As we spin towards summer, the length of the day continues to increase, but more slowly. Early in May the sun rises about 6 am and sets about 8:10 pm; near the end of May it rises about 5:45 am and sets about 8:25 pm. (All times are EDT.)
Phases of the Moon: Full on the evening of May 5; 3rd quarter on May 12, New on May 20; and 1st quarter on May 28. On May 20 the Moon will partially cover the Sun in what is called an annular eclipse. The eclipse will not be visible from Maryland, but if you are planning to be anywhere north-west of a line from central Texas through central Wisconsin, you might be able to see it in the late afternoon before sunset. Be sure to use safe viewing techniques (eg. Observing Solar Eclipses Safely).
Evening Sky: Auriga (the Charioteer), with its bright star Capella, and Gemini (the Twins) (bright stars Castor and Pollux) dominate the western sky after sunset. At this time of year in the evening, the Big Dipper very high in the sky, making it quite easy to use the Pointer Stars to find the North Star, aka Polaris. Bootes (the Herdsman) with its bright star Arcturus is well up in the north-east. Mars is high in the sky towards the south-west, still near the bright star Regulus in Leo (the Lion). Saturn is high in the sky towards the south-east, near the bright star Spica in Virgo (the Young Woman). Venus is at it most brilliant in early May, gleaming high above the western horizon, poised for its plunge back towards the Sun and its Transit of the Sun on June 5th. Early in May Venus sets more than 3 hours after sunset. As the month progresses, Venus appears lower and lower in the sky, until by the end of May it will set a mere 30 minutes after sunset. Viewed with good binoculars or a small telescope, the sunlit disk appears as a fat crescent early in the month; during the month, the crescent will grow in length while it thins to a sliver, as Venus “catches up with” the Earth as the two planets move in their orbits around the Sun. The crescent phases of Venus were one of the key observations that lead Galileo to conclude that the Earth is not the center of motion for the solar system.
Pre-dawn Sky: For those who start the day early, or are just getting off the 6pm-3am shift, the pre-dawn sky holds some treats. Early in May, Sagittarius (the Archer) is rising in the south-east, and Scorpius (the Scorpion), harbinger of summer, with its bright red star Antares, is well up towards the south. The name “Antares” is of Greek origin, and means “not Mars”. If you are out between 1 am and 2 am in the morning early in May, take a moment to compare Antares with Mars, which is low on the western horizon, and you may understand why that star received such a name. In addition, Saturn is still high in the sky towards the south-west, and does not set until around sun rise. If the sky is dark enough in your area, you may see the faint glow of the Milky Way rising up from Scorpius and Sagittarius and arcing above the eastern horizon through Aquilla (the Eagle) and Cygnus (the Swan).
That’s all for this month. And, as the great Jack Horkhemier used to say, Keep Looking Up!
Helen Hart, 30 April 2012; references: the Astronomical Calendar, Starry Night planetarium program, USNO Astronomical Data Services.