This is Dudley Observatory’s Skywatch Line for Friday, July 21, through Sunday, July 23, written by Sam Salem.
On Friday, Sun rises at 5:36am and sets at 8:27pm; Moon rises at 3:43am and sets at 6:50pm. New Moon occurs on Sunday at 5:46am. Mercury is higher in the evening twilight this weekend. Its greatest elongation from the Sun occurs on July 25. Jupiter shines at magnitude –1.9 high up in the west-southwest as darkness falls. At magnitude 0.2, Saturn is still an easy find low down in southern of constellation Ophiuchus. The best time to see Saturn in a telescope is when it reaches the meridian at around 11pm. Venus clears the east-northeast horizon at magnitude –4.1 around 2:42am on Friday.
Use the Summer Triangle asterism 1st-magnitude stars Vega, Altair, and Deneb to point you to the Teapot of Sagittarius and the Galactic center. Gaze southward once it gets dark to find “The Teapot” which marks the general direction of the Galactic center so heavily veiled by intervening stars, star clusters and nebulae. The Teapot makes up the western half of the constellation Sagittarius, the Archer. Draw an imaginary line from Deneb through Altair to star-hop to the Teapot of Sagittarius. This year the Milky Way center lies about midway between the spout of the Teapot and the planet Saturn. Use Saturn to guide you to the Teapot and the Galactic center.
The Teapot of Sagittarius sits below constellation Scutum, the Shield, and the bright star Vega shines high above it. Scutum is located in a rich region of the Milky Way and requires a dark sky to be seen. Look southward late in the evening toward the richest part of the Milky Way to find the very small constellation Scutum. It has only four stars that make up the constellation outline. The Polish astronomer Johannes Hevelius named it Scutum Sobiescianum, meaning the shield of Sobieski, in 1683. He named it for Jan III Sobieski, a Polish king who led his armies to victory in the Battle of Vienna. Scutum is one of two constellations named after real people. The other one is Coma Berenices, named for an Egyptian queen.
On these July evenings, look high overhead for Corona Borealis, or the Northern Crown. The semi-circle of stars lies between the two bright stars Arcturus and Vega. Arcturus reaches transit altitude of 66 degrees south almost an hour before sunset and it sets around 2:30am. Vega is still high in the east on July evenings. Vega reaches transit altitude of 68 degrees south around 11:30pm. Corona Borealis constellation is easy to pick out in a dark sky. It makes the shape of the letter C in the middle of which is a white jewel of a star called Alphecca or Gemma. Gemma is a Latin name, which means a “gem”. The gem of the Northern Crown is also called Alphecca, from an Arabic phrase meaning “the bright one of the dish”. Gemma, or Alphecca, is an eclipsing binary system. It consists of a smaller star that passes in front of a brighter star every 17.4 days, as seen from earth.