Observing by Degrees

Posted by Kelly on April 18th, 2013

In this image of Jupiter, the Moon, and Venus taken on February 26, 2012, Jupiter and the Moon are less than five degrees apart and the Moon and Venus (below) are about 12 degrees apart.

In this image of Jupiter, the Moon, and Venus taken on February 26, 2012, Jupiter and the Moon are less than five degrees apart and the Moon and Venus (below) are about 12 degrees apart. Credit: Kelly Kizer Whitt

Often when I’m discussing what gorgeous sky events you should be on the lookout for, I describe them in terms of degrees. Perhaps Jupiter is just three degrees away from the Moon, or Saturn and Venus have a close encounter placing them only a half degree apart in the sky. But what, exactly, is a degree?

The sky is split up into 360 degrees. To get a general idea of how to measure degrees, amateur astronomers use their fist held at arm’s length. Extend your arm out from you, make a fist, and the distance from the top of your hand to the bottom is about 10 degrees. This works even though people have different size hands, because a child with a smaller hand will have a shorter arm length and therefore still block out about 10 degrees of sky with this trick. To check it for yourself, extend your arm and fist out toward the horizon. Then place your other arm and fist on top of the first and alternate them until you have counted to nine. You have just measured 90 degrees and your arm should now be straight up over your head, as it is 90 degrees from the horizon to the zenith.

To measure one degree, use your pinky extended at arm’s length. Your three middle fingers extended should block out five degrees. For 15 degrees, use your index finger and pinky stretched out (as if you’re head banging at a rock concert), and to find 25 degrees, use your pinky and your thumb stretched apart.

You can teach this method to kids by using 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 five 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 handle of the dipper and you have 25 degrees.

People often do a poor job of estimating how large objects are in the sky. How many degrees across do you think a Full Moon is? 10 degrees? Five degrees? One degree? In actuality, the Full Moon is a mere half degree across. How about the Sun? The Sun is larger, right? It looks huge as it sets on the horizon as an enormous ball of magenta. So how many degrees is the Sun? The answer, of course, is the same as for the Moon. They both take up a half degree in our sky. Just like with a smaller fist that is closer to the observer and measures the same amount as a bigger object that is more distant, 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.

This information can also have a practical use. If you are out hiking and it’s getting late and you want to know how much longer you have before the Sun sets, you can measure the distance to 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. (Of course the Sun is not really moving, it is only appearing to move in the sky as Earth turns.) The one caveat with measuring the Sun as it sets is that it doesn’t make a direct beeline for the ground in most parts of the world, but this will still give you a general approximation of how much daylight you have left to find your way back to your car.

For objects that are closer together than one degree, the measurement is often further divided by arcminutes or even arcseconds. There are 60 arcminutes in one degree, and 60 arcseconds in one 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.

Mizar has another companion that is even closer than Alcor. Mizar’s double star is a mere 14.4 arcseconds away. But the unaided eye can’t really see separate objects closer than about 4 arcminutes without help. Arcminutes are written with the same symbol as feet (‘) and arcseconds are written with the inch notation (“).

One person has commented

mike schuster said,

Nice job! Lots of interesting information.

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