• Moiya McTier

Astronomy Fun Fact #85

Space is huge and has no single reference point, which means there are no set directions. So how do astronomers keep track of the positions of space objects?

There are a lot of different coordinate systems, each with their own origin (or central point) and way of defining directions in space. The different systems are used in different fields of astronomy research, but there are two coordinate systems that are more commonly used than all the others. I'll describe one today and the other tomorrow.

The first system is called the Equatorial Coordinate System. You can think of it like a giant version of the longitude-latitude system we have here on Earth, projected out into space. But instead of latitude, we use declination to measure an object's position above and below the circle projected out from the Earth's equator.

And instead of longitude, we use right ascension, which can be measured in degrees or hours (calling them hours is just a way of splitting up the 360 degrees of a circle into 24 equal chunks to make it more analogous to a day on the clock). The zero point for right ascension is the line drawn between Earth and the Sun at noon on March 21st.

Declination and right ascension just tell you an object's position on the 2-D sky. They don't actually tell you how far away that object is, which is necessary to know exactly where that object is in 3-D space.

This coordinate system is really only useful for objects that can be seen from Earth and have a more-or-less fixed position relative to our solar system.

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