Skywatching Activities, April and May 2011
by Robert Victor
Several bright stars of April evenings will depart by the end of the school year. Watch them go! The April Sky Calendar and Evening Skies star map are available at www.pa.msu.edu/abrams/ABDNHAMar-AprSkies/.
Each year in the April evening sky, a large oval of bright stars, called the Winter Ellipse, or Winter Hexagon, heads toward the western horizon. Rigel, in the foot of Orion, the Hunter, will be the first of these stars to depart near the end of April. Next, in order of disappearance from mid-northern latitudes, are:
- in early May: Aldebaran, eye of Taurus, the Bull
- in mid-May: Sirius, the “Dog Star”; and Betelgeuse, shoulder of Orion, will actually be inside the Hexagon
- in early June: Procyon, the Little Dog Star
- in late June: Capella, the “Mother Goat” star; and Pollux, one of the Gemini Twins (accompanied by his fainter twin, Castor).
All but Capella and the Twins will be gone by the start of summer. These stars will appear lower each evening in spring as a result of the Earth’s revolution around the sun.
Student Activity Suggestion:
Provide each student with a star map and a checklist of these stars. Have them note on each clear evening whether or not they were able to observe each star. Ask the students to start searching for these stars within 45 minutes to an hour after sunset. It is especially important to start looking that early to catch a star approaching the western horizon during the week or so before its departure.
There are four additional bright stars shown on the April star map. The handle of the Big Dipper locates two of them, when you “Follow the arc to Arcturus and drive a spike to Spica.” Water leaking through the bottom of the Big Dipper’s bowl would fall on the back of Leo, the Lion, whose brightest star is Regulus. Finally, Vega, the leading star of the Summer Triangle, is just rising in the northeast.
After Vega rises and before Rigel sets, we have 11 of the 15 brightest stars (of first magnitude or brighter) ever visible from mid-northern latitudes, the greatest number possible. (Another selection of 11 will appear on our December map, after Sirius rises and before Altair sets.)
The only other bright object in the April early evening sky is Saturn. On April 3, this planet aligns with the sun and Earth in a straight line so that Saturn appears at opposition to the sun and is above the horizon all night.
Some 59 years ago, when as a 12-year-old I started noticing the positions of the planets, Saturn appeared in nearly the same place against the stars. Now, two Saturn-years later, in the spring of 2011, we can again watch Saturn appear to retrograde or move west against the same background. Note the 3rd-magnitude star Gamma in Virgo not far to Saturn’s upper right. In the second week of June, Saturn will pause just a quarter of a degree from that star.
The four other bright planets will form wonderful groupings best seen through binoculars very low in bright morning twilight. Drawings of the planet gathering as seen from the latitude of southern California on each morning of the month appear on the May Sky Calendar at www.pa.msu.edu/abrams/skycalendar/. Descriptions of various groupings appear in the calendar’s left margin notes.
May 11 is the date of the most compact trio of Venus, Jupiter, and Mercury, all within 2.1 degrees. Venus and Jupiter, the two brightest planets, appear closest that morning, and fainter Mars will appear within 6 degrees to Venus’ lower left, as shown in the calendar diagram for May 11.
Observers can simply follow Venus in the morning sky in the weeks and days leading up to the gathering on May 11. Sky-watchers can also determine well ahead of time exactly where the gathering will appear in relation to the local horizon features at their selected observing sitesusing the following procedure:
- Observe the sun rising over the local horizon scene on Sunday, April 10, being sure to choose a place with a relatively unobstructed view in that direction.
- Note the time when half the sun’s disk has risen above the distant horizon profile.
- Subtract about 1 hour 44 minutes from that time, and on May 11 at this earlier time, Venus will pop into view in nearly the same place!
Here is a table of time adjustments for previewing the rising of Venus on various dates in May by viewing sunrise about a month earlier.
View to preview Look earlier
sunrise Venus rising by this time interval
on this date on this date to catch Venus rising
April 5 May 7 1 hr 48 min
April 10 May 11 1 hr 44 min
April 15 May 15 1 hr 40 min
April 20 May 19 1 hr 35 min
April 25 May 23 1 hr 31 min
Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs.
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