Skywatch Line for Friday, July 16, through Sunday, July 18, 2021

This is Dudley Observatory’s Skywatch Line for Friday, July 16, through Sunday, July 18, written by Sam Salem.

On Friday, Sun rises at 5:32am and sets at 8:31pm; Moon rises at 12:25pm. The Moon completes the first quarter of its orbit around Earth at 6:10am on Saturday. Its 90-degree angle away from the Sun will cause us to see the Moon half-illuminated on its eastern side. At first quarter, the Moon rises around mid-day and sets around midnight.

As darkness falls on Friday and Saturday, the Moon introduces you to Spica, the brightest star to in the constellation Virgo the Maiden. Once every month, as the Moon makes its monthly rounds in front of the constellations of the zodiac, the Moon sweeps by Spica. From a distance of 262 light-years away, Spica appears to us on Earth to be a lone bluish-white star in a quiet region of the sky. Spica consists of two stars, and maybe more. The pair are both larger and hotter than our Sun. They’re separated by 11 million miles. They orbit their common center of gravity in four days. Earth is 93.3 million miles from our Sun. The two stars in the Spica system are individually indistinguishable from a single point of light, even with a telescope. The dual nature of this star was revealed only by analysis of its light with a spectroscope.

On Saturday, the dim and distant dwarf planet Pluto will reach opposition. The Earth will be positioned between Pluto and the Sun. While at opposition, Pluto will be located 3.10 billion miles, or 277 light-minutes from Earth and it will shine with an extremely faint visual magnitude 14.3. That’s far too dim for visual observing through backyard telescopes. Pluto will be located in the sky about midway between Saturn and the bright star Nunki in Sagittarius’ Teapot asterism.

On Sunday, the main belt asteroid designated (2) Pallas will halt its regular eastward motion in front of the distant stars and begin a retrograde loop that will last until early November. Pallas’ visual magnitude of 9.7 will allow it to be seen in amateur telescopes starting in late evening. On Sunday, Pallas will be positioned in the eastern sky, less than half a degree to the right of the magnitude 6.65 star HIP116417 and the magnitude 7.35 star HIP116431, which sit near the ring of stars that forms the western fish in Pisces. The asteroid and those stars will appear together in the eyepiece of your telescope.

The Coathanger, or Brocchi’s cluster, is a tiny asterism. This star formation looks exactly like its namesake and is easy to make out through binoculars, you need to know just where to look. The Summer Triangle asterism can help point the way. The cluster is located along a line between two Triangle stars, Vega, and Altair.

On July 17, 1850, the first photograph of a star, other than our Sun, was taken. At Harvard Observatory, the observatory director, William Cranch Bond and a Boston photographer John Adams Whipple took a daguerreotype of Vega. Astrophotography began in 1840, when John William Draper took an image of the Moon, using the same daguerreotype technique. The technique was based on polishing a sheet of silver-plated copper to a mirror finish, treating it with fumes that made its surface sensitive to light and exposing it in a camera for as long as it was necessary. The plate was then developed in a current of magnesium vapor, which adhered to the light-struck parts of the plate.

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