# Compensating for Atmosphere

**00:00**
The sun and the moon aren’t the only considerations. The other thing that affects your viewing is the amount of atmosphere you’re looking through. This is known as the air mass, meaning the mass of air between you and your target.

**00:12**
As you might guess, this is related to how high above the horizon your target is. If it’s near the horizon, you’re staring at it through a lot of air. While if it’s directly above you, there isn’t as much.

**00:27**
A quick estimate for the amount of air mass is the secant of the zenith angle,

**00:33**
and in case trigonometry is far enough in your past that secant only vaguely rings a bell, here’s a quick reminder.

**00:40**
If you think of the unit circle as the Earth, then the secant is where the azimuth hits the circle’s tangent, a rough approximation for the amount of atmosphere through which you’re looking.

**00:51**
In fact, this approximation is so common that there’s a method for it on the `AltAz`

object. Let’s go add more information to our optimal viewing graph.

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