Finding Your Way Across The Sky

Hey, sky fans! I’m going to be taking some time off for the next week or two. I’m mostly going to see how many lobster rolls I can eat at a time, and then try to break that record. So I won’t be posting much, though I have a couple lined up for this coming week. I really appreciate all of you for reading this summer. I raise my… soup? Who has soup in Aug…? Okay, cruel world, I raise my soup to you.

One of things I’ve always tried to do here is be sure to define all of the astronomy words I use as often as I can. A place I haven’t done that, though, is what I mean when we talk about measuring distances in the sky. I’m not talking about how far it is to the Moon (239,000 miles), the Sun (92.9 million miles; 1 astronomical unit) or the star Sirus (8.6 light years).

Just by standing outside, you can’t really tell how far away objects are. They could all be 200 miles, or 200 gazillion. There’s no real way to know without equipment that I don’t carry with me. They all appear to us to be… there. I mean distances between things as they appear in our sky.

What I mean is, if you were to draw a line on the sky from one thing to another, how long would that line be? Eight feet? Nine centimeters? Two fathoms? None of these really work. Lots of people use degrees for these measurements. For instance, you’ve probably noticed a few times when I’ve said things like “Mars and Saturn will be eight degrees apart,” or “The Moon will move its usual 13 degrees from tonight until you see it tomorrow night.”

It’s really simple, actually. You probably remember that the distance around a circle, its circumference, is divided into 360 equal parts called degrees. So, 1/360 of the way around is one degree. A quarter of the way is 90. Halfway is 180. Spheres are the same idea, just in three dimensions. So, it’s all a bunch of 360-degree circles going all over the place and turning into a ball. Each degree is then divided into 60 arc-minutes; each arc-minute into arc-seconds. Using fractions like “63 and a half degrees” or “63.5 degrees” is fine, too.

Let’s think of the Earth as sitting at the middle of the big sphere that is the entire sky. The distance from a point on the horizon to a point on the directly opposite horizon, say, from due east to due west, is 180 degreeshalf of a circle. From the horizon to straight up overhead is 90 degreesa quarter circlehalf the distance from horizon to horizon. You can measure the distance between any two points in the sky in degrees, too. Altair to Deneb in the Summer Triangle? About 40 degrees.

This is called the angular distance. Since the Earth’s surface usually gets in the way, from the ground you can only ever see at most about half of the big sky sphere at a time. So, the furthest apart any two things can be is 180 degrees.

Lucky for you, you’ve probably got a great measuring tool with you all the time: your hand and arm. All you need to do is hold your arm straight out against the sky, and count how much of your hand it is from one thing to another:

  • The width of finger is about 2 degrees
  • The width of three fingers is about 5-6 degrees
  • The width of your whole closed fist held out against the sky is about 10 degrees
  • The width of your whole fist held with your thumb and pinky out is about 25 degrees

Not everyone hands and arms are the same size, it’s true, but this works for almost everyone because of how your hand looks to you at the end of your arm. Most people’s hands and arms are in the same proportions. So people with bigger hands tend to have longer arms, which in turn, make their hands seem a little smaller because they’re further away. The opposite true, too. People with smaller hands often have shorter arms, so their hands are closer and seem bigger.

Now all you need to do is count how many fists it is from Arcturus to the Regulus. Six fists at about 10 degrees each is about 60 degrees.

Measuring angular distances with your hand at arm’s length

This actually brings to mind a bigger question about how to find your way around the sky when you’re standing outside looking up, or in the case of this week’s Venus-Jupiter excitement, looking out at a pleasing, just-slightly-more-than-level angle.

When I was younger, as I started falling in love with astronomy, I tried and tried to learn  about where the stars were in the sky. The terms that I read over and over in books were declination and right ascension.

Astronomy uses a coordinate system, and those two are the same sort of idea as latitude and longitude are here on the ground. I’m not as good at math as I’d like to be and, while these aren’t math, they’re… math-ey.  I just couldn’t grasp them. Even now, I only have the thinnest idea of how they work, and I don’t think about them much. Coordinate systems are great, don’t get me wrong. If you’re walking around a big city, it’s really easy to say an old ’80s video game store you’re looking for is on 3rd avenue between 28th & 29th street, and much more helpful than saying you can find that old dusty copy of “Pitfall!” at a specific address. For me, though, in the sky, these things never worked out. Believe me; I tried. For what I like to do, mostly naked eye stuff, close enough is close enough.

Instead, I learned where lots of really bright stars are, the constellations they’re in or the asterisms they’re near, and the time of year they’re up in my nighttime sky.

Here’s something you can do if you’re in the northern hemisphere. The two most important constellations with my method are Ursa Major and Cassiopeia. They both have big, prominent asterisms, recognizable groups of stars: Ursa Major has the Big Dipper; Cassiopeia has that terrific W-shaped pattern. What’s more, one of them is almost always visible in the northern sky. They sit directly opposite each other in the sky with Polaris, the North Star, between them. So if you can find either of them, you can figure out what direction you’re facing with a snap of your fingers or a wiggle of your nose.

cass urs
The Big Dipper and Cassiopeia are directly across Polaris from each other

With the coordinate system out of the way, you just need to keep in mind that Spica is the brightest star in Virgo, rather than knowing its coordinates. This is like knowing Albuquerque is the biggest city in New Mexico. You don’t need to know it’s at 35 degrees north latitude, and 106 degrees west longitude.

Remembering brightest stars in constellations is just like remembering the brightest stars in The Breakfast Club, plus then you get to have conversations like this one with your friends:

Janet: Is that Betelgeuse?
You: No, it’s July. That’s Spica.

Maybe your friend’s name isn’t Janet.

There are other navigational terms, too, but we’ll talk about those later on, when they come up. For now, though, let’s keep it here. This works really well for me, and I’m able to do pretty much everything I want in the sky with it. Maybe it’ll work for you, or maybe you’ll prefer using some other method. Please, please, do whatever you like, whatever makes you happy, inspired, and keeps you looking up. Just like so many things in life, the best one is the one you use.

See you soon, and clear skies, everyone!


2 thoughts on “Finding Your Way Across The Sky

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