
The Solar rises at Stonehenge. Credit score: Wikimedia Commons.
Historic archaeological websites (like Stonehenge) have been constructed in order that the Solar would shine by a sure opening on some special occasion (sometimes a solstice). Wouldn’t Earth’s precession change the geometry of the scenario through the years?
Michael C. West
Bethesda, Maryland
In case you are fortunate sufficient to face within the heart of Stonehenge on the summer time solstice, you will note the Solar rise over the Heel Stone, because it has for millennia. The Heel Stone was erected exterior the primary ring of stones round 2600 b.c. This rock marked the summer time solstice dawn level, and it nonetheless does some 4,500 years after it was put in place.
To grasp how the “wobble” of the 26,000-year-precession cycle has no noticeable impact on the Solar’s rise and set factors, think about the celestial sphere of stars that surrounds us. Your horizon, working from north by east, south, west, and again once more to north, is a circle. Now, think about turning on a vivid mild at Earth’s heart and projecting our planet’s equator onto the celestial sphere. Earth’s equator then turns into one other circle — the “celestial equator.” Lastly, hint the Solar’s obvious yearly path in opposition to the background stars to make one final circle. That is the ecliptic. The Solar crosses the celestial equator twice every year on the March and September equinoxes. The Solar’s highest level, 23½° above the celestial equator, happens on the June solstice. The Solar’s lowest level, 23½° beneath the celestial equator, represents the December solstice.

Precession causes Earth’s poles to hint a circle on the celestial sphere, shifting our present North Star, Polaris, nearer to or farther from the North Celestial Pole each 13,000 years. However precession doesn’t have an effect on the ecliptic or the angle between the celestial equator and the ecliptic. In consequence, it doesn’t have an effect on the Solar’s place relative to the horizon. The connection between these three circles stays the identical because the millennia cross, so a monument like Stonehenge continues to mark the rising and setting of the Solar after 4,500 years.
Raymond Shubinski
Contributing Editor
This text was first printed in 2014 and has been up to date.