer in southern Chile. Although we're in the same time zone and might both be using our telescopes at the same moment, the stars over my head are below my counterpart's horizon, and vice versa. That's why we print two evening sky charts each month, one for 40° north latitude and another for 35° south.

S&T Sky Chart
Each issue of Sky & Telescope magazine contains a foldout chart of the Northern Hemisphere's evening sky for that month. It is computed for an observer at 40° north latitude.

There's a further complication caused by Earth's daily spin: the constellations visible from a given location are constantly changing, with new ones rising in the east as others set in the west. That's why we print dates and times on our star charts. Finally, the naked-eye planets don't stay put. This is especially true of Venus, which moves across a large swath of sky each month. We compromise by plotting the position of the planets for midmonth.

The bottom line is that unless you live at exactly 40° north or 35° south, and unless you restrict your stargazing to midmonth at exactly the time for which your hemisphere's chart is plotted, what you see in the sky won't quite match what's on the map. Veteran amateur astronomers know this instinctively and make mental adjustments with little effort. But beginners do not, and trying to learn the constellations with charts that don't match the sky faithfully can be extremely frustrating.

From Ink to Electrons

When we redesigned our Web site in 2002, we looked for ways to use the site to complement what we print in the magazine. For the reasons just outlined, it seemed obvious that we should create an electronic sky chart modeled on the ones in Sky & Telescope but customizable for any location on Earth, on any date, at any time. So that's what we did!

Sky & Telescope's Interactive Sky Chart is a Java applet based on the same data and algorithms we use to plot our monthly evening sky charts. Its principal creators include senior editor Roger W. Sinnott and associate art director Steven Simpson, the brains behind our printed charts, as well as engineers and programmers at the Interactive Factory in Boston.

In case you're unfamiliar with Java applets, these are self-contained applications written in the Java programming language pioneered by Sun Microsystems (S&T: June 2001, page 58). When you use a Java-enabled browser to view a Web page that contains an applet, the applet's code is transferred to your computer and executed by the browser's Java Virtual Machine. Java applets are supposed to work on all computers and operating systems, but because the major players in the computer industry haven't yet agreed on uniform standards for the language, this "platform independence" hasn't been fully realized. To find out whether our sky-chart applet will work on your computer, review our system requirements.

Although we developed our interactive sky chart mainly to provide newcomers to astronomy with S&T-style constellation maps for their particular location and observing time, we soon discovered that the applet has many other uses. Some should appeal to more advanced amateurs and, especially, to teachers and students at all grade levels. By changing the observing location, date, and time in certain ways, you can use the applet to gain a new appreciation of the mechanics of the solar system and the sky's daily, monthly, seasonal, and annual cycles. You can also use it to do a bit of astronomical detective work. Let me walk you through some examples to show what I mean.

Getting Started

Choose Location Dialog
Before you can see an accurate representation of your sky, you have to set your observing location. You can enter your latitude and longitude if you know them, or let our geographic database figure them out from your city, state or province, and country.

On the main sky-chart page you'll find links to launch the "Interactive Sky Chart." You'll also see a link to Sky Chart Help, which you should read if you get stuck or want more detailed instructions.

Click one of the "Interactive Sky Chart" links. This will launch the sky-chart applet (which will take a minute or two to load if you have a slow Internet connection) and open a Choose Location dialog box. Enter "Denver" in the City box, choose "Colorado" from the State/Province list, and select "USA" from the Country list. Then click the Submit button. Next you'll see a Choose Time Zone dialog box; note that Denver is reported to lie almost exactly at 40° north, the latitude of Sky & Telescope's northern evening sky chart. Pick "Mountain Time" from the list, check the Daylight Saving Time (DST) box, and click Submit again. Now you'll see a sky chart for this evening (whatever the date) at 9 p.m. local daylight time in Denver.

Choose Time Zone
After entering your observing location, you specify the corresponding time zone and indicate whether daylight saving time is in effect.
On the right side of the screen is a circular map of the constellations labeled All-Sky Chart, with a green, four-sided frame near the edge labeled West. The part of the sky inside this frame appears at a larger scale in the rectangular window labeled Selected View in the upper-left corner of your screen. Under that you'll find Location and Date & Time displays.

