Summer Solstice

For anyone located in the Northern Hemisphere, as I am, today is the longest day of the year (in the Southern Hemisphere this is the Winter Solstice). The scientific term for this phenomenon is solstice. The actual solstice (the moment when the earth’s tilt is pointed directly at the sun) occurs about 10 minutes from the moment I’m writing this post, at 5:26 AM Pacific Time (12:26 PM UTC).

The solstice is also the day (again, in the Northern Hemisphere) when the sun rises and sets closer to north than on any other day. In fact, at any location north of the Arctic Circle, the sun never sets today. At my location (near San Diego, California) the sun will rise this morning at about 58 degrees east of north (estimated by Brunton compass); tonight it will set at the same angle west of north. Our view of both sunrise and sunset here is a little distorted by the fact that we live in a valley and are surrounded by mountains — we can’t see the actual horizon in any direction.

Many people believe that summer is when the Earth is closest to the sun, and winter when it is furthest. A simple way to prove to yourself that this is not true is to consider that right now (summer for us) it is winter down in the Southern Hemisphere. The Earth can’t simultaneously be at its closest and furthest from the sun! In fact, nearly all of our seasonal climate variations are caused by the tilt (about 23 degrees) of the Earth’s axis, relative to its orbit around the sun. There is a tiny variation caused by distance; tiny because the Earth’s orbit is almost perfectly circular. There’s also a tiny effect caused by the precession of the Earth’s axis (i.e., its angle of tilt changes slightly), and a few other even smaller effects — but all of those smaller effects are trivial when compared to the effects of the simple tilt.