Carnegie Skywatch

March/April Night Skies

by Jenny Pon

Stars Hidden by the Moon

When the moon passes in front of a star or planet and covers it, we call it an occultation. Two take place in March. During the evening of March 4, Aldebaran (the bright red star marking the eye of Taurus the Bull) will disappear along the dark eastern limb of the nearly first quarter moon at 7:26 pm EST, as viewed from Pittsburgh. During morning twilight on March 26, Jupiter will rise already occulted by the moon. Appearing low in the east, the waning crescent moon will slowly uncover Jupiter. The planet will reappear along the dark western limb of the moon at 5:44 am EST. Both occultations are best viewed through a telescope. However, the events can still be viewed and enjoyed through a pair of binoculars held steady, or with the naked eye.

Each month the moon passes near several bright stars of the zodiac, sometimes occulting some of them, as is the case on March 4 with Aldebaran. Look for these occultations: Aldebaran: March 4, 31, April 1, 27, 28 (dusk). Pollux: March 7, 8, April 4 (dusk). Regulus: March 10, April 6, 7 (dusk). Spica: March 14, 15 (overnight), April 11 (dusk). Antares: March 19, 20, April 15, 16 (dawn).

Lunar Eclipse

A penumbral lunar eclipse will occur during the night of March 12. The Earth’s shadow is made up of a dark central portion known as the umbra that is surrounded by a lighter portion known as the penumbra. During this eclipse, the moon will pass only through the penumbra and north of the umbra. Observers will notice a slight shading on the moon’s southern portion. Greatest eclipse occurs at 11:30 pm EST, with 73% of the moon’s diameter in the penumbral region of the Earth’s shadow.

Using the Stars As A Calendar

Stars disappear into and emerge from the solar glare on the same dates each year, as seen from a particular latitude.

By carefully observing stars near the western horizon after sunset or near the eastern horizon before sunrise, ancients used the sky as a calendar to determine when to begin such activities as plowing, planting, and harvesting.

During April, the bright winter stars appear above the western horizon after sunset, allowing viewers to see the seasonal westward march of the stars as the ancients saw it. Try observing daily at a fixed time after sunset, say 45 minutes later. Keep track of these stars through June: Rigel, Aldebaran, Sirius, Betelgeuse, Procyon. You will notice these stars appear progressively closer to the western horizon each evening, and then finally disappear into the twilight glow. The final observation of a star, on the last date possible before its disappearance into the solar glare, is called the heliacal setting of that star. Keep a log on each of the five stars listed and try to discover for yourself the last possible date you can observe each star with the unaided eye and with binoculars only. The reason for this apparent westward motion of the stars is the Earth's orbital revolution around the sun.

The Planets

Evening planets: March will give us a last glimpse of planets seen in the evening sky for awhile. Other than an appearance by Mercury during late June and July, the early evening sky will be devoid of planets until September. March begins with faint Mars low in the west at dusk with Saturn over 20 degrees to its upper left. To the upper left of Saturn on March 1 is the moon. During March, look early each evening and watch Saturn appear lower in the sky and approach Mars. Binoculars will help spot Mars close to the horizon. Mercury joins the pair as early as March 6 and begins its best evening appearance for the year for northern observers. On March 10, Mars appears one degree to the left of Mercury. On March 21, Mercury appears 5 degrees to the right of Saturn. On March 22 and 26, the trio of planets will form an isosceles triangle low in the west at dusk.

Morning planets: For most of March, Venus is the only naked-eye planet in the morning sky. Venus shines brilliantly low in the ESE an hour before sunrise. Jupiter joins Venus in the morning sky in late March. The reappearance of Jupiter from behind the moon on March 26 may provide the first glimpse of the planet in the morning. Thereafter, Jupiter climbs higher in the sky each morning, appearing to the lower left of Venus. Watch during the first three weeks in April as Jupiter closes in on Venus. By April 23, Jupiter passes Venus by only 0.5 degrees (about a moon’s width). In reality they are many millions of miles apart, with Jupiter the one farther behind. They only appear close together in the sky because they lie nearly along the same line of sight as seen from Earth. The crescent moon joins the pair on the 23rd, making this particular Venus-Jupiter conjunction a rare event. Don't miss it! After the 23rd, Jupiter continues to climb higher into the sky, appearing farther away from Venus. By April 30, the two planets will be nearly 7 degrees apart, with Jupiter to the upper right of Venus. Mercury makes a faint appearance late in April. Binoculars will be necessary to catch a glimpse of it hugging close to the eastern horizon far to the lower left of Venus.

Moon Phases

First Quarter Mar 5 3:41 am Apr 3 3:18 pm

Full Mar 12 11:34 pm Apr 11 6:23 pm

Last Quarter Mar 21 2:38 am Apr 19 3:53 pm

New Mar 27 10:14 pm Apr 26 7:41 am

For current information on astronomical events, call the Skywatch Hotline at 237-3400, then press 1-3-4 within the menuing system.

Planetarium Shows

Check the Highlights section of this magazine for information on current shows in the Henry Buhl, Jr. Planetarium.

Jenny Pon is planetarium astronomer and digistar system manager for the Henry Buhl, Jr. Planetarium & Observatory at Carnegie Science Center.

Using the Star Map

This star map is like a photograph of the sky taken at a particular time on a certain date. Each day the stars in the sky reach the same positons as those shown on the star map by about 4 minutes earlier. This adds up to about an hour every two weeks. This subtle shift in time is caused by the Earth’s orbiting around the sun. Therefore, the approximate Eastern Standard Times this star map will match the sky are: March 1, 11 pm; March 15, 10 pm; March 31, 9 pm; April 15, 8 pm; April 30, 7 pm. Add an hour for Daylight Savings Time in April. The map is useful up to an hour either side of the times listed.

To read the map in the dark and to preserve your night vision, place a small brown paper bag over a flashlight. Hold the map in front of you and rotate it so that the edge of the horizon circle that corresponds to the direction you are facing appears at the bottom of the circle. For example, if you are facing south, rotate the map so that the edge corresponding to south appears at the bottom of the circle. The stars above this horizon on the map now match the stars you are facing in the sky.

The map’s center corresponds to your overhead point. Therefore, a star located halfway from the edge to the center can be found in the sky about halfway from horizon to overhead. (Saturn plotted for February 1.)

Copyright 1998 Carnegie Magazine All rights reserved. Email: carnegiemag@carnegiemuseums.org