Constellations

Thousands of points of light, seemingly scattered at random: how is anyone supposed to find their way? People of all cultures had the same idea. They connected striking stars into figures, gave them names and told stories about them. Hunters, bears, queens and dragons. These constellations are still the map of the sky today, and whoever knows a few of them is never completely lost at night.

Patterns as a map

So that all astronomers talk about the same sky, the International Astronomical Union divided the entire sky without gaps into 88 official constellations with fixed borders. Every star belongs to exactly one constellation, just as every place on Earth belongs to exactly one country. Famous patterns like the Big Dipper are actually not a constellation of their own, but only the most striking part of a larger one, in this case Ursa Major, the Great Bear.

Finding Polaris

The most important trick in the northern sky: find the Big Dipper, seven bright stars shaped like a box with a handle. The two box stars farthest from the handle are called Merak and Dubhe. They are known as the pointer stars. Take the gap from Merak to Dubhe and lay it out about five more times beyond Dubhe, and you arrive at Polaris. It stands almost exactly above the Earth's north pole, so it always marks the direction north, all night, all year. And it reveals even more: its height above the horizon equals your northern geographic latitude. By the way, it is not particularly bright, just reliable.

Constellations are perspective

As familiar as the patterns look, they exist only as seen from Earth. The stars of a constellation are usually not close to each other at all, but at completely different depths of space. In the Big Dipper, Merak is about 80 light-years away, while Dubhe is around 123 light-years. In Orion, several hundred light-years lie between Betelgeuse at roughly 550 light-years and Rigel at about 860 light-years. Viewed from another star, the same stars would form entirely different figures. So constellations are not families in space, but chance patterns of our line of sight.

The zodiac

The sun, the moon and the planets always move through the same narrow strip of constellations in the sky, because the whole solar system is roughly as flat as a disc. This strip is called the zodiac. Classically twelve constellations are counted there, from Aries to Pisces, though the sun's path actually grazes a thirteenth, Ophiuchus. Because the sun travels through the whole zodiac once in the course of a year, it stands in front of each of the twelve classic constellations for about one month on average. This has nothing to do with astrology and horoscopes, it is pure geometry of the orbits.