The Summer Triangle

Date: July 15-30
Time: 8:45 pm, one hour after sunset
Place: the eastern sky

The Summer Triangle is visible in the east this month. The Triangle is what astronomers call an asterism, or “unofficial” grouping of stars. It connects three major constellations in a triad of bright, easily visible sky lights.

The brightest star is Vega, part of the constellation Lyra. Next is Deneb, the tail of Cygus the Swan. Dimmest, due to its lower elevation, is Altair, the eye of Aquila the Eagle.

We say the Summer Triangle is “rising” since it appears for the first time each year, after sunset, in early Summer. As time goes by it’ll drift west by 1° per day, completing a 360° circuit around the sky in exactly one year. At this time, one year from now, the Summer Triangle will be in exactly the same position.

This westward drift is common to all constellations. It’s a result of Earth’s orbital motion around the Sun. You can follow the Triangle for the next six months. By late December it will have drifted so far to the west it’ll set at the same time the Sun rises.

Each season has a prominent constellation, or asterism, for which it is known. Recognizing these patterns helped early humans anticipate the annual cycles of nature. When to plant, when to hunt, when to harvest, when to gather wood for the winter — all these decisions were guided by the sky.

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