![]() Remember that the celestial sphere is only an illusion, made from the way celestial objects all appear to be on a sphere around us - and all these objects are so far away that Earth’s size is insignificant in comparison. Q: Wouldn’t my horizon actually be slightly less than half of the celestial sphere, because Earth “sticks up” a bit in the middle (as shown in part (a))? A: No.Another fun way to address this idea is by asking students to think about the question, “What’s north of the North Pole?” The answer is nothing, since you cannot travel north of the North Pole in other words, it is in essence a meaningless question. Q: Why isn’t the horizon marked with the directions north, south, east, and west? A: Because if you are at the north pole, all directions are south! This can be a strange concept to students, but you can point out that if you start anywhere on Earth and walk due north, you will eventually reach the North Pole – and if you then continued walking in the same direction, you’d find that you were heading south.Q: Isn’t the circle marking the horizon in Figure 2.10 the same one that we called the celestial equator in Figures 2.7 and 2.10? A: Yes if you were at the North Pole, your horizon would trace out the celestial equator.Teacher Notes (Figure 2.10): If your students ask you, here are answers to three common questions that arise with these diagrams and the related discussion note that these questions are not addressed in the main text because they are beyond the scope of what we expect 8th grade students to learn: Credit: Adapted from The Cosmic Perspective. (b) This diagram is the same as the one to the left, except this time showing your local horizon instead of the entire Earth. Figure 2.10 – (a) If you lived at the North Pole, your local sky would be the northern half of the celestial sphere, with your “up” pointing directly toward the north celestial pole. Moreover, your sky would always consist only of the northern half of the celestial sphere, so that you could never see any stars or other objects that are in the southern half of the sphere. In other words, for the sky at the North Pole, all stars are circumpolar. Therefore, you would see all stars making daily circles around your sky, never rising or setting. Notice that, as viewed from the North Pole, the daily paths of all celestial objects are parallel to the horizon. Earth itself would be blocking your view of the southern half of the celestial sphere, so your sky would consist only of the northern half, which would be making its daily circles as shown. Imagine that you are standing at that position. ![]() If you add a little stick figure person standing at the North Pole to Figure 2.9, the person would be oriented as shown in Figure 2.10. We’ll start with the local sky at the North Pole, which is probably the easiest local sky to understand. (b) This diagram shows the same thing, but we’ve removed the stars and made the sphere transparent, which will make it easier to see how the sky varies at different latitudes. Figure 2.9 – (a) We imagine the celestial sphere rotating around us each day, but it is really Earth that is rotating, not the sky. Still, for purposes of understanding what we see in the local sky, it’s useful to use the model of a celestial sphere rotating around us.Īs you’ll see, this daily circling of the sky explains all the daily motions we see for celestial objects in the sky, including why some objects just follow daily circles without rising or setting, and why all other objects rise in the east and set in the west. ![]() Of course, today we know that the celestial sphere is just an illusion, and that the reason it appears to rotate around us is because Earth is rotating in the opposite direction - from west to east - once each day. The fact that celestial objects rise in the east and set in the west (or, for circumpolar stars, circle around the north or south celestial pole) each day led the ancient Greeks to assume that the entire celestial sphere rotates around us once each day, going from east to west as shown in Figure 2.9. ![]()
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