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# Lunar Phase Simulator – Background 5

## The Horizon Diagram

Note that the exact position of the horizon plane depends on the location of the observer. This is why the appearance of the sky is varies for people on different places on earth at any given time. A horizon diagram is one tool we can use to show what an observer's sky looks like. A horizon diagram shows the sky as if it is an upside-down bowl viewed from the outside.

In this system, the point directly overhead is called the zenith (the nadir is the opposite point directly below the observer). The celestial meridian is the imaginary line that runs in the sky from the north to the south. If you are standing on the earth looking directly south, then the meridian is the imaginary line that starts behind you, goes through the zenith, then sets in front of you. It separates the eastern (left) and western (right) portions of the sky.

We can locate an object on this 'bowl' by first describing the direction it is in (north, east, south, or west, or any degree in between). Then we can describe how high in the sky it is, that is, its altitude, which is usually measured in degrees. The horizon itself is at 0° and the zenith is at 90° – all other visible objects are in between.

In this lab we always show the sun and moon on the celestial equator, which is a circle that rises in the east, reaches its highest altitude at the meridian, and sets in the west. This is where the sun and moon can be found on average, but in fact their positions vary quite a bit.

So, for example, the sun in the diagram below is crossing the meridian. It is found in the south direction and has an altitude of about 45° (ie. it is about halfway between the horizon and the zenith).