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?

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!

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.