Stellar Cartography

Stellar cartographers are responsible for mapping the accurate locations of stars, galaxies, and other celestial objects.  This sounds fairly simple to do–just look up into the night sky and make a map of what you see.  Except that unlike terrestrial cartography, all the objects you see in the night sky are in three dimensions, hundreds, thousands, millions, and sometimes billions of lightyears apart from one another.  To make matters even more confusing, every celestial object is moving in different directions at different speeds.  And even though we don’t notice it, the Earth and the rest of the Solar System are moving as well.

To help keep track of everything, stellar cartographers have divided the sky into constellations, each with official borders.  There are 88 official constellations, as designated by the International Astronomical Union in 1922.  In 1930, their borders were drawn by Eugène Delporte, a Belgian astronomer who discovered 66 asteroids during his career.  In the picture of the constellation Orion, above, the dotted lines delineate the borders between constellations, with Orion highlighted in white.  Click on the image above for the bigger picture, from the IAU.

Orion consists of seven main stars, all named on the map: Betelgeuse, Rigel, Bellatrix, Mintaka, Alnilam, Alnitak, and Saiph.  Stellar cartographers also give the major stars in each constellation designations based on Greek letters.  This system is called the Bayer Designation. The Bayer designations of the seven main stars in Orion are Alpha (α), Beta (β), Gamma (γ), Delta (δ), Epsilon (ε), Zeta (ζ), and Kappa (κ) Orionis, in the same order as the list above.  All the other major stars in the constellation also have Greek letters assigned to them.  These Greek letters are also indicated on the map.

The map also contains a few objects with a name that has a letter “M” followed by a number.  These are Messier Objects, named for French astronomer Charles Messier.  In 1781, Messier published a sky catalog with 103 bright objects he had studied. 

Finally, as I mentioned earlier, the stars in Orion look like they’re on the same plane, but in fact they are widely distributed through space in three dimensions.  For a great visualization of this distribution, take a look at this image created by Don Dixon over at Cosmographica.


4 Responses to “Stellar Cartography”

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Cartographia is a blog about how we use maps to represent the world around us, and how people interpret maps today and throughout history. Please feel free to send any questions, comments, or recommendations directly to me at

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