Friday, August 10, 2012

How Big is a Constellation

One of the most difficult things many people find in stargazing is learning the constellations. Most people can find Orion, and the "Big Dipper" here in the northern hemisphere, which pretty well passes for Ursa Major (the Big Bear.)

But others are more difficult.

Most constellations are not as well defined as the most popular ones. The three stars of Orion's belt are not only bright, but they're all about the same brightness. Which makes them easier to pick out. Then, finding the other bright stars of the body isn't that hard. Most folks don't bother picking out the stars of the head, shield, and so on.

The Big Dipper is an asterism (sort of a non-official constellation, the words asterism and constellation mean the same thing but they're used differently.) It is made up of seven stars of roughly equal brightness that stand out from the background of the sky around them.

But other constellations don't have nice patterns like this, with groups of stars of roughly equal brightness that are also brighter than other stars in the area, and with a nice fairly dark buffer space between them and the next constellation over.

Another thing that makes it hard to find constellations, even when you use a star chart, is having a sense of how large they appear in the sky.

Some are very large and spread out. Scorpius has a tail that stretches off across the sky. Ophiuschus, the Snake Handler, covers a large area next to Scorpius, along with the Snake he's holding. But the Fox and the Arrow are small.

Start With What You Know

Before wandering too far out into unfamiliar territory, it usually helps to start with the constellations you know. That way, you already have a sense of where something is in the sky, and a general idea of what its boundaries are. Knowing more there can help, too. If there is a constellation you already know, compare the version of it you recognise in the sky with the version of it on the star charts. Are there large parts of it you aren't counting in when you see it? Where does it begin and end?

Then, starting there, and considering the orientation (which changes over time), can you find some other constellation next to it from the chart?

If you've already taught yourself to find the legs of the Big Bear coming off the Big Dipper, and the nose of the bear, then you're prepared to find the Hunting Dogs, a pair of stars beneath the curve of the Big Bear's tail. It's not much of a constellation, just a pair of stars, visually, but it is another constellation.

Likewise, knowing both the Big Dipper and the Little Dipper, there's that line of stars running along between them. Those belong to Draco, the Dragon.

Further south, once you've found the limits of Orion's limbs, you can easily find the Rabbit beneath his feet. And the Big Dog to one side of him, with the bright Dog Star, Sirius, in it.

Pushing out your knowledge step-by-step in this way is a good way to learn constellations, because you can learn how to locate them again later, in a month or two, when the position of the constellations in the sky has changed.

Further Afield

There are a few constellations that look like what they're named for. Scorpius is one of them. The bright red star Antares makes it easy to pick out in a July/August sky, with the head and claws nearby in a nearly straight line of stars that, while not all that bright, are easy to associate with each other because they're about the same brightness.

From those, the Scorpion runs back through bright red-orange Antares then sweeps around in a long fish hook shape with a sort of barb on the end. (This constellation is Maui's Fish Hook to the Pacific Islanders.) A constellation that looks like its name, with a bright, distinctive star in it makes a good starting point when not working off a familiar constellation's side.

Then, once it is learned, other nearby constellations can be learned.

The Power of Asterisms

Just as the Big Dipper isn't an official constellation, but just a part of the Big Bear, there are other asterisms in the sky. Asterisms are more commonly named for what they look like than the traditional constellations. And many of them make up enough of the actual constellation they're part of that finding the asterism is about as good as saying you've found the constellation itself.

The Teapot is an asterism that occupies the main part of Sagittarius, the Archer. The Teapot shape is not hard to see once it "pops" for you. If you hold your hand out at arm's length but bend it so that it's at about a right angle to your arm, it will about cover the area of the Teapot. The Teapot lives behind the tail of the Scorpion.

It has a narrow right triangle as a spout, with the cloudiness of the Milky Way looking like steam coming from the spout. The body is a large trapezoid, which shares one side with the spout. On top (it's upside-down in the southern hemisphere) is a triangle of stars that's just a bit flat of an equilateral triangle which forms the lid of the Teapot. Finally, the handle is another trapezoid which is short and wide, with its bottom the side of the body that's opposite the spout.

If you're in mid-northern latitudes it will be in the south near the horizon during summer.

There are other asterisms like the Summer and Winter Triangles (the names are northern hemisphere centric) where each of the stars in the triangle is in a different constellation. By learning to find these, you get a foothold into three constellations apiece!

The Winter Triangle has a star in Orion, the other stars are in the Big Dog (Sirius, the brightest star) and the Little Dog (Procyon, the dimmest star in the Triangle.)

The Summer Triangle has a star in the Harp, the Swan, and the Eagle. Vega, in the Harp, is the brightest star. Deneb is a middling-bright star in the Swan, at the tail of the constellation (another constellation that has a resemblance to its name.) Altair is a bright yellow star in the Eagle. It forms one end of a pair of much smaller triangles that make up the wings of the Eagle.

Looking Gets You There

It's not easy to learn your way around, unless you have somebody who's patient, already knows their way around, and there's a green laser pointer for them to use. Even at that, once the sky moves over the course of a few weeks, you will need to get re-oriented. But once you take the time to learn on your own, or through a guide, you will be able to star expanding on what you know quickly.

Just as it's easier to find your way to more places around town, the more places you already know and can use as waypoints, you can find your way around more of the sky for each constellation you add to your personal list.

Pretty soon the individual ones you know that are away from each other start growing into ones you know that run into other groups of constellations you know. Then you're looking at the areas in the middle of the constellations you know and wondering what those stars are. Whole sections of the sky become familiar.

Learning the constellations is satisfying and enjoyable in and of itself. I still spend most of my time under the sky with no telescope or binoculars or other "distractions". But the side benefit is that once you do take out an instrument, you can now easily find many items to look at if you know at least a handful of the current constellations in the sky. Some of the best things to look at are placed in easy to aim at places in the constellations. Or, if you have a computerized telescope, you can more easily and more surely give it its guide stars to get its computer started off.