In article , says...

It seems what I'm after is misinterpreted. I was talking with a friend
about some variable star work he does at his domed observatory. Somehow
I asked him at what is the lowest elevation he surveys. He said 30
degrees, about two atmospheres of thickness. He then said," maybe near
the horizon, it's 47 thickness." That's what I'm looking for.

How many thickness of atmosphere will I see from zero to ninety degrees
above the horizon.

Some other useful approximations can be found he

http://en.wikipedia.org/wiki/Air_mass_%28astronomy%29


A particularly simple approximation, which yoelds results right down to
the horizon which aren't that far off, is:

Air mass = 1 / ( cos(z) + (1/40)*exp(-11*cos(z)) )

This formula yields a horizontal air mass (cos(z) = 0) of 40