LearnLight & spectraSome maths
Extinction and airmass
The dimming and reddening of starlight by the atmosphere, which increases with how much air it has to cross.
Why it matters
It is why objects low in the sky are fainter, redder and blurrier, and why any target list is really a list of when things are high enough to be worth shooting.
The maths, in layers
1 · Intuition
Straight up you look through one atmosphere. At 30° altitude you look through two; at 10°, nearly six.
2 · The equation
X ≈ sec(z), z = 90° − altitude
- X
- airmass, in units of the zenith path
- z
- zenith angle
In practice
This is the reason the altitude curve matters more than the rise and set times. Thirty degrees is the usual floor.