This is the Dudley Observatory Skywatch Line for Wednesday, October 21st, and Thursday, October 22nd, written by Louis Suarato.
Look over the south-southwestern horizon after sunset on Wednesday for the 33% illuminated, waxing crescent Moon. To the Moon’s upper left are Jupiter and Saturn. Thursday night, the nearly First Quarter Moon moves below the 2 gas giants to form a triangle. Saturn and Jupiter are 6 degrees apart. In the east, Mars shines at magnitude -2.45 after rising at 5:43 p.m., Wednesday. Thursday night, all four Galileo moons will be visible to the sides of the Jupiter until Io begins to transit the planet at 11:30 p.m.. Jupiter’s Great Red Spot begins its transit across the central meridian at 8:31 p.m., Wednesday. Venus rises in the constellation Virgo at 4:07 a.m. Thursday.
Look to the west of Mars for one of the most prominent constellations of Autumn. Easily identified by its Great Square asterism, Pegasus is the seventh largest constellation. The four stars that comprise the Great Square of Pegasus are located in a relative empty sky, void of bright stars, so are easy to see. The four bright stars are Alpheratz, the upper left star in the square in the northeastern corner and actually part of the constellation Andromeda. Markab is located at the southwestern corner of the Square. Algenib is located at the southeastern corner. Scheat is at the northwestern corner and is a bright giant red star visible through binoculars. Algenib is located at the southeastern corner of the Great Square. Algenib is located 335 light-years away and is the dimmest of the four stars. M15 is a globular cluster located in Pegasus. Discovered by Jean-Dominique Maraldi in 1746, M15 shines at magnitude 6.4 and is 33,600 light-years from Earth. It is one of the oldest known globular clusters, estimated to be 12 billion years old. M15 is one of the most densely packed globular clusters in the Milky Way, comprised of over 100,000 stars. In 1928, a planetary nebula known as Pease 1, was discovered by Francis G. Pease at the center of M15. Only telescopes with an aperture of 12 inches or more can detect Pease 1. Look for M15 4 degrees to the northwest of the star Enif, the brightest star in the constellation.