# How come I can point to two places with telescope and have the same coordinates?

Absolute beginner here. I read through the telescope manual, watched many videos, and got a pretty decent understanding of DEC and R.A.

However, I don't understand one thing (where theory doesn't match what I see).

On the equatorial mount, you can change DEC by moving the telescope to the left or the right. The picture DEC was set to point to Polaris (~90), and DEC goes down to the left and the right.

Let say I lock R.A. (so it's a constant), And I turn the telescope to the left to set DEC to 50. It points at some place.

I don't release R.A. (so it's still constant) and turn the telescope to the right to set the same DEC (50) only on the right side. And now the telescope points to a different location.

Per my understanding, DEC + R.A. is a coordinate system. As a result, I am puzzled how two points can have exactly the same coordinates.

• @DaddyKropotkin Let say I am pointing at the Northern Celestial Pole. Per my understanding, if change declination to 50 by turning right or left, it's still will be 50𝑜 (positive). And I believe only if I point the whole telescope downwards (facing ground) it will be −50𝑜. Sep 17 at 1:16

It might be easier to visualise if you look at a start chart: (Finder Chart for Polaris (credit:- freestarcharts))

In your example you've locked the RA axis at 0h, and your Dec is pointed at the centre - 90°. Now, when you move the Dec axis one way or the other, it's going to change the RA at some point. With the mount I have, when it's at the home position, the Dec axis is perpendicular, so if I were to move it left then the RA would be 18h, and if I were to move it right then the RA would be 6h.
It's possible your mount will move towards 0h when you move the Dec axis one way, but the other way would change RA to 12h.

Now imagine doing as Dr Chuck's answer says: let's say you're on the Earth and facing due north: 0°, and your latitude is 90°, so you're on the North Pole. Now, what happens if you "lock" your rotation and step to the left? Suddenly due north is to your right, even though you haven't changed your rotation!

When you look at the sky, try to imagine an equatorial grid overlaid on it, like you see in a star chart, and you should be able to see how passing through the celestial pole flips the right ascension.

For further reading, look up the concept of a "meridian flip".

• Thank you. I think between yours and @Dr Chuck answer I understand it now. Sep 17 at 17:32

When you are at the North Pole (on earth), you can only go south. If you to 60 degrees latitude, you might be in Canada, Russia or various other countries.

Its the same with the telescope.

• I agree it's a part of the answer. However, let say you configured your telescope properly (did a northern pole alignment and pointed to a bright star, and set R.A. correctly). This means that you should be able to point it at any location and get specific (unique) coordinates. However, I can easily go through a northern pole and change DEC in one direction vs another and get the same coordinates for two points. It looks like I am missing some rule of telescope usage (something like "If you go through northern pole, you need to configure R.A. spinner again) Sep 17 at 13:58
• You are correct. "If you go through the pole, you need to configure R.A. circle again." Likewise, you need to reset the R.A. circle after a sufficient length of time has elapsed. If you do not move the scope at all (and assuming any motor it has is turned off), the circle says it is pointed at the same R.A. all night. But as the sky rotates, the R.A. is changing continuously. Sep 17 at 15:40