Sir Norman Lockyer (1836-1920), a professional astronomer, tried to date Stonehenge by astronomical means. The technique he attempted to use was astronomical dating. Over a period of several thousand years, the midsummer sunrise position had changed. Today, it rises slightly east of the position that it rose in prehistoric times. The sighting line was marked by the axis of the Avenue, and pegs were set at intervals along the Avenue. Since the sun no longer rises in the same position, we have to measure the alignment of the stones with optical instruments to calculate the position of the sunrise as it used to be.

The sun's declination at the solstices of the present day is about 23.5º, but the angle of the ecliptic changed over the century. It has decreased to its present value from about 24.1º degree in 4000 BC. The difference in the sunrise position between AD 1900 and 2000 BC is about 0.85º. The rate of this change is about 0.02º per century. This calculation comes from a trigonometric equation.

cos A= sind-sinfsinh.
cosfcosh

(A-azimuth, h-the angular elevation of the Sun at sunrise and sunset, f(phi)-latitude, d(delta)-declination of the Sun)

By measuring the azimuth of the axis of the Stonehenge Avenue and comparing it to the present value, we will know the date of the monument. Using this method, Lockyer estimated the date of the Stonehenge Avenue alignment was 1680 BC.(a span from 1900 to 1500 BC)

Reference:

Wood,John Edwin. Sun, Mood, and Standing Stones. Oxford University Press, London, 1978.