REFRACTION, ASTRONOMICAL. The effect of refraction in the appearance of the heavenly bodies is to make them appear higher in the sky. Thus refraction increases the altitude, but it does not alter the azimuth. In the figure S is the true position of the star, and S' the apparent. The refraction is greatest at the horizon, where it is about 37'. This is evident since the angle of incidence is here the greatest. From the hori zon to the zenith the refraction constantly de creases, very rapidly near the horizon and more slowly at greater elevations. Tt varies approxi mately as the tangent of the zenith distance down to a zenith distance of about and is entirely independent of the distance.
The effect of re fraction upon the ris ing and setting of the heavenly bodies is to make them appear to rise earlier and set later than they would if there were no at mosphere. The hori zontal refraction var ies from 35' to 39'. Therefore when we see the sun's lower limb rising the whole disk is really below the horizon. In this
way the refraction accelerates the sunrise in our latitudes from 2 to 4 minutes. At sunset the sun is delayed an equal amount. Thus the total effect of refraction is to increase the length of the day at the expense of the night from 4 to 8 minutes, according to the inclination of the sun's diurnal circle to the horizon.
Refraction also explains the elliptic appear ance of the sun's and moon's disks when near the horizon. Owing to the rapid increase in the amount of refraction near the horizon, the lower limb appears more elevated than the upper, thus shortening the vertical diameter.
Another effect of refraction when the air is much disturbed is to make the stars 'dance,' especially when seen through a large telescope with high power. This is to the constant displacement of the image by the varying re fraction.