The All-Sky Chart's center represents the sky directly overhead. Its circular rim represents the horizon; compass directions are labeled around it. A star that's plotted on the map halfway from the edge to the center will be found halfway up the sky — that is, halfway from horizontal to straight up.

The Selected View shows about as much sky as you can take in at once with your unaided eyes; it measures 50° wide by 40° tall. Compass directions are abbreviated along the bottom, and two markers partway up the right edge indicate your viewing altitude, from 0° at the horizon to 90° overhead.

Let's change the date to October 16, 2002, and the time to 8:30 p.m. to match the circumstances of the northern evening sky chart in our October 2002 issue. In the Date & Time display at the lower left of your screen, click once on the name of the month; it will become highlighted. Now click the + or – buttons to change the month to October. Next, highlight the date and use the + or – buttons to change it to 16. Make sure the year reads 2002. Then do the same thing for the time, highlighting first the hours, then the minutes, and using the + or – buttons to set it to 8:30 p.m.

Sky Chart
Sky & Telescope's Interactive Sky Chart offers two simultaneous views of the heavens: an all-sky chart (at right) and a close-up of the area within the green frame, which can be repositioned by the user. As shown here, the chart is set for mid-October, shortly after dusk, for an observer at 40° north latitude — the same circumstances depicted on the Northern Hemisphere's Sky map in the magazine's October issue.

Now compare the All-Sky Chart in your Web browser to the one in the magazine: they match, right down to the position and phase of the Moon for midmonth! (Although the Moon's phase is indicated properly, the orientation of the terminator — the line between the illuminated and shadowed parts of the disk — is not; it is always shown as vertical.) If you wish, you can choose which objects and labels get displayed on the chart by clicking on the Show Advanced Display Options link under the Date & Time area and making your selections.

Now would be a good time to play with the applet to see how it behaves. Click anywhere inside the green box on the All-Sky Chart, then drag the box around the sky and watch what happens in the Selected View. Or do the opposite: click within the Selected View and move the cursor around while you watch what happens on the All-Sky Chart. Be as adventurous as you'd like. If you run into trouble, click the Help button at upper right or close the applet, return to the main sky-chart page, and start over.

Play and Learn

The real fun begins when you change the location, date, and time and watch how the chart responds. Here are some activities to get you started; no doubt you can come up with many more - especially if you're a teacher!

Spin control: It's a mid-October evening in Denver. Highlight the hours value in the time and start clicking the + button repeatedly while watching the All-Sky Chart. You'll see the stars rise in the east and set in the west, swinging around Polaris, the North Star, as they arc across the sky. (If the one-hour increment is too jarring for you, use one-minute increments instead.) If you keep clicking through the daylight hours, you'll see the Sun follow a similar arc, reaching its highest point due south at midday.

Note how the constellations between Polaris and the northern horizon never set — they just circle the pole, always above the horizon, even in daylight. These are the circumpolar constellations for anyone at a latitude of 40°.

Change the date to September 23, 2002, the autumn equinox, and the time to 6 a.m. Click through the hours and note that there are about 12 each of day and night — that's what equinox means! Now change the date to December 21, 2002, the winter solstice, at 6 a.m. Click through the hours again — sure enough, there are fewer hours of daylight and more of nighttime darkness. And did you notice that the Sun doesn't climb as high up at noon in December as it does in September?

It's just a phase: Let's stay in Denver awhile longer. Change the date to October 5th, when the Moon is new, and set the time to 6:30 p.m., right at sunset early in the month. Note the new Moon near the Sun on the western horizon. Now highlight the day of the month and click the + sign repeatedly. As the days progress over the next two weeks and sunset comes a bit earlier each day, note how the Moon waxes from crescent to gibbous to full as it makes its way through successive constellations of the zodiac. On the 20th, when the Moon is full, it's coming up in the east just after sunset. After this date, if you want to watch the Moon wane toward new again, you'll have to change the time to a later hour; try 6:30 a.m., just before dawn. Neat, huh?

