Skywatch Line for Wednesday and Thursday, April 30 and May 1, 2025, written by Alan French

This is Dudley Observatory’s Skywatch Line for Wednesday and Thursday, April 30 and May 1, 2025, written by Alan French.

The Sun rises at 5:50 A.M. on Wednesday and sets at 7:55 P.M. On Thursday it rises at 5:49 A.M. and sets at 7:56 P.M. This Thursday has 15 ½ minutes more daylight than last Thursday. In April we gained 1 hour, 22 ½ minutes of daylight.

The Moon reached new Sunday afternoon and is now moving toward first quarter, so a crescent Moon will grace the evening sky. On Wednesday night look for the Moon in the west, above Jupiter and very close to the star Elnath. At 9 P.M. the Moon will be 14 ½% in sunlight and 29 degrees high, shining at magnitude -5.0. Bright Jupiter, at magnitude -2.0, will be just over 6 degrees below the Moon and a little to its south. Elnath, at magnitude +1.67 is just over 2 degrees to the lower right of the Moon. Elnath, also know as Beta Tauri, is the second brightest star in the constellation, the 25th brightest star in the night sky, and lies 134 light years from Earth.

Claudius Ptolemaeus Pelusiniensis (today known as Ptolemy) observed at Alexandria between 127 and 151 A.D. He wrote important works on geography and astronomy, notably one known as the “Almagest,” which became the authoritative reference on astronomy in Europe and the Near East. His star catalog expanded on work done by Hipparchus 250 years earlier. In Ptolemy’s catalog Elnath was shared by two constellations, Auriga and Taurus.

Johann Bayer published the first complete star atlas, “Uranometria,” in 1603. Its 51 plates covered the heavens and the atlas included a catalog of 1,277 stars, each with an estimated magnitude. Bayer also assigned a Greek letter to the brighter stars in each constellation, a practice continued today. Like Ptolemy, he included Elnath in two constellations, giving it two names, Beta Tauri and Gamma Aurigae. When constellation boundaries were formalized, it kept one, Beta Tauri.

Beta Tauri is one of only two stars to have shared two Bayer designations, the other being Alpheratz, sharing Andromeda and Pegasus. (Ptolemy also had it in both constellations.) It was named Alpha Andromedae and Delta Pegasi. Today it is just Delta Pegasi, officially residing within the constellation Pegasus.

By Thursday night at 9 P.M. the Moon will have moved eastward into Gemini and lie 40 degrees high. Its journey around Earth and resulting march of sunrise across the lunar landscape will have brought more of its visible face into sunlight, now 23.6% illuminated. The Moon will not set until just after 1 A.M. Friday morning. It will reach first quarter next Tuesday.

These are good days to get in some lunar observing at a convenient hour and under more agreeable temperatures.