This is the Dudley Observatory Skywatch Line for Wednesday, July 4th, and Thursday, July 5th, written by Louis Suarato.
The Last Quarter phase of the Moon occurs at 2:18 p.m. Wednesday, about 2 hours after it sets for the day. The Moon will rise again at 28 minutes past midnight. Try to spot Mercury before it sets with M44, the Beehive Cluster, at 9:40 p.m., in the northwest. Mars, Saturn, Jupiter, and Venus all share the sky until Venus sets at 10:53 p.m., ahead of the constellation Leo, and its brightest star, Regulus. Look for Jupiter above the southwestern horizon before midnight. Saturn will be embedded within the Milky Way in the south-southeast. Mars rises at 10:27 p.m. in Capricornus.
Celebrate the 4th of July by looking for NGC 6946, also known as the Fireworks Galaxy. NGC 6946 is a face-on spiral galaxy between the constellations Cepheus and Cygnus. This 9.60 magnitude galaxy is about 22.5 million light-years away. Discovered by William Herschel on September 9, 1798, the Fireworks Galaxy spans 40,000 light-years, about one-third the size of our Milky Way. Eight supernovas have been observed to explode within this galaxy during the past century. Robert Burnham, Jr. describes NGC 6946 as having “at least four well defined arm segments, and several fainter branches or “sub arms”.” Look for The Fireworks Galaxy about 20 degrees to the north of Deneb, the tail star in Cygnus the Swan, It can also be located by looking 2 degrees southwest of the star, Eta Cephei.
In addition to July 4th being the anniversary of the signing of theDeclaration of Independence, it is also the birthdate of astronomer Henrietta Swan Leavitt. Born in 1868, Leavitt studied the periods of pulsating stars, known as Cepheid variables. These stars regularly vary in brightness from a few days to several months. By studying 1,777 variable stars in the Magellanic Clouds, Levitt determined that “since the variables are probably nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness.” This became known as the Period-Luminosity relation, and is used to calculate the distance of galaxies.