Saturn's ringsbetween 35" and 45" across, depending on distance from EarthRemember that when you look at something far away, it appears smaller than an identical object that's closer to you. The Sun is far larger than the Moon, but from Earth it looks like it's the same size as the Moon because it's farther away. The Hercules Cluster appears to be a little bit smaller than either of them -- even though it's many times bigger than our entire solar system, containing over a hundred thousand red giant stars!Earlier we looked at a rough method of measuring angular distances and sizes using your outstretched hand. If you want to know the angular size of an object that's too small for that method, you can look at them through a telescope or binoculars and compare them to the size of the field of view.If you don't know the size of your field of view, look through your telescope at an object whose size you already know. The image below is a simulation of the view through an 8" Orion SkyQuest Dobsonian reflector with its included 25mm Sirius Plossl eyepiece. The Moonappears to be a bit smaller than half the width of the field of view. Since we know that the Moon is approximately half a degree across, we can estimate by eye that the field of view is a bit more than a degree in diameter.Remember that this is still a rough estimate -- the angular size of the Moon's disc (regardlessof what phase it's in) changes a little bit over the course of a month, because its orbit around the Earth isn't perfectly round. Sometimes it's a little bit farther away and looks smaller; sometimes it's closer and looks larger. In a future installment, once we've learned a
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few more concepts related to measuring the sky, we'll learn some more accurate methods ofmeasuring your field of view.One warning about terminology: "field of view" (or FOV) is a term that refers to more than one thing. The box an eyepiece comes in usually states the apparent field of view(often abbreviated to AFOV). This is the angle that you will see if you hold the eyepiece up to your eye and look through it by itself. Look at the opposite wall of your room through an eyepiece with a narrow AFOV, and you might see part of your window; look through an eyepiece with a wider AFOV, and you might see the whole window plus a bit of floor and ceiling. The AFOV of an eyepiece is determined by various factors in its optical configuration, and typically ranges from around 50° (for inexpensive eyepieces that come included with telescopes) to over 100° (for high-end specialty eyepieces).This does notmean that you will see 50° or 100° of sky when you look through a telescope with that eyepiece! When you put the eyepiece on a telescope, you're introducing more optical components into the system. The true field of view(or TFOV) is the width of the bit ofsky that you see when you look through a telescope with an eyepiece attached. The same eyepiece on a different scope will have a different TFOV; so will the same scope with a different eyepiece. In the example above where compared the field of view with the width of
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