Look up one cloudless night and see the stars. You ought to be impressed, particularly if you are away from city lighting and can see the Milky Way. But what you see is only a tiny fraction of the entire cosmos.
To be exact, there are about 5000 stars that a human can see without a telescope. Not all of these are over the horizon at any one time. Unless you live near the equator, you will never see some of them unless you travel.
What we do see, we currently group as constellations, and these are a human invention. Stars exist as objective facts. The view from Earth is also an objective fact. The apparent star groupings are imposed by human culture; yet many of the groupings arise naturally from what we see.
With our unaided eyes, we see a few thousand stars. All of them are close neighbours in the vastness of the galaxy. Mostly within a few tens or hundreds of light-years of us, within a galaxy with a diameter of at least 100,000 light-years.
Most visible stars are faint: from cities we may not see them at all. The brighter or nearer of the stars strongly outshine the rest and seem to make patterns. The rotation of the earth makes these patterns seem to rise and set every day, though those close to the pole (Earth’s axis of rotation) are visible every night. (Also above the horizon during the day: but the sky is too bright for them to be seen.) But since Earth and the other planets developed from a flattened disk of gas and dust, our planet rotates close to what’s called the Plane of the Ecliptic, and this determines the pattern of how we see the planets appear to move. From an Earthly viewpoint, the planets are ‘wandering stars’, which however only wander within a single distinct band of stars.
With geometry, people were able to work out that the sun would also be seen as moving within this narrow band if the daytime stars were not drowned out by the much brighter sky. The Babylonians (or possibly some earlier people) did these calculation, creating the concept of a zodiac. Also creating a tradition of astrology that was absurd nonsense in itself, but did ensure that Western Europe as heir to Babylonian knowledge paid a lot of attention to the apparent movements of planets within that zodiac.
What do these zodiac signs mean? Probably nothing, but just possibly the star-beasts and other concepts assigned to them were not random. The constellation Virgo is one of twelve traditional signs in a zodiac we get from the Greeks, but which the Greeks based on a Babylonian system of which we have just fragments. As I mentioned, the Virgo Cluster is a kind of super-centre for many galaxies including ours. Perhaps Virgo the Virgin was originally some sort of Great Mother – similar transitions happened when Christianity took over populations where some sort of mother-goddess had long been reverenced. Dianna of the Ephesians was reinvented as Mary Mother of Jesus, a woman mentioned only in passing in the actual gospels. But assigning a Mighty Virgin to that interstellar direction fits better than any other constellation-beast would have.
Another zodiac sign is Sagittarius, traditionally a centaur firing an arrow. And it is also the direction of the centre of the galaxy. The location of the galaxy’s black hole, which shoots out jets of matter. Also Andromeda (outside of the zodiac) is traditionally a chained-up lady: our Milky Way and the Andromeda Galaxy are in a sense chained together by gravity. The two of them will eventually merge, though since the Andromeda Galaxy is larger it should dominate, whereas the chained Andromeda was due to be eaten by her arriving monster.
Many years ago, it occurred to me that these might be echoes of some alien contact filtered through human misunderstandings. But this seems weaker now that I learn that we’ve not actually destined to end up as part of a unified Virgo Supercluster. But a multi-limbed centaur firing an arrow is a passable version of what primitive minds might have made of a giant black hole, with its tendency to shoot out jets of plasma. Still, with 48 constellations in Ptolemy’s ancient list, a few ‘hits’ could be expected to occur be sheer chance. It may mean nothing, but I thought it worth recording.
For constellations, numbers and definitions vary a lot. The Babylonians seem to have had 17 or 18 constellations just for the zodiac. Their constellations also do not cover the whole of the sky. Significantly, the area left unnamed is the part of the sky that would not have been visible from Babylon at the time the definitions were probably first devised. Ptolemy reorganised this into 48 constellations, the basis for the modern system. But there were also a number of attempts to either rebuild the system or to slot in extra constellations, as well as mapping new constellations like the Southern Cross in areas that Ptolemy would not have seen.
The official body controlling astronomical names is the International Astronomical Union. Formed in 1919, it settled on a system of 88 constellations covering the whole of the sky. This included three or perhaps four to replace the ancient and over-large ‘Argo Navis’: the Ship of the Greek Argonauts. Carina (the keel, or the hull, of the ship), Puppis (the poop deck, or stern), and Vela (the sails) are the main replacements. The area now known as Pyxis (the mariner’s compass) occupies an area which in antiquity was considered part of Argo’s mast, though the Greeks knew nothing of compasses. (They were invented in China and arriving much later in the West.)
