The sky is filled with a clockwork of motions that ticks off the seconds, days, and aeons as precisely and regularly as the finest timepiece humans have ever made. The motions in the sky are a combination of the Earth’s orbital motion around the Sun, the spin of the Earth on its axis, and the fact that the North Pole of the Earth is not pointing straight up from its orbit.
One consequence of these three facts is that the Sun’s position in the sky changes over the course of a year. Each day it rises and sets at a different point on the horizon, and tracks across the sky along a different pathway. The equinoxes are the days during the year when the Sun rises and sets directly in the East and West. The solstices are the days during the year when the Sun rises and sets at the point furthest North and South on the horizon.
Sometimes modern landscapes align with the celestial clockwork, possibly by design but often by accident. Many towns and cities are laid out on a grid system. In Chicago, the streets are laid out (more or less) on a rectangular grid aligned to the compass points, and a “Chicagohenge” can be photographed on the Spring Equinox in March, and on the Fall Equinox in September!
No matter where you are, there will be opportunities to use your city as a henge for photographing the ever-changing motion of the Sun. The Sun is always on the move, marching up and down the horizon. With some careful planning, observations, and simulation (such as with a planetarium simulator like Stellarium, http://stellarium.org), you can figure out when the sunrise and sunset will line up with the streets in your own town.
The truth is, it is not a life and death matter if I don’t pay attention to the daily motion of the Sun. I can really just do astronomy for fun --- because it is cool, and it lets me capture some beautiful pictures that I can use to impress my friends and woo members of the opposite sex.
The Spring Equinox is just around the corner! So go out with your cell phone and catch a sunrise or a sunset, directly down your nearest East-West street. Make sure you tweet the picture to the rest of us, so we know it is spring and can start to think about planting our gardens!
Read the full post at Write Science.
Written by Shane L. Larson.