Glossary (Blog #0)

In this “zero-eth” Messier Pro, I am just going to define a few terms that you might not be familiar with unless you’re already familiar with amateur astronomy. I’m going to post a link to this on the side of the page, and I will come back and add to this glossary as I keep finding things that I forgot to define …

ASTERISM:
An asterism is a pattern of stars that is not a CONSTELLATION (which see). I’ll make up patterns of stars to help find some objects off the beaten path, and I’ll introduce some identified asterism as we go.

BRIGHTNESS:
Brightness describes how an object appears from Earth, and so doesn’t actually say anything about the object itself. There are stars that are bright because they are close to us, and stars that put out much more light, but are dimmer because they are farther away.
Brightness is measured in magnitudes, in an idea going back to the Greek astronomer Hipparchus. Hipparchus labeled the brightest visible stars as “stars of the first rank”, and the dimmest stars as “stars of the sixth rank”. Tradition is a powerful force, and this has remained the pattern even as we now have technology able to measure just how much light is coming off of a star. The faintest stars that can be seen with the naked eye (see “named stars”) are sixth magnitude, and first magnitude stars are some of the brightest stars in the sky. The magnitude system is not linear because the eye’s response is not linear. The system is defined such that a star with a magnitude of m = 1.00 provides 100 times as much light as a star with m = 6.00. This means that brightnesses can be unintuitive to the point of frustration. The brightest star, Sirius, has a magnitude of -1.4, the planet Venus can get as bright as -4, and the Sun has a visual magnitude of -26.

CONSTELLATION
A constellation is an arrangement of stars that has long been considered to represent some figure or object. Most cultures had their own set of constellations (some cultures named individual stars, some probably had their own but got culturally co-opted by some other group), but the “official” set was settled upon by the International Astronomical Union (the same people who disqualified Pluto) in 1930 based upon the Roman constellations taken from the Greeks, some of which they took from the Babylonians, Indians, and more. There are 88 accepted constellations.

DOUBLE STAR
A double star is a system in which two stars are actually orbiting around a common center of mass. There are many sets of double stars that are close enough to Earth to make interesting things to see, there are some that are interesting because the orbits of the two stars actually cause them to eclipse each other as seen from Earth, and there are also some stars (called “optical doubles” that have been identified as double stars, but were later discovered to have simply been in the same line of sight, and nowhere near each other.

NAMED STARS
There are about six thousand stars visible to the naked eye in the sky. (I wonder if this is going to mean I get some hits if someone googles “naked”? Hmmm … naked naked naked naked …) Less than two hundred stars have individual names, primarily because remembering six thousand names is a little problematic. Still, the brightest stars all largely have names, and the commonly accepted names are largely from the Arabic. (I am not sure why Greeks named constellations and Arabs named stars. Perhaps Greek navigators didn’t need to be as precise navigating in the Mediterranean as Arabic travelers needed to be in the Indian Sea and aiming for small oases in big deserts.)
When I post star maps, I’ll post the names of all identified stars. If I miss any, I’ll try and fix that. This brings up an interesting point. For some years, there have been companies that allow you to “name” a star for a person. This is official – as far as that company is concerned, but nobody else knows about it. Have something cheaper! If you want to name a star, pick a star that has no name associated with it and send it to me (one per person, please), and I’ll mark it one the Messier Pro maps.

TIME:
When I refer to how the sky will appear on a certain night, unless I specifically refer to a certain time, I will be speaking of a time an hour after twilight has ended. This will generally be about two hours after sunset, and it will change over the course of the year. This “observing time” will be marked on the post, although any descriptions I give will be good for some time before and after “observing time” (unless I refer to something close to the western horizon).