Twice during the year, September 21 and March 21, the length of day and night are equal because the tilt of the Earth's axis (in relationship to the sun) is nullified and both the Northern and Southern Hemispheres receive equal quantities of sunlight. An animation showing this is here. [Natural History; v103, 26-27; 1994.] [Astronomy; v20, 52-54; year?.]
