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The trouble with it all is that the Earth rotates!
Imagine all the stars sitting still in the universe, and the only motion in existence being the rotation of the Earth around its own axis, causing night and day and night and day all over again. Though a completely false assumption, for the purpose of setting up a telescope you can put yourself in this frame of mind.
What would happen if you looked at “Some Other Star” in Figure 1? Forget everything else on the diagram. If you were looking at that star, with a telescope glued in place with crazy glue, no gears or any movable parts in the mount, at the same place were “Telescope A” is diagrammed, the star would move out of the telescope’s field of view rather quickly. Not what you were looking for.
Now imagine you looked at Polaris, as shown by “Telescope B” in Figure 1 instead: Since Polaris is sitting right on the axis of the Earth’s rotation, the star would seem suspended in the same spot – i.e. the star would appear to rotate around itself, but at the exact same location. You could glue a telescope to the ground focused on Polaris, and every night you’d see it, barring clouds. Effective, but boring.
This would only work North of the Equator – the Earth is not transparent, and therefore you’d have a blocked view at Telescope “C”.
Another interesting point is that Polaris is not exactly over the point of rotation – in part because of gyroscopic precession, Polaris currently ends up slightly on the wrong spot. Polaris will be the closest to the North Pole in 2105. However, let’s assume for now that Polaris is perfectly positioned. Correcting for the error will be discussed later.
If you now look closer at the diagram, you will notice that there are dotted lines going from Polaris to the two telescopes A and B. You will notice that those dotted lines end up pointing at something curious on the tripods of the telescopes – a very short little red line at an angle on telescope “A”. Now imagine that you are at telescope “A”, looking at Some Other Star. IF YOU ROTATED THE TELESCOPE, WHICH IS NOW NO LONGER FIXED IN SPACE, AROUND THE SMALL RED LINE (AND THEREFORE THE DOTTED LINE) AT THE SAME RATE OF THE EARTH’S ROTATION, “SOME OTHER STAR” WOULD REMAIN FIXED IN THE TELESCOPE’S RANGE OF VIEW!
Let that sink in. Ponder it. Think about it. Once you see it, you will understand the simplicity of the equatorial mount. In some of the subsequent diagrams, the polar axis will be pointed out in red.
Figure 1 – Different points of observation from the Earth’s surface. Terribly out of scale.
Now that the notion of something having to point to Polaris to create an axis, in order to rotate the telescope to keep things stationary, sinks in, there is a whole new benefit we gain from this setup.
Forget the rotation of the Earth for one second. Instead, imagine that you are on the North Pole, looking at Polaris, as shown in Figure 2. Unless you were lying down on the ice beneath you, you’d have to really bend your neck to look straight up at the star. That’s because from your vantage point Polaris is exactly at 90 degrees from the surface, straight up. This also happens to be 90 degrees from the Earth’s Equator.
Now assume for the time being that the Earth is standing still. If you now lowered your gaze and looked at Some Other Star, the angle would be much less than 90 degrees, and your neck would be better off. Note that this angle remains constant over time, i.e. if you knew the East/West direction to turn and knew the angle of your neck to look down, you’d always find that Some Other Star right there, in the same spot. By having a fixed point at Polaris, you can now reference the location of Some Other Star, or every other star, for that matter.
Now place the Earth into rotation. While you are still on the North Pole, you’d quickly realize that Some Other Star would still be at the same angle in terms of the curve in your neck, but the position of the star would change East to West over time. More on this later!
Figure 2 - Gazing at the stars from the North Pole.
Figure 3 – All the ways a German Equatorial Mount can twist and turn.
Figure 4 – Everything looks straight from the side. The red dashed line overlays the polar alignment axis.
Figure 5 – Everything looks straight from the front.
Figure 6 – Winged nut holding vertical axis in place.
Figure 7 – Setting the local latitude.
Figure 8 – Trying to compensate for the error in Polaris’s position over the North Pole.
Figure 9 – The axis of declination with scale set at 90 degrees.
Figure 10 – The axis of ascension with scale.
Figure 11 – The Big Dipper has a good marker in the sky for setting the axis of ascension scale.
You have already set up the scope! Now here is an interesting tidbit: The mount is in parallel with the Earth’s axis of rotation! Figure 1 suffers from being terribly out of scale, so the dotted lines appear at an extreme angle to the Earth’s axis of rotation. However, as we move Polaris further and further away (Figures 12 and 13), it becomes more and more evident that the line of sight to Polaris is actually quite parallel to the axis of rotation. If you slowly rotate the scope around this axis (the control is shown in Figure 14), objects you look at should stay fairly centered over time.
In addition, you can now use star charts to find any star or other heavenly object using the right ascension and declination coordinates.
Figure 12 – The line of sight seems more parallel with the axis of rotation than in Figure 1.
Figure 13 – The further you remove Polaris, the more parallel the lines become. Polaris is 360 light years away from Earth, so no diagram can even begin to approximate the true proportions of the distance.
Figure 14 – Control to compensate for the Earth's rotation over time. On many scopes you can attach an electric clock drive to the other side of the mechanism to make the continuous time adjustment automatic.
Entire document © 2004 by FunnyYouAsked.com
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