Astro-Lesson: Summer Solstice

The Sun as it appeared just a few minutes before today's solstice occurred. (Credit: NASA/SDO)

As of 17:16 UTC (1:16 p.m. Eastern US time) today, June 21, 2001, the Sun will reach the furthest north of its travel across the sky. This event is known as the summer solstice in the north, and consequently has the most daylight of the year. In the southern hemisphere, it would be the winter solstice and the shortest day.

The summer solstice is generally used to either mark the beginning of summer or the midpoint, neither is really right or wrong. Seasons are kind of an abstract notion as it is. But from this point on, the remaining days of the year will get slightly shorter until we reach the winter solstice on December 22.  But, that doesn't necessarily mean cooler days, as anyone who has experienced the month of August knows. (And again, for all of those in the southern hemisphere, just the opposite is true.)

Solstice and Equinox. (Credit: NASA)

What causes this change in seasons has everything to do with the tilt of the Earth. The distance of the Earth from the Sun has nothing to do with it, though I know many people who have been incorrectly taught so or envision it this way because the Earth's orbit is slightly oval. In fact, for the northern hemisphere, summer occurs when the Earth is furthest away from the Sun. And if distance were the cause, then both hemispheres would have the seasons at the same time.

What really happens is that the Earth appears to "wobble" around the Sun on its tilted axis. And because one hemisphere is pointed tilted more towards the Sun due to this wobble, it gets more direct sunlight and longer days. This happens until the Sun reaches a certain number of degrees (currently 23° 26′ 16″) north or south of the equator, and then the Sun appears to travel back across the sky to the next point. These lines at north and south are known as the Tropic of Cancer and Tropic of Capricorn, respectfully. And tracing the Sun's path between these two points, every day at the same time, leads to the formation of a figure-8 pattern called an analemma.

Solar Analemma over Athens, Greece. The lowest point is on the winter solstice, the highest is the summer, the crossings are the spring and fall equinoxes. (Source)

Anyways, I hope you have enjoyed the longest day of the year hear in the northern hemisphere. It tends to be a well celebrated day all around the world, and has been since ancient times. Sites like Stonehenge and the ruins of ancient cultures, like the Greeks and Mayans and Egyptians, all have indications of marking the importance of the summer solstice. It's an astronomical event!

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