The Constellations

[HenryC]

Prior to the popular TV series “Star Trek”, most Americans, even educated ones, could not tell you the difference between a star and a planet. Every week, Capt Kirk and his crew would visit a different planet orbiting a different star and have a new adventure, and soon the distinction between the two became obvious. A star was a huge hot fireball like the Sun, and a planet was a small rocky body like the Earth.

But there is still much confusion about even the most basic and fundamental astronomical terms. To this day, people who should know better often get “Galaxy” and “Solar System” confused, using the terms interchangeably. And there are too many people who may know the difference between the Senate and the House, or who know that a whale is not a fish, who still don’t understand what a “constellation” is, or could even name one, for that matter.

A constellation is an arbitrary piece of sky. It is roughly comparable to the geographical term for a vague indeterminate area, like “New England”, “Upper West Side”, or “The Middle East”. The concept is found in all ancient cultures, and is used to designate a piece of the night sky where a recognizable pattern of stars can be found. For example, we talk about Orion the Hunter, or the twelve constellations (Signs) of the Zodiac. Constellations are a useful mental device to help memorize star patterns, which is a quaint and obscure hobby today but at one time was probably a real survival skill, useful for hunters, sailors, farmers, and witch doctors.

Different civilizations often called the same constellation by different names. For example, the Greeks and Romans called Ursa Major the Great Bear, while the Anglo-Saxons called it the Wagon, and the ancient Hebrews, the Plow. We call it the Big Dipper. The pretty little star pattern the Classical world called Delphinus the Dolphin the Old English called Job’s Coffin. But usually the referred star pattern was striking, memorable, and stood out easily from the surrounding stars. Sometimes, it even physically resembled its namesake: Scorpius actually looks like a scorpion. In other cases, the traditional name is a real stretch. Orion the Hunter always looked like a bow tie to me.

Most constellations today go by the names given to them by the ancient Greeks, although many originated much further back in antiquity. They usually are named for characters from mythology. It makes little scientific difference, of course, but it is fascinating to think that we still routinely use names and terms that may go back to prehistory. In the Southern Hemisphere, where European and Middle Eastern astronomers could not see until the Renaissance, the constellations have names picked by navigators and explorers of the Age of Discovery. The constellations of the southern sky are not gods and goddesses, but are often named after nautical terms, mechanical objects, or the new and exotic creatures of Africa and South America. Down under you can find Pavo the Peacock, Camelopardalis the Giraffe, Sextans the Sextant, Carina the Keel, Fornax the Furnace, Mensa the Table, and so on.

The constellation concept was used as a quick-and-dirty way to point out a place in the sky where a star or planet might be. There were no hard, fast and fixed boundaries, but you could say “Aldebaran is the bright red star in the eye of the Bull” and people knew what you were talking about. The Bull, Taurus, by the way, was confronted by Orion and his two hunting dogs, each represented by his own constellation, Canis Major and Canis Minor. Very often a group of constellations told a little story, Perseus and his squeeze Andromeda are both up there, right next to Cassiopeia and Cepheus (her royal Mom and Dad). Even his flying horse, Pegasus, is nearby, as well as the sea monster.(The kraken is there, too!) On the other side of the sky, Sagittarius the Archer aims his bow at the heart of Scorpius the Scorpion. Although to my imagination, I think the former looks more like a teapot, and the latter like a longshoreman’s hook.

Of course, astronomers eventually decided to systematize the whole system and give the constellations precise boundaries and to document them exactly. Early astronomers had started designating stars by constellation, using Greek letters to list them in order of brightness, followed by “Flamsteed Numbers” when the Greek alphabet ran out of characters. The old, traditional Arabic names, like Aldebaran, now exist side by side with the Bayer designation, Alpha Tauri. Betelgeuse is also known as Alpha Orionis,
and Antares is synonymous with Alpha Scorpii. So it is possible for a bright, well-known star, to have three valid names, as well as a host of variant spellings and nicknames, and a host of obscure catalogue numbers. Sirius, Alpha Canis Majoris, is also known as the “Dog Star”. And its tiny, faint white dwarf companion, Alpha Canis Majoris B, is also called “The Pup”.

Today, the 88 constellation boundaries are carefully marked off on star charts. They are all polygons marked by lines that go N-S, E-W, and they are designated by convention by the precise celestial coordinates of each corner. Unfortunately, in order to remain consistent with earlier research as well as tradition and history; when this system was systematized in the 1930s, the shapes had to be extensively gerrymandered so they look like the boundaries of Florida congressional districts.

The coordinates of each corner of each constellation polygon are carefully listed in a book, and never change. Unfortunately, the celestial coordinate system changes due to precession as the earth wobbles on its axis. So the coordinates change, but the stars don’t, and as the coordinate system slowly slides around, the constellation boundaries shift across the sky, and some stars suddenly find themselves in a new constellation. You can’t just change the Bayer Greek-letter designation or Flamsteed number because it may be mentioned by that moniker in the literature. The result is mass confusion. Not only that, since the constellation boundaries never change, but the coordinate system is always changing due to precession, the boundary lines no longer proceed neatly N-S and E-W.
They all look twisted and warped on a map.

Its one of the charms of astronomy. Mind-numbing nerdy precision conflicting and coexisting with antique lore and Babylonian mysticism.

[BruceW]

That was a fun read!

[Craig]

Always look forward to Henry's posts! Always interesting and informative! Those roots of navigation should never be forgotten!