Airmass
From Free net encyclopedia
- For Air mass in metrology, see Air mass
The airmass in astronomy quantifies the path length that the light from a celestial source must travel through the Earth's atmosphere to get to the observatory, relative to that for a source at the zenith.
When the zenith distance is less than 60 degrees, a good approximation is given by assuming a plane parallel atmosphere. In this case, the airmass is simply the secant of the zenith distance, which is often denoted "sec(z)".
By definition, a source at the zenith has an airmass of 1. A source at a zenith distance of 60 degrees (i.e. at an an altitude = 90 - zenith distance = 30 degrees) has an airmass of 2.