Rise/set search for observers above the ground

Rise/set times for observers above the ground

This release of Astronomy Engine supports searching for rise and set events of celestial bodies when the observer is a significant height above ground level, for example in a balloon or airplane. This causes the apparent horizon to dip below the observer's horizontal plane, which affects the timing of rise/set events. There is now an additional parameter to the SearchRiseSet functions called metersAboveGround. The observer parameter's height field is still the number of meters above mean sea level, but metersAboveGround tells how high the observer is with respect to an idealized flat ground level.

Atmosphere model

As part of this new capability, I had to calculate relative air densities at different elevations above mean sea level. This was needed for calculating refraction of light rays from the horizon toward the observer. As a by-product of this work, there is a new function Atmosphere that implements the U.S. Standard Atmosphere model of 1976. Atmosphere provides idealized air temperature, pressure, and density as a function of elevation.