You can easily measure these angles when stargazing by using your hand as your ruler. Angles this small are near or beyond the limits of ordinary human vision, but they become useful when using a telescope to make observations.įor casual stargazing, observers think about much larger angles. There are sixty arc minutes in a degree, and sixty arc seconds in an arc minute. For this purpose, astronomers use arc minutes and arc seconds. Most objects in the sky are smaller than this, so it is often convenient to use a smaller measure of angle. The Sun and the Moon, for example, subtend an angular diameter of half a degree. We do this by measuring the angle an object subtends in the sky. If you can just cover the object with your index finger, then you know that the object subtends about one degree.Īlthough we can't easily measure the physical sizes of celestial objects, we can measure their apparent sizes. To gauge angles of distant objects, hold your hand at arms length and compare the appearance of the distant object to the size of your hand. The Sun appears to be the same size as the moon because the Sun is much farther away simply because the Sun is both 400 times larger in diameter and 400 times farther away than the Moon. The Sun and the Moon look about the same size in the sky, even though the Sun is really much larger. Orion's pattern would disappear if we could view it from any other angle or if we could perceive the depth, because the stars would project differently.īecause depth perception is lost, measurements of size are much more difficult. The stars in the constellation Orion, for example, are at a variety of distances, but the differences are imperceptible to us on Earth. As we think about how we would expect to perceive the rotation of the Earth, we can use this second perspective to guide us.Įverything we see in the sky, we see as though projected onto the celestial sphere. To an observer on Earth, these two perspectives appear the same. In the other perspective, the Earth stands still and the celestial sphere rotates once per day. In one perspective, the celestial sphere itself remains still while the Earth turns inside it. The celestial sphere can be seen from either of two perspectives. Our first and most basic look out into the universe is completely stripped of any depth perception. Brighter stars appear closer stars in nearly the same direction appear nearby each other, even if they are separated by great distances. Our particular position among the stars gives us a particular view. The observer is always taken to be at the center of the celestial sphere, even though the observer isn't at the center of the Earth. It can be taken to be infinite (or at least really big), with an infinitesimal Earth at the center. The celestial sphere is an imaginary hollow globe that encloses the Earth. We will use the reference points of the celestial sphere as the basis for several coordinate systems used to place celestial locations with respect to one another and to us. When we refer to the celestial sphere, we are imagining that everything we see on the sky is set on the inside of a huge spherical shell that surrounds the Earth. Working with the celestial sphere offers a convenient way of describing what we see from Earth. The concept of the celestial sphere provides a simple way of thinking about the appearance of the stars from Earth without the complication of a more realistic model of the universe. Only a few centuries ago astronomers came to realize that the stars are actually very far away, scattered throughout the Milky Way Galaxy, rather than attached to the inside of a vast sphere. Because of this, many ancient civilizations believed that a dome really did enclose the Earth. The stars appear to move together across the sky during the night, rising in the east and setting in the west, as if they are affixed to the inside of a dome.
This creates the appearance that all of the stars have the same distance. Depth perception fails us for the distant objects we see in the sky. If you look out from an empty field into a dark sky, you will get the impression that you are standing on a flat plate, enclosed by a giant dome.
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