Orion Upside Down
In the wee hours of a mid-October night, Orion, the Hunter, stands on his head as seen from Sydney, Australia. This is a detail from the 'maximized' (full-screen) version of the sky-chart's Selected View looking slightly east of north.
Changes in latitude: Change the date back to October 16, 2002, and the time to 7:30 p.m. Now click the Change button in the Location display to reopen the Choose Location dialog box. Scroll down to the World by City area. Enter "Sydney" in the City box, select "Australia" from the Country list, and click Submit. At the Choose Time Zone dialog, select "Canberra, Melbourne, Sydney: UT+10 hours," leave the DST box unchecked, and click Submit again. You should see an All-Sky Chart very similar to the Southern Hemisphere's sky map on page 90 of the October 2002 issue. Now advance the time in one-hour steps as you did before. Objects still rise in the east and set in the west, but now they arc highest in the sky in the north rather than the south. Can you find the south celestial pole, the point in the sky around which the constellations rotate? (You won't find a "South Star" there!) Find Orion in the predawn sky and center it in the green box on the All-Sky Chart. Look at the Selected View: compared with the view from the Northern Hemisphere, Orion is upside down!

Click the Change button in the Location display. Scroll down to World by City and specify Quito, Ecuador, on the planet's equator. Choose the time zone "Bogota, Lima, Quito: UT-5 hours" and leave the DST box unchecked. Once again, advance the time in one-hour steps. With the Selected View pointing due east, you'll see constellations rise straight up. Now look due west: the stars dive straight down to the horizon!

Return to the solstice on December 21st, at 6 a.m. As you advance the time hour by hour, you'll see that even on this date there are 12 hours of daylight and 12 of night. On the equator, this is true year-round!

North Polar Sky
This maximized all-sky view shows the stars above the North Pole on the December solstice, when night lasts 24 hours. Polaris, directly overhead, is at the chart's center. Deep-sky object labels have been turned on using the advanced display options. If the sky-chart applet were a little smarter, the cardinal directions labeled on the horizon would all read 'South,' and the houses and trees in the Selected View would be replaced by icebergs.
Leaving the date in late December, call up the Choose Location box again. This time, scroll down to the Worldwide by Latitude & Longitude area. Enter 90° 00' N latitude and any longitude; then specify any time zone — it doesn't matter which one. Welcome to the North Pole! Do you see Polaris directly overhead, at the center of the All-Sky Chart? (Pay no attention to the houses and trees around the horizon in the Selected View — they're illusions brought on by the cold.) Watch the horizon in any direction in the Selected View as you advance the clock hour by hour: the constellations move parallel to the ground, like horses on a merry-go-round, and the Sun never rises. Now switch to 90° south, leaving everything else the same. Welcome to the South Pole, now enjoying 24 hours of sunshine!

Unidentified flying objects: On July 4, 2002, millions of Americans gathered in parks and along riverfronts to enjoy traditional Independence Day fireworks. As the twilight sky darkened, a brilliant beacon gleamed in the west. Was it a UFO? I'm sure many nonastronomers thought so. But a quick check of our interactive sky chart for 9 p.m. on July 4th from any U.S. city shows that the beacon was none other than the planet Venus, still in the midst of a truly dazzling performance in the evening sky.

What Next?

Printable All-Sky View
This is the printable version of the all-sky view for October 16, 2002. You'll get better results if you can print the normal colorful view.

As already noted, our sky-chart applet doesn't show the Moon's true orientation, though it does correctly represent the lunar phase. The chart also shows the five naked-eye planets with symbols that are all the same size, belying the huge range of brightness among them. We accepted these compromises to keep the applet from getting too big and taking too long to download. When high-speed Internet connections become more common, we'll address these limitations in a future release of the applet.

If you click the Create PDF button under the All-Sky Chart or the Selected View, you'll get a black-on-white version you can print without emptying your printer's ink cartridges.

If you want more power in a sky-charting program — fainter stars and planets, more deep-sky objects, the ability to zoom in and out — you should consider buying one of the full-featured planetarium programs advertised regularly in Sky & Telescope. We didn't design our sky-chart applet to compete with these programs, merely to bring the magazine's monthly evening sky charts to life.

So what are you waiting for? Download our sky-chart applet, pick a location anywhere on the planet, and explore the night sky. Remember: on the Web it's never cloudy!