Determining Degrees in Observing
Finding Distances Between Astronomical Objects
Amateurs astronomers need to know how to measure degrees in observing in order to find out how far away stars, planets, and other objects are from each other.
How big is a degree in the sky? How can you quickly determine a rough estimation of degree? Sky guides often report planetary conjunctions in separation of degrees, and binary stars in terms of arcminutes or arcseconds. Just how far apart does this mean?
The general rule amateur astronomers use is that the width of your fist from top to bottom held at arm's length equals about 10 degrees. The difference in the size of peoples' fists is generally proportional to the length of their arms. Therefore, a child with a small fist and small arm measuring 10 degrees should be as accurate as an adult measuring 10 degrees. If you want to do a rough check, extend your arm and fist out toward the horizon. Then place your other arm and fist on top of the first, and alternate, trying not to wobble, until you have counted nine fists. Because it is 90 degrees to overhead, your ninth fist should be pointing straight up. For someone with a relatively flat horizon (outside a hilly or mountainous region), you should be able to measure 180 degrees from horizon to horizon.
To measure other angles between objects in the sky besides 10 degrees, there are a few other finger tricks you can use. At arm's length, a pinky measure about 1 degree, and your three middle fingers measure about 5 degrees. To find 15 degrees, use your index finger and pinky stretched out (pretend you are at a hard rock concert), and to find 25 degrees, use your pinky and your thumb stretched apart.
Check your hand measurements with a well-known object in the sky: the Big Dipper. The end two stars in the bowl, the ones that are used to find Polaris, are about 5 degrees apart. The top two stars in the bowl of the big dipper are 10 degrees apart. And finally, using the same star in the bowl of the Big Dipper that you used for the first two tests (Dubhe, the spot at which water would pour out if it were a real dipper) plus the end star in the dipper and you have 25 degrees.
How wide do you think the full moon looks? What would be your guess as to how many degrees across a full moon is? Most people estimate the moon to be much larger than it is. In actuality, the full moon is a mere half degree across. How about the sun? The sun is larger, right? Although harder to see because of its brightness. But imagine a setting sun: How large would it be in the sky? How many degrees would it take up? The answer, of course, is the same as for the moon. They both appear a 1/2 degree in our sky. Just like with a smaller fist that is closer to the observer and measures the same amount as a bigger fist on the end of a larger arm, the sun and moon are different sizes but different distances away. The two objects appear to be about the same size in our sky, and this is why we get those fleeting total solar eclipses.
You can tell how long until the sun will set by measuring its distance from the horizon. The sun moves about 15 degrees across the sky in an hour. Moving 15 degrees an hour for 24 hours would equal 360 degrees, or a full day from sunset to sunset (or sunrise to sunrise, if you wish). Of course the sun is not really moving, it is only appearing to move in the sky as Earth turns.
For binoculars, the field of view can range from 5 to 10 degrees. The number is usually written on the side of the binoculars. For example, a pair might say 10x50 and 5 degrees. A pair of binoculars this size could show 10 full moons sitting side by side. For astronomical observing, a larger field of view is usually preferred.
Once you have a good grasp of degrees, you simply have to know that degrees are further divided down by arcminutes. There are 60 arcminutes in 1 degree, therefore the moon and sun are each 30 arcminutes across. Arcminutes can also be divided. 60 arcseconds make up 1 arcminute. Going back to the Big Dipper, the stars in the handle named Mizar and Alcor are separated by just 12 arcminutes. People with good eyesight can see the two separate stars without optical aid. Stars closer than this usually require binoculars or a telescope to split. Mizar has another companion that is even closer than Alcor. Mizar's double star is a mere 14.4 arcseconds away. Arcminutes are written with the same symbol as feet (') and arcseconds are written with the inch notation (").
