# How to calculate the altitude of the Celestial Equator from an observer's latitude? Exam question

When observing from a latitude of 50 degrees North on Earth, the Celestial Equator will have a maximum altitude of: Answer: 40 degrees

I've looked this up and it says something about the observer's co-latitude. However, I do not understand why this is the case. Could someone explain this please?

Perhaps you have heard that the altitude of the celestial pole (3 in the figure below) is equal to your latitude. In other words, angle 5 is the same as your latitude. Of course, the angle between the celestial pole (3) and the celestial equator (4) is 90 degrees (angle 6). From this, you should be able to calculate the angle of the equator (angle 7).

Item 1 is the horizon, and 2 is the celestial sphere.

If you're at the equator ($0$ latitude), where is the Celestial equator?
If you're at the Pole ($90$ latitude), where, the Celestial equator?