NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction question
From: Frank Reed CT
Date: 2004 Nov 13, 19:53 EST
From: Frank Reed CT
Date: 2004 Nov 13, 19:53 EST
Alex E wrote:
"My altitude over the sea level is 530 m. This should somehow affect refraction (decrease the magnitude of the correction?)."
Yes. The air is thinner at altitude. There are two ways to handle this: either enter a lower air pressure than the usual pressure reported by weather services (which is adjusted to sea level by convention) or apply a separate correction in addition to the temperature and pressure corrections. It works the same in the end either way. The altitude correction multiplies the total refraction by approximately
exp(-h/32000feet)
or, nearly enough,
exp(-h/10km).
If you ever find yourself sailing on Lake Titicaca in South America, this would be a major factor. For an example, let's suppose the refraction for an object you're observing is 5 minutes of arc using the tables in the usual manner and with standard air pressure (sea level convention). Suppose your altitude above sea level is 3200 feet. For small-ish values of its argument, the exponential function is well approximated by exp(x) = 1 + x so for 3200 feet the values of exp(-h/32000feet) is just about 1-0.1 or 0.9. That means that the refraction would be reduced from 5 minutes of arc to 4.5 minutes of arc.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"My altitude over the sea level is 530 m. This should somehow affect refraction (decrease the magnitude of the correction?)."
Yes. The air is thinner at altitude. There are two ways to handle this: either enter a lower air pressure than the usual pressure reported by weather services (which is adjusted to sea level by convention) or apply a separate correction in addition to the temperature and pressure corrections. It works the same in the end either way. The altitude correction multiplies the total refraction by approximately
exp(-h/32000feet)
or, nearly enough,
exp(-h/10km).
If you ever find yourself sailing on Lake Titicaca in South America, this would be a major factor. For an example, let's suppose the refraction for an object you're observing is 5 minutes of arc using the tables in the usual manner and with standard air pressure (sea level convention). Suppose your altitude above sea level is 3200 feet. For small-ish values of its argument, the exponential function is well approximated by exp(x) = 1 + x so for 3200 feet the values of exp(-h/32000feet) is just about 1-0.1 or 0.9. That means that the refraction would be reduced from 5 minutes of arc to 4.5 minutes of arc.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
