How to use Constellations to tell Time

Before the advent of watches and clocks, groups of people throughout the world used the rising and setting of stars to mark off the hours of the night. Although time keeping devices are ubiquitous in the modern world, telling time by the stars is still an enjoyable activity, and in certain situations, a valuable skill. This article will focus on easy to find stars and constellations, with an emphasis on patterns visible to observers in the Northern Hemisphere.

For beginners, one of the most easily recognized constellations is Ursa Major, the Great Bear. The most noticeable part of the Great Bear is an asterism known as the Big Dipper. Most of the seven stars of the Big Dipper are of second and third magnitude, meaning they can be seen even under hazy conditions or in areas with substantial light pollution. So that the next few paragraphs make sense, please take a moment to refer to the diagram of the Big Dipper in relation to Polaris, the North Star, which marks the tail of Ursa Minor, the Small Bear or Little Dipper. Finding north is not complicated, assuming you have a compass. Failing that, take notice of the position of sunrise or sunset, and keep in mind the mnemonic Never Eat Shredded Wheat to orient yourself with respect to the cardinal directions.

In the early evening in spring, the Big Dipper is directly above the North Star, just as it appears in the diagram. Six hours later, the Big Dipper has moved 90 degrees and is now to the left, or due west of the North Star. Six hours after that (right around dawn), the Big Dipper is directly below the North Star. In twelve hours, the Big Dipper has moved around half of a great circle (180 degrees) with the North Star in the center.

In the winter, the Big Dipper is to the right, or due east of the North Star. Twelve hours later, it has reached its summer position. In the span of one hour, stars move 15 degrees to the west, in a counterclockwise direction relative to the North Star. 15 degrees is approximately the distance from your thumb to the tip of your pinkie with your arm outstretched.

For more advanced sky watchers, as long as you know the season of the year, you can usually estimate the time of night accurately from the positions of a few bright stars.

In late winter and early spring, Arcturus in Bootes can be found in the eastern sky by following the curve of the Big Dipper’s handle. Arcturus is a bright orange star and is relatively easy to spot. Four hours later, another bright star called Vega, rises in the northeast. Alternatively, you can look to the northwest and spot the Pleiades and Taurus setting in the early evenings of March and April.

In summer, three stars called the Summer Triangle dominate the night sky. They are Vega, in Lyra, the Harp; Deneb, in Cygnus, the Swan; and Altair, in Aquila, the Eagle. Other noticeable constellations in the summer sky form part of the zodiac: Leo, the Lion in the southwest; followed by Virgo, the Virgin; then the dim constellation Libra, followed by Scorpio, the Scorpion, with its bright red star Antares. Sagittarius, the Archer, rises in the southeast a few hours after the Scorpion, and is visible nearly to dawn.

The coming of autumn is marked by two bright stars – Capella, in the constellation Auriga, the Charioteer; and Fomalhaut, the only bright star in Piscis Austrinus, the Southern Fish. The important thing to remember is that although these two stars are located in the extreme northeast and southeast parts of the sky, they rise roughly at the same time. Fomalhaut set around 2 am, around 2 hours before Capella has reached the half way point in its course across the sky. Another constellation easily recognized in the northeastern sky in late summer and early fall is Pegasus, the Winged Horse, marked by four stars called the Great Square. Pegasus is visible throughout the winter months, setting by early spring.

The winter sky is dominated by the constellations Orion, the Hunter; and Canis Major, the Big Dog. Orion contains the bright red star Betelgeuse and the brilliant blue star Rigel. Three stars marking the hunter’s belt run midway between these two along the sky’s celestial equator. Southeast of Orion is Canis Major, which contains Sirius, the brightest star in the night sky. By December, Orion is visible just after sunset, and sets around sunrise. Sirius

Southern Hemisphere – For observers south of the equator, many constellations will still be visible, with the notable absence of the Big and Little Dippers. Constellations straddling the celestial equator, like Orion, are useful guides, at least for 6 months or so each year. Certain constellations are circumpolar in relation to the south celestial pole , meaning they can be seen on any clear night of the year. The ones with bright stars include Centaurus (the Centaur), which contains Alpha and Beta Centauri; Crux, the Southern Cross, with the stars Acrux and Becrux; Carina, the Keel, which contains Canopus, the second brightest star in the night sky; and the southern region of Eridanus, the River, with its bright star Achernar.

In terms of position, Achernar is southwest of Orion. Canopus is almost directly due south of Sirius, the Dog star. The Southern Cross and Centaur rise around 6 hours after Orion, so Orion is close to setting by the time these constellations become visible. As a final tip, keep in mind that an hour or so after Orion sets, the Scorpion rises. Conversely, an hour or so after the Scorpion sets, Taurus the Bull, with its bright star Aldebarran rises.