There are 88 official constellations, and the lines between them were drawn by one Belgian astronomer and published in 1930. Before that, the sky's borders were a matter of habit.

I find this one of the more surprising facts in astronomy. Most of us assume the constellations have always been fixed, like the countries on an old map. They have not. The shapes are ancient, but the borders are surprisingly recent, and they matter a lot when you ask whether the Sun passes through twelve constellations or thirteen.

Shapes are old, borders are new

The patterns of stars that we call constellations go back thousands of years. Different cultures drew different pictures in the same stars. For a long time there was no single agreed list, and astronomers drew their own charts with their own names, some of which were invented by mapmakers as recently as the 1600s and 1700s to fill gaps in the southern sky.

The problem with that is practical. If two astronomers report a nova "in Hercules" and their charts disagree about where Hercules ends, they cannot be sure they mean the same place.

1922: the list

The International Astronomical Union (IAU) is the body that names things in astronomy. At its first General Assembly in 1922 it adopted a standard list of constellations, each with a three-letter abbreviation (for example Ori for Orion). The list had 89 entries, because it still included Argo Navis, the great ship of the Argonauts. Argo was later split into three constellations, Carina, Vela and Puppis, which is how the modern count of 88 arrived. The list had no borders yet.

1928 and 1930: the borders

The IAU adopted constellation boundaries in 1928, and they were published in 1930. The work was done by the Belgian astronomer Eugène Delporte, who drew boundaries for each constellation so that every point in the sky belongs to exactly one of them. He used arcs of right ascension and declination, which is why many constellation borders are made of straight, right-angled steps rather than natural curves. The Wikipedia page on the IAU designated constellations has the details, and there is a short biography of Delporte.

The result is a sky in which every point belongs to exactly one constellation. That is very useful for astronomers, and it is the reason a modern telescope database can say which constellation a star or planet is in.

Why some constellations are huge and others tiny

The borders were drawn to respect older traditions, so they inherit the older unevenness. Hydra, the water snake, is the largest of the 88, at about 1,300 square degrees of sky. The Southern Cross, Crux, is the smallest, at about 68 square degrees, though it is one of the most famous. That imbalance is a historical accident, not a design.

What it means for the zodiac

Here is where it gets interesting. The path the Sun follows through the year, the ecliptic, crosses thirteen of these official constellations, not twelve. The extra one is Ophiuchus. The ecliptic runs through it for about two and a half weeks in late November and December, in the constellation table I use (30 November to 17 December). I explain that in Ophiuchus: is it the 13th zodiac sign?.

Also, the Sun spends very different amounts of time in each one. It crosses Scorpius in about six days and Virgo in about six weeks. That unevenness is why the 13-sign zodiac has signs of very different widths, and why the twelve-sign zodiac, with its tidy 30-degree slices, is a different thing entirely. I compare them in the 13-sign zodiac explained.

Two questions that get mixed up

  1. Which constellation is the Sun in front of today? This is astronomy, and the IAU borders answer it.
  2. Which zodiac sign is the Sun in? This is a choice of system. Tropical astrology answers it by counting 30 degrees from the March equinox, without reference to the constellation borders at all.

Neither one is wrong. They just answer different questions, and the confusion starts when people treat them as the same one.

Did the borders ever get updated?

The 1930 boundaries have been kept ever since. Because precession slowly shifts the coordinate grid (see why the zodiac drifted), the borders are defined at a fixed reference date, the epoch B1875.0, and are converted when needed. The borders stay put against the stars while the coordinates drift.

Why I find this useful

Whenever someone tells me "astronomers say the zodiac is wrong", I like to have the facts ready. Astronomers say there are 88 constellations and the ecliptic crosses 13 of them. They do not say that anyone's Sun sign is wrong, because a sign is not an astronomical object. It is a convention, and this kind of history is a good reminder of how many of our "obvious" pictures of the sky are conventions.

See where your planets fall

My Solar Blueprint reading can show your Sun, Moon and planets in the 13 IAU constellations, next to the twelve-sign systems, so you can see how the same sky reads in each. I write each reading myself with my own tools and check it before it goes out. If you enjoy this sort of detail, the ADHD MindFlow community is a friendly home for curious minds.

Astronomy facts here are checked against the sources linked. Astrology is a way to reflect, not science, and it does not predict events.

Astrology is a way to reflect, not science, and it does not predict events. Astronomy facts here are checked against the sources linked. Read our sources, commercial disclosure and editorial approach.