This is Dudley Observatory’s Skywatch Line for Friday, May 29, through Sunday, May 31, written by Sam Salem.
On Friday, Sun rises at 5:21am and sets at 8:25pm, the first quarter Moon sets at 1:32am and rises at 11:53am. Moon reaches its first quarter phase on Friday at 11:30pm. As darkness falls on Friday and Saturday, watch the Moon as it travels in front of the constellation Leo the Lion. As stars come out, the Moon is high in the southwest below the belly of Leo’s stick-figure lion pattern. The Moon forms a nearly equilateral triangle with Leo’s Regulus to its lower right and Gamma Leonis, or Algieba, slightly fainter, more directly to the Moon’s right.
Look for Mercury in evening twilight, in the west-northwest. By this weekend, Venus has dropped out of sight. Mercury fades from magnitude –0.6 to 0.0. In a telescope it is a tiny “half-moon” shape, 7 arc-seconds in diameter.
Mars, Jupiter, and Saturn shine in the southeast to south before and during early dawn. Jupiter, the brightest, is on the right. Saturn glows pale yellow 4 degrees to Jupiter’s left. They both rise now around midnight. They straddle the border of Sagittarius and Capricornus. Mars, in dim constellation Aquarius, is 35 degrees to 40 degrees to the left of Saturn as dawn begins. It has been slowly brightening and enlarging. In a telescope, Mars is now 9 arc-seconds wide, a little gibbous disk. Neptune, at magnitude 7.9, in Aquarius, is fairly low in the east-southeast before dawn begins.
Capella is still up in the northwest in twilight, but it sets in the northwest fairly soon after dark. That leaves Vega and Arcturus as the brightest two stars in the evening sky. Both are magnitude 0. Vega shines in the east-northeast. Arcturus is very high toward the south. A third of the way from Arcturus to Vega, look for semi-circular Corona Borealis, the Northern Crown, with 2nd-magnitude star Alphecca, or Gemma, the jewel of the crown. Two thirds of the way from Arcturus to Vega is the dim Keystone of Hercules. Use binoculars or a telescope to examine the Keystone’s top edge. A third of the way from the edge’s left end to the right is 6th-magnitude M13, one of Hercules’s two great globular star clusters. In binoculars it’s a tiny glowing cotton ball. A scope resolves some of its details. M13 consists of several hundred thousand stars in a swarm about 140 light-years wide.
Friday marks the 101st anniversary of the solar eclipse permitted observation of the bending of starlight passing through the Sun’s gravitational field as predicted by Albert Einstein’s theory of relativity. Separate expeditions of the Royal Astronomical Society travelled to Brazil and off the west coast of Africa. Both made measurements of the position of stars visible close to the Sun during a solar eclipse. These observations showed that, indeed, the light of stars was bent as it passed through the gravitational field of the Sun.