Atmospheric Refraction

The deviation of the light or electromagnetic wave from the straight line as it passes through the atmosphere because of the difference in the air density as a function of altitude is called atmospheric refraction. The atmospheric refraction will make the astronomical objects to appear very high in the sky than they are in reality. It is not only affecting the light rays but also affects the electromagnetic radiation.  For example blue is more affected than red in the visible light. The amount of atmospheric refraction is a function temperature, pressure as well as humidity (the amount of water vapour is important in the mid infrared wavelengths). 

Values

Atmospheric refraction is zero in the point of pinnacle; it is less than 1’ at 45⁰ apparent altitudes, and 5.3’ at 10⁰. If the altitude decreases the atmospheric refraction will quickly increase, reaching 9.9’ at 5⁰ altitude, 18.4’ at 2⁰ altitude, and 35.4’ at the horizon. Atmospheric refraction is slightly greater than the visible diameter of the sun, in the horizon. The sunset and sunrise refers to the time at which sun upper limb appears or disappears from the horizon. The standard rule for the sun is -50’ for the true altitude: -34’ for the refraction and -16’ for the sun semi diameter.

The day to day variation in the weather will affect the accurate sunrise and sunset times as well as moonrise and moon set times. Due to this reason, it is meaningless to give the rise times and set times to greater accuracy (Meeus 1991, 103). More accurate calculations will be helpful in finding day to day changes in rise times and set times, that would occur with the standard value of refraction.  The atmospheric refraction is 34’ on the horizon, but only 29’ at 0.5⁰above it. The sun setting or sun rising seem to be flattened by about 5’.

Identifying leveling errors using Atmospheric refraction

This error vary as the square of the distance, but opposite in sign (+ or -) that caused by the earth curvature. The effect of the atmospheric curvature is only one-seventh of that caused by earth curvature. Earth curvature errors are produced by the unbalanced sights in the leveling. In the first and second order leveling, the effect of atmospheric refraction is minimized by taking the BS (Bureau of statics) and FS (federal survey) readings in quick sequence and it is better to avoid reading near the ground, they should be taken atleast 2 ft from the ground level.

Questions:

• What is atmospheric refraction?
• What are the values of atmospheric refraction?