Constellations

What a constellation really is

Look up on a clear night and your brain starts joining dots almost automatically. People in every culture did the same, turning groups of stars into hunters, animals and heroes. These pictures are the old meaning of the word constellation.

Modern astronomers use a more exact meaning. A constellation is a defined area of the sky with fixed borders, a bit like a country on a map. Every star, galaxy or comet you can point to lies inside exactly one of them.

The stars in a pattern are usually not close to each other in space. They only line up from our point of view. One star in Orion may be a few hundred light-years away while its neighbor in the picture is more than twice as far.

Why there are exactly 88

Many of today's names come from the 48 constellations listed by the Greek astronomer Ptolemy around 150 CE. European sailors and mapmakers added more in the 1500s to 1700s, especially for the southern sky, which was invisible from the Mediterranean.

By the early 1900s different star atlases disagreed about names and borders. In 1922 the International Astronomical Union (IAU), the global body of professional astronomers, settled on a list of 88. Between 1928 and 1930 it fixed their boundaries along lines of the sky's coordinate grid, so the whole sphere is covered with no gaps or overlaps.

The biggest is Hydra, a long snake stretching across a large part of the spring sky. The smallest is Crux, the Southern Cross, which is easy to spot from the southern hemisphere despite its size.

Asterisms: patterns inside patterns

Some of the most famous shapes in the sky are not official constellations at all. These are called asterisms, which simply means a recognizable star pattern.

The Big Dipper (the Plough in Britain) is seven bright stars that make up the rump and tail of Ursa Major, the Great Bear. The Summer Triangle joins three bright stars from three different constellations: Vega in Lyra, Deneb in Cygnus and Altair in Aquila. The Winter Hexagon does the same with six bright winter stars.

Asterisms are often the best way to begin, because they are bigger, brighter and simpler than the full constellations around them.

Constellations versus zodiac signs

As Earth orbits the Sun, the Sun appears to move against the background stars along a path called the ecliptic. The constellations along that path are the zodiac constellations of astronomy.

The ecliptic actually crosses 13 constellations. The 13th is Ophiuchus, the Serpent Bearer, which the Sun passes through for a couple of weeks in late November and December. The real constellations also have unequal sizes, so the Sun spends very different lengths of time in each.

Astrology's 12 signs are equal 30-degree slices of the year, set up about 2,000 years ago. Since then, a slow wobble of Earth's axis called precession has shifted the dates by roughly a month. That is why the Sun is usually not in 'your' sign's constellation on your birthday. Astronomy measures where things are; astrology is a belief system and has no scientific support.

How to learn them

Start with one bright anchor pattern you can find every time, then use it to find its neighbors. Learning two or three new constellations per month is a realistic pace.

Go out on a moonless night, away from bright lights, and give your eyes time to adjust. A planisphere (a rotating paper star wheel) or a star chart app set to night mode will show you what is up at your location and hour.

Do not expect the stick figures to look like their names. A 'bear' or a 'swan' only makes sense once you know which stars to connect. Patience and repeat visits matter more than any equipment.

Easy constellations by season

For mid-northern latitudes, these are good first targets in the evening sky. Orion, the Hunter, rules winter, with three stars in a neat row for his belt and the red star Betelgeuse at one shoulder.

Ursa Major, home of the Big Dipper, stands high in spring evenings and is visible all year from much of the United States, Europe and Türkiye. Cassiopeia, a bright W or M shape, sits across the North Star from the Dipper and is easiest in autumn.

In summer, look for Cygnus, the Swan, flying along the Milky Way with Deneb at its tail. Low in the south you will find Scorpius, whose curved tail and red heart, the star Antares, really do look like a scorpion.