2009-09-22 05:11:07.000 – Mike Carmon, Staff Meteorologist
Today at 5:18 PM EDT, a special astronomical event will occur, and you probably wouldn’t notice unless you were told.
In case you blinked, I can tell you that today is September 22nd, which (for 2009) means the first day of astronomical fall, and yet another summer has drawn to a close.
What occurs at 5:18 PM? The autumnal equinox (for the Northern Hemisphere). Because our educational observer has left us for the last half of the week, I’ll attempt to explain how this works:
The earth revolves around the sun, but also rotates on its own axis as it revolves. In addition, the earth is tilted 23.5 degrees on this axis, which plays a vital role in giving us seasons. As the earth revolves around the sun, the tilt causes different hemispheres to be closer or further from the sun at different times of year. Around June 21st, the axis is tilted in such a way that the Northern Hemisphere is closest to the sun. Hence, June 20th or 21st is the first day of astronomical summer for the Northern Hemisphere. At this time, 23.5 degrees North latitude receives the most direct sunlight on earth. This means the sun’s rays have to pass through the least amount of Earth’s atmosphere at this point, so essentially, this is where the most heat is being applied. This is known as the summer solstice. The Southern Hemisphere is now furthest from the sun, so for those folks, it is the first day of winter and their winter solstice. In the above picture, the location of the earth on the far left represents the June solstice.
The opposite holds true for the December solstice (the far right on the above picture).
However, on September 22nd or 23rd every year (the earth in the foreground), the tilt of each hemisphere is neither towards nor away from the sun. The equator (0 degrees latitude) receives the most direct sunlight on earth, which signals the beginning of a transitional season for the temperate regions (in this case, autumn for the Northern Hemisphere and spring for the southern hemisphere). On the day of the equinox, the geometric center of the sun is in the sky for exactly 12 hours everywhere on Earth. As a result, every location in the world has approximately 12 hours of daylight and 12 hours of night. It is not exact, however, due to the fact that the sun is not simply one geometric point, but a three-dimensional sphere. So, the center of the sun may have set, but the upper edge of the sun ‘disk’ is still visible. This adds a little more time to daylight. The same is true for sunrise as well, as the upper edge of the disk is visible before the center point clears the horizon. Also, refraction of the sun’s rays causes the sun to appear higher in the sky, so the upper edge of the disk appears above the horizon before it actually reaches the horizon. Again, vice-versa applies for sunset. These subtle characteristics lead to a length of day longer than 12 hours on the day of the equinox, despite the fact that the geometric center of the sun is in the sky for exactly 12 hours.
The same holds true for the March equinox.
It is both the tilt of the Earth and the revolution of the Earth around the sun that contribute to the changing of seasons.
Mike Carmon, Staff Meteorologist