The familiar constellation of Orion, the three stars in the center are known as Orion's belt.
- The official definition of a constellation is an area of the sky with defined boundaries, all stars and any other objects within that boundary are considered part of the constellation.
- Historically and in common language stars that form patterns in the night sky are also referred to as constellations.
- In modern scientific language stars that form patterns are known as asterisms, asterisms usually fall within a single constellation and bare the same name but they can also be part of multiple constellations.
- There are 88 official constellations which are recognized by the International Astronomical Union.
- Some constellations are only visible in the northern hemisphere, while others are only visible in the southern hemisphere.
- Constellations that are visible in both hemispheres may appear upside down in the southern hemisphere.
- A few constellations can be viewed all year long but most are seasonal and can only be viewed at certain times of the year.
- Distant galaxies and nebulae also form parts of constellations.
- The sun is the only known star in our galaxy which is not part of a constellation.
History of Constellations
More than half of modern day constellations are based on the writings of Ptolemy.
For as long as mankind has walked the Earth it is likely we have always recognized patterns formed by stars in the night sky. Some European cave paintings that date back over 10,000 years bare the marks of star formations that are familiar to us even to this day. The earliest official records of constellations were made more than 3,000 years ago by the Babylonians, an ancient civilization based in what is now modern day Iraq. The twelve signs of the zodiac are based on their observations and those of their predecessors.
In the 4th century BC these zodiacal constellations and others recorded by the Babylonians were introduced into ancient Greek culture by the astronomer Eudoxus of Cnidus. Some five hundred years later the Greco-Roman scientist Claudius Ptolemy listed 48 constellations in his book Almagest, these constellations and the names he gave them exist to this day and are the basis for all the 88 official constellations listed by the International Astronomical Union.
Understanding Star Charts
To find constellations you’ll need a star chart, these give you a snapshot of what the night sky will look like at any one time and at any one location. The star chart above shows you how the night sky will look above most of the United States at 8 pm in late January. These maps may seem rather baffling and confusing at first but they’re actually very simple to use. To simplify matters the chart above only shows constellations but normally they will also indicate prominent stars, galaxies, nebulae and planets.
The first thing you may notice is that east and west seem to be the wrong way round, but if you imagine holding the chart above your head, which is how they are designed to be used, it becomes apparent this is not the case. The outer edge of the chart indicates the horizon, so the further the stars are from the edge the higher they will be in the sky.
The center of the chart shows the stars and constellations that will be directly overhead, so the map above shows you that the constellations of Auriga, Taurus and Perseus will be directly above you at that time.
To find your bearings it is helpful to find Polaris, the star which always points north. First find the famous Big Dipper, which is part of the constellation Ursa Major and visible all year in the Northern hemisphere, draw an imaginary line through the outer two stars of the Big Dipper’s bowl and you will come to Polaris, which is the brightest star in the constellation of Ursa Minor. In the Southern hemisphere it is helpful to find the Southern Cross, which always points south, this is done by drawing a line through the bright stars Alpha and Beta Centauri.
Once you have found your bearings you can start searching out constellations and the objects they contain. Using the chart above if you look south you’ll notice the constellation of Orion, perhaps the most recognizable constellation of them all. As well as the Orion nebula the constellation also contains the bright supergiant stars Betelgeuse and Rigel. Overhead and to the west you’ll find the Andromeda constellation which contains the Andromeda Galaxy, the most distant object that can be viewed with the naked eye. There are many other fascinating objects to look out for and a star chart will be essential in guiding you around the night sky.
Stars do not stay fixed in the night sky, as the Earth rotates they change position, as a result the night sky will look different at midnight from what it did several hours before or after. Most constellations are also seasonal, meaning that ones that are visible in winter may not be visible in summer and vice-versa, so sky charts usually come in seasonal versions. Constellations may also be drawn slightly differently on each chart and it’s also worth noting that there are separate charts for the Northern and Southern hemispheres.