The magazine Astronomy Now recently had an article called Constellations that ceased to be. It starts off with successful additions to fill the gaps, from the 16th century onwards. These include Columba (the dove), Monoceros (the unicorn) and Leo Minor (the smaller lion). In Late Roman times, the Emperor Hadrian’s male lover Antinous was given a chunk of Aquila (the Eagle) and it almost made the grade as an official constellation, but in the end got rejected. So too did Robur Carolinum (Charles’ Oak), invented in 1679 by Edmund Halley and including the famous star now known as Eta Carina. Likewise Taurus Poniatovii (Poniatowski’s bull), proposed by a Pole to honour a Polish king.
As I said, constellations are an interesting hybrid of the two worlds I began by defining. The view from Earth is an objective observation of the nearby stars. But our current view lines up stars that often have little connection to each other. And over hundreds of thousands of years, this view will change considerably:
- the stars Castor and Pollux in Gemini (Twins) are not twins at all. Castor is a gravitationally bound group of six stars that include two bright white suns similar to Sirius. Stars close enough to seem a single star at their distance of 56 light-years away. Pollux by contrast is a single yellow-orange giant 34 light-years from the Sun. It is less massive than either of the two main stars of Castor, but it is also near the end of its lifetime and therefore must be much older. (Large stars burn much hotter and brighter than small stars, and so don’t last as long. It is a weird world, as if elephants were born and died in a single day while mayflies lived for centuries.)
The Castor system is reckoned to be 370 million years old: Pollux 724 million years. Like most stars or groups of stars in the galaxy, both have their own distinct orbit around the galaxy. Tens of millions of years ago, they would not have been close to our sun or to each other. Tens of millions of years in the future, they will be far apart again. But Pollux will by then have blow off a Planetary Nebula and will end its days as a White Dwarf. (It is too small to become a supernova.)
- Orion is more of a natural group. Most of its bright stars are probably recent products of the Orion Molecular Cloud Complex, of which the Orion Nebula is just a small part. But the three bright stars of Orion’s Belt are not in fact close to each other: their distances are 736, 915 and 1340 light years from Earth. From most viewpoints other than Earth, they would not form the near-perfect straight line we see in our own night sky.
- Ursa Major is dominated by the seven stars of the Plough or Big Dipper. This consists of five stars that are part of the Ursa Major Moving Group, and two unrelated stars at either end that are going somewhere else. The Ursa Major Moving Group is about 300 million years old; stars that formed in a single star-forming region but are now drifting apart. The five brightest stars will maintain the core of the current Plough, but the different motions of the other two means that the overall shape formed by the seven bright stars changes markedly over tens of thousands of years.
Bram Stoker used this in his novel The Jewel of Seven Stars. A mummified Egyptian queen has a jewel showing the Plough as it looks now, subtly different from what it would have been in her day. (Being a horror story, she also comes back to life, of course.)
- Centaurus is a southern constellation that most Europeans have heard of, because of the Alpha Centauri system, the three closest stars to Earth. We see a single star, but this is actually a close pair of stars. We do not see Proxima Centauri, which is much dimmer and was only discovered in 1915. Proxima is separated by such a gap that not everyone agrees that it is truly part of the same system as the other two, though their motions in orbit round the galaxy are suspiciously similar.
When I visited Australia and New Zealand, I was surprised to see that Alpha Centauri had an apparent twin, Beta Centauri. Beta Centauri looks slightly less bright: it is in fact vastly brighter and more distant.
- Cassiopeia: the five brightest stars of this constellation make up a W shape that even a novice can spot. But these are five unrelated stars that just happen to line up from the viewpoint of Earth.
Interestingly, if humans went to the Alpha Centauri system, we could look back and see our sun making a zigzag patters with the stars of Cassiopeia: a distinctive group of six.
One could give the same treatment to the other 83 officially-recognised constellations. Their bright stars are often not associated with each other: they just happen to line up from the viewpoint of Earth. From other stars in the galaxy, we would see a different pattern. From an Earth-like planet a few thousand light-years away, almost every star we can see with the naked eye would be invisible. The night-sky would consist of a completely different set of stars, arranged in other patterns.
All of this is just Europe’s tradition. Other cultures have their own systems. Some also derived from the Greeks or Babylonians. But the patterns are wholly different for the Chinese.
I notice also that most believers in mystic star-signs recycle the same set of ancient ideas, but with extras that have no source and come from their own whims. Doris Lessing did this with great literary skill, improbable politics and a poor knowledge of astronomy in her Canopus in Argos novels. Argos is an ancient Greek city: she has confused it with Argo Navis, the officially-abolished constellation which did indeed include Canopus. She also seems to be referencing astrology, where planets that happened to line up with vastly more distant stars are said to be ‘in’ that constellation of the zodiac. But Canopus is a distant, bright and short-lived star: all of the constellations would have lost their meaning from an Earthly viewpoint long before Canopus would line up with a different set of stars.