Observing the Stars

141029_ALMA_tauri_01Identifying objects in the night sky for the first time can be a daunting experience. A good way to start is by learning to gauge distance and finding a few bright stars and key constellation. Once these reference points have been identified, imaginary lines can be drawn outwards from them to find other stars and patterns, which in turn act as useful stepping-stones to the rest of the sky.

Scale in the sky

It can be difficult to judge how large a feature will appear in the sky by looking at a map. Fortunately, a hand held at arm’s length acts as a convenient measuring scale. For example, an index finger easily covers the Moon or Sun, both of which are only half a degree across, the back of a hand is about 10 degree across, the width of the bowl of the [Big Dipper]. To cover the Great square of Pegasus [16 degree across], the figure must be splayed.

Northern Skies

The Plough, or Big Dipper, which is high in northern-hemisphere skies in spring, is a key pattern in the northern sky. Two stars in the bowl of the Plough, alpha and Beta Ursae Majoris, point to the North Pole star, Polaris. On the other side of the celestial pole is the W-shaped constellation Cassiopeia. The Plough’s bowl can also be used to locate the bright star Vega, which is prominent in northern skies in summer. A line extended from the curving handled of the Plough leads first to Arcturus, a beacon of spring skies, and then to Spica, the brightest star in Vigro. Also to the south of the bowl of the Plough are leo and the stars Castor and Pollux in Gemini.

Mid-latitude skies

Orion is a prominent constellation in the evening skies if northern winter and southern summer. The line of three stars that forms Orion’s Belt points to the brightest star in the sky, Sirius [in Canis Major]. Sirius marks one corner of a huge triangle [known in the northern hemisphere as the Winter Triangle]. The other corners are marked by the bright stars Betelgeuse [in Orion] and procyon [In Canis Minor]. A line from Rigel through Betelgeuse points to castor and Pollux in the neighboring constellation of Gemini. On the other side of Orion is Almdebaran, the brightest star in Taurus; beyond Aldebaran, in the same direction, is the Pleiades open cluster. Almost directly to the north of Orion lies Capella, the January evenings from mid-northern latitudes.

Southern Skies

Two bright stars in Centaurus and the familiar shape of crux, the Southern Cross-all of which are at their highest on April and May evening-are the starting points for finding a way around southern skies. A line from Alpha through nearby Beta Centauri points to crux, the smallest constellation in the sky but also one of the most distinctive. The Southern Cross is not to be confused with the slight larger False Cross, which is closer to the bright star Canopus. Unlike in the northern hemisphere, there is no bright star near the south celestial pole, but the longer axis of Crux points towards the pole, as dose a line at right angles to the one linking Alpha and Beta Centauri. Canopus and Achernar from a large triangle with the south celestial pole.


Posted on February 22, 2015, in Uncategorized. Bookmark the permalink. Leave a comment.

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