Skywatch Line for Wednesday and Thursday, March 20 and 21, 2024, written by Alan French

This is Dudley Observatory’s Skywatch Line for Wednesday and Thursday, March 20 and 21, 2024, written by Alan French.

The Sun rises at 6:58 A.M. on Wednesday and sets at 7:08 P.M. On Thursday it rises at 6:56 and sets at 7:09. This Thursday has 20 ½ minutes more daylight than last Thursday.

The Moon reached first quarter early last Sunday and is headed toward full. Wednesday the Moon rises at 2:36 P.M. in the east northeast and is 83% full. At 8:00 P.M. it will be 54 degrees above the southeastern horizon, 85% sunlit, and in the constellation Cancer.

Thursday the Moon, 90% full, will rise at 3:41 P.M. in the east northeast and will be high in the east southeast at 8:00 P.M. in the constellation Leo. The Moon will reach full next Monday. This seems appropriate since Monday comes from the Latin “dies Lunae,” meaning Moon’s Day.

Mercury, in the evening sky, is approaching its maximum eastern elongation, its greatest distance east of the Sun, which it reaches late Sunday. To spot Mercury you will need to look soon after sunset, you will need a good view low to the west, and you will need clear skies near the horizon, free of haze.

Thirty minutes after sunset on Wednesday, at 7:38 P.M., Mercury will be 11 degrees above the horizon and slightly north of due west. If you imagine bright Jupiter, 35 degrees high toward the west, as the center of a clock, Mercury would be at about the 5:30 position (also remember that a fist held at arm’s length spans 10 degrees across the knuckles). Mercury is at magnitude -0.6, but twilight may require binoculars to spot the innermost planet. As it reaches 8:00 P.M. the skies will be darker and Mercury will be 7 degrees above the horizon. It is a toss up whether the darker skies benefit more than the thicker atmosphere penalizes.

Thirty minutes after sunset on Thursday, at 7:39 P.M., Mercury will still be 11 degrees above the western horizon and at magnitude -0.6. At 8:00 P.M. it will be 7 degrees above the horizon.

Due to its closeness to the Sun Mercury never appears high in dark skies. It is also small and distant. These factors make it difficult to study from Earth. When I was young, our knowledge of Mercury was limited. Like our Moon, Mercury was thought to be tidally locked, but with one side always facing the Sun. Its day was thought to be the same length as its year, 88 Earth days. We now know that Mercury rotates slowly, with one day on Mercury being equal to 58.65 Earth days, so one day on Mercury is equal to two-thirds of its year. This spin-orbit resonance means it rotates three times in two revolutions around the Sun.

NASA’s Mariner 10 made three flybys of Mercury, two in 1974 and one in 1975, and revealed that the planet is heavily cratered, like our Moon in appearance. A second NASA spacecraft, MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), made three flybys of Mercury, the first in 2008, before entering orbit around Mercury in 2011.