# Plotting constellations

Suppose you wanted to write a program to plot constellations. This leads down some interesting rabbit trails.

When you look up data on stars in constellations you run into two meanings of constellation. For example, Leo is a region of the night sky containing an untold number of stars. It is also a pattern of nine particular stars connected by imaginary lines. It’s easier to find data on the former, say sorted by brightness.

Are the nine brightest stars in Leo-the-region the nine stars of Leo-the-stick-figure? Not exactly, but close.

Wikipedia has an article that list stars in each constellation region, and star charts that have constellations as stick figures. If the stars on the chart are labeled, you can cross reference them with Wikipedia.

On a particular star chart I have, the stars in Leo are labeled with their Bayer designation. Roughly speaking the Bayer designation labels the stars within a constellation with Greek letters in descending order of brightness, but there are inconsistencies. The nomenclature goes back to Johann Bayer (1572–1625) and has its flaws.

The stars in Leo, in line-drawing order, are

1. δ Leo
2. Denebola (β Leo)
3. θ Leo
4. Regulus (α Leo)
5. η Leo
6. γ Leo
7. ζ Leo
8. μ Leo
9. ε Leo

You can look up the coordinates of these stars here. Line-drawing order does not correspond to brightness order, so without a labeled star chart you’d have some research to do. My chart labels all the stars in Leo (the stick figure), but not, for example, in Virgo.

γ Leo is actually two stars, and Wikipedia ranks the brightness of the stars a little differently than Bayer did, which is understandable since brightness could not be objectively measured in his day. Wikipedia also inserts a few stars in between the stars listed above.

Here’s a plot of Leo using the data referenced above.

## 5 thoughts on “Plotting constellations”

1. Those are beautiful. Thanks.

As you said, I wanted to roll my own just to see what’s involved.

2. Those are nice, Jonathan. I am impressed that the IAU page has gone with the “modern” shapes inspired by H.A. Rey rather than the more usual “traditional” shapes John is referring to. I grew up with Rey’s book, and his constellation patterns usually make much more sense.

3. That’s fantastic! I’d never heard of Rey’s update.

I wonder if this isn’t a re-invention. Maybe the ancients saw the same patterns Rey describes, or similar, but somehow what was passed down was a simplification. Maybe it was something like Hebrew vowels: you’re supposed to know that they’re there even though they’re not explicit.

4. I’m curious too, John. Your idea makes a lot of sense; I hadn’t thought of that.

Some star names refer to the part of the constellation they belong to: Deneb and Denebola mean “tail” (of Cygnus, the Swan, and Leo, respectively); Spica means a “head of grain” being held by Virgo; Betelgeuse means “Orion’s hand”; Fomalhaut means “mouth of the fish”; and so on. So the ancients saw meaning in the positions.

Rey’s shapes make a lot of sense, and I’ve always preferred them, but I have to admit that sometimes the traditional shapes are actually easier to trace in the sky because they connect brighter stars. So maybe they’re simplifications as you suggest.