Why are constellations not always drawn the same way?

Quadrans Muralis, the Mural Quadrant, does not appear on any modern star charts. Elijah H. Burritt included it in his 1835 edition of Geography of the Heavens. The stars that make up this extinct constellation lie within the boundaries of Draco and Hercules. Credit: Michael E. Bakich Library

When I use my astronomy software or look through various books, I notice that not everyone draws constellations the same way. Why?

John Hinkamp
Andros Island, Bahamas

Picturing familiar images in the sky has always been a simple way to track the yearly progress of the stars. Around the 2nd century AD, the Greek astronomer Claudius Ptolemy listed 48 “official” constellations in his book Almagest. There is no original copy of Almagest It has survived, so we don't know how he connected the stars to form figures. In the 16th century, European explorers added new constellations to fill out their maps. In the late 1670s, Edmond Halley, of comet fame, placed a large oak tree on one of his maps to honour Charles II of England and the Royal Oak behind which he supposedly hid after the Battle of Worcester. That constellation, along with many others, quickly fell into disuse.

One of the most influential star atlases is that of Johann Bayer. Uranometrymade in 1603. It was the first to encompass the entire celestial sphere. Bayer set the standard for the classic images we associate with constellations today.

However, there was no rule determining which stars were included in a constellation. All star charts used wavy lines to encompass not only the classical images, but also all the faint stars in a given constellation. Then, in 1928, the International Astronomical Union (IAU) imposed boundaries based on lines of right ascension and declination, dividing the celestial sphere into 88 regions that make up the modern constellations. However, the IAU did not set standards for the star patterns of the constellations. Even in the 21st century, star chart makers have artistic license to connect the dots as they see fit.

So no two sets of maps will ever be exactly alike. You can even create your own constellation shapes, as long as they stay within the boundaries designated by the IAU. Orion still looks like a big butterfly to me.

Raymond Shubinski
Contributing editor

This question and answer originally appeared on February 2011 affair.