Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Astronomical Jargon (or "How you can sound like an astronomer, too!")

Michael Richmond
Feb 8, 2013


Those crazy astronomers! If you listen to one of their conversations, you'll find that it is sprinkled with -- no, chock full of -- strange terms and phrases that make no sense. If you intercept their E-mail messages, you'll see what appears to be a secret code: MCG +01-3-34, J053432+220051, 2012 DA14. What's going on?

Don't worry. It's not a secret plot to take over the world; it's just the native lingo. Astronomers have typically spent five or ten or twenty years working in a specialized field, studying a small subset of objects in the sky, writing and reading papers which discuss these objects in minute detail. Because we -- like most people -- are LAZY, we have come up with a number of ways to use a few words or characters to express complex ideas. For example, instead of writing

the big spiral galaxy just north of a spot between the two stars at the end of the handle in the Big Dipper

we prefer to write


It saves 108 characters. If you have to mention this galaxy five or six times in the course of a lecture, this can really add up!

Astronomers aren't really any different from other specialists. For example, here's a small sample of a recent posting from Sauropod Vertebra Picture of the Week

But the ventral body profile still had to meet the distal ends of the pubes and ischia, which really can't go anywhere without disarticulating the ilia from the sacrum (and cranking the pubes down would only force the distal ends of the ilia up, even closer to the tail - the animal still had to run its digestive and urogenital pipes through there!).
and here's one from a baseball website I often visit
331 / 417 / 559
1599 BB / 696 K

Just look at those numbers. God damn. 

If astronomers are wierd, then paleontologists and baseball fans are wierd, too.

Of course, they all probably ARE kind of wierd, when you think about it.

It is of course impossible to cover all the strange terms which astronomers use in just one short lecture ... but I'll try to mention some of the most common ones, and give you a few tips so that you might be able to figure out some of the others yourself.

Where have all the meters gone?

Astronomers tend not to use the ordinary units -- meters, kilograms, miles, pounds -- when discussing their favorite objects. Why not? Because astronomers prefer not to use scientific notation; we like it better when all the numbers lie in a small range, say, between 0.01 and 1000. For example, instead of writing the mass of the star Vega as 4.25 x 1030 kg, we prefer to write 2.14 solar masses. See -- isn't that easier?

            2.14           vs.      4.25 x 1030

Keep that goal in mind as we walk through the garden of astronomical units.

But notice that we are not actually mentioning the astronomical unit.

Systems of time
In our everyday lives, we make appointments to meet for lunch at 12:30 PM. Occasionally, we include time zones: 12:30 PM Eastern Standard Time, or 12:30 PM EST. It turns out that these civil times are designed to follow the Sun: in general, noon will occur when the Sun is close to its highest point in the sky, and midnight in the middle of the night (duh). But astronomers have several ways of keeping track of time which are more convenient for specific purposes:

  • Universal Time is good for keeping everyone synchronized, no matter where she lives. If you are trying to describe the exact moment when one star began to eclipse another, you don't want to get caught up in time zones.
  • Local Sidereal Time is a clock which follows the stars, not the Sun. If I want to know if the Andromeda Galaxy is high above the horizon right now, just tell me the LST.
  • Julian Date counts time in days, rather than in hours or seconds or minutes. Moreover, it has a starting point -- a time when the JD = 0 -- far, far in the past, long before any written astronomical records. That means that just about every astronomical event will have a positive date, and THAT makes it very simple to compute the time between two events.

    Quick: how many days was it between these two events?

                   Athletics win World Series      Oct 26, 1911
                   Titanic sinks                 April 15, 1912
                     time between  =               

    Not so easy, is it? Try again with Julian Dates:

                   Athletics win World Series      2,419,335
                   Titanic sinks                   2,419,507
                     time between  =               

Durations of time
Humans spend most of their time worrying about hours and days; sometimes weeks; occasionally years. Historians might ponder centuries or even millenia.

But astronomers have a much broader span of time to cover -- the entire history of the universe. You'll hear them use these terms:


Space is big, really bi--- oh, heck, let's go straight to the source.
Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.
         Q:  Who wrote this?

       the answer 

As a result, astronomers have devised a set of units which are reasonably well matched to commonly encounted distances in space. Among them are the

Sometimes, astronomers describe the distance to an object in an indirect manner. There are good reasons for the following approaches, but it can be really frustrating at first glance.


Once again, the usual units -- kg, tonnes -- just aren't big enough. In order to keep the values in a reasonably small range (most of the time, anyway), astronomers choose to measure mass with


Well, this is one area in which astronomers have failed to come up with a big set of distinctive units. For the most part, astronomers measure EVERYTHING in terms of the amount of energy that the Sun emits each second:

The Milky Way Galaxy, for example, has a luminosity of roughly 200 billion .

Why don't astronomers create a unit which represents the luminosity of an entire galaxy? If they did, they wouldn't have to keep including factors like 1011 when talking about galaxies, or 1013 when talking about quasars.

Why don't they? Beats me.


I do not think it means what you think it means

"Hot". "Cold". "Hard". "Soft."

Simple words, right? You may use them every day, and you may think you know what they mean. But if you were to hang out with astronomers, you'd soon hear them used in completely new and mystifying ways. Let's examine several very familiar words and phrases which astronomers have, um, bent to serve their own purposes.

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Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.