The sky — astronomy

Ursa Major and the Big Dipper

The Big Dipper is a well-known pattern of seven bright stars inside the constellation Ursa Major. Its two outer bowl stars, Dubhe and Merak, point to Polaris: extend the line between them about five times. That gives you true north.

A constellation and an asterism

Ursa Major (Latin for the Great Bear) is one of the 88 constellations recognised by the International Astronomical Union (IAU). By area it is the third largest, after Hydra and Virgo. A constellation is an officially defined region of the sky, not merely a figure drawn between stars (see the 88 constellations).

The Big Dipper is something different: an asterism, a recognisable pattern of stars that is not a constellation in its own right. It is made of seven bright stars within Ursa Major, four forming the bowl and three the handle. From the far side of the bowl to the tip of the handle they are Dubhe, Merak, Phecda, Megrez, Alioth, Mizar and Alkaid. The constellation itself is far larger than the Dipper and contains many fainter stars that trace the bear’s head, body and paws.

From mid-northern latitudes such as Bulgaria, the Big Dipper is circumpolar, meaning it circles the north celestial pole without ever setting. On spring evenings it rides high overhead; on autumn evenings it sits low in the north.

How to find Polaris

This is the Dipper’s most useful trick:

1. Find the Big Dipper and its bowl, the part shaped like a pan.
2. Identify the two stars on the outer edge of the bowl, farthest from the handle: Dubhe and Merak. These are known as the Pointer stars.
3. Imagine a line from Merak through Dubhe and extend it onward.
4. After about five times the distance between the two Pointers, you reach a moderately bright, rather isolated star. That is Polaris, the North Star.

Polaris belongs to Ursa Minor (the Little Bear) and marks the end of the Little Dipper’s handle. It is not especially bright, but it lies less than one degree from the north celestial pole. As a result, the other stars appear to wheel around it during the night while Polaris barely moves. More details are in the article on Polaris.

A handy rule: the altitude of Polaris above your horizon is roughly equal to your latitude. In Bulgaria that means about 42 to 44 degrees above the northern horizon. South of the equator Polaris cannot be seen, and observers there use other guides, such as the Southern Cross.

If the Dipper is hidden by clouds or buildings, look the other way: on the opposite side of Polaris lies the W-shaped Cassiopeia. The two patterns balance each other around the pole, so one is usually easier to spot than the other. For observing tips, see How to watch the sky and light pollution.

Mizar and Alcor

The second star from the end of the handle is Mizar. Close beside it, under dark skies and with sharp eyesight, you can see the faint star Alcor. The pair is a classic naked-eye double, separated by roughly 12 arcminutes, about one fifth of a degree. For centuries, being able to split the two was treated as a small test of good vision.

Through a telescope the story deepens. Mizar turns out to be a double star, and spectroscopy later showed that it is a system of several stars, which made it one of the early examples of such discoveries. Alcor also has a companion. So Mizar and Alcor together form a multi-star system rather than just two points of light. Whether the two are tightly bound to each other has been debated, but modern studies treat them as members of the same group.

Another curious fact: five of the Dipper’s stars, Merak, Phecda, Megrez, Alioth and Mizar, travel through space in roughly the same direction and belong to what astronomers call the Ursa Major Moving Group. Dubhe and Alkaid move differently. Over many thousands of years the Dipper’s shape will slowly change.

A pair of binoculars is a good way to enjoy the area (see Binoculars or telescope).

Names around the world

Few parts of the sky have as many names:

• The Big Dipper: the common name in North America, after a ladle for scooping water.
• The Plough: widely used in Britain and Ireland.
• Charles’s Wain: an old English name, picturing a farm wagon.
• The Great Wagon (Голямата кола): the familiar Bulgarian name, also found in other Slavic languages.
• Saptarishi: in Indian tradition the seven stars are linked to seven sages.

The bear image comes from the Greek tradition and the Latin name Ursa Major. Interestingly, several Indigenous peoples of North America also tell of a bear in the bowl, pursued by hunters in the handle stars. The same seven stars therefore inspire different stories across the world, and what they share is that people have long used them as a compass and a calendar.

To try it tonight, step out to a dark place, let your eyes adapt for about ten minutes, and face north. With the Dipper as your guide you can find Polaris, and from it every direction on the compass.

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