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September 23, 2011 / Dave Gorham

Where is the Sun, Little Darling?

It’s the first day of Autumn today (officially, Autumn arrived at 0904 UTC, or 0304 CST) and the Northern Hemisphere is on a familiar path to winter. So today, where exactly is the sun?

Sorry, trick question! Technically, the sun is in the same spot it always is. The better question then would be, “Where is the Earth?” although that too, would be a trick question. Of course, the Earth’s orbiting the sun as it always does. It’s the tilt of the Earth upon its axis and the orbit around the sun that brings about the change of seasons and the placement of the Earth relative to the sun. Actually, we’re always moving in six different directions at once, but that’s for another day.

As the Earth rotates around the sun, it also spins about its own axis. Image: Wikipedia

So where does that leave us today? It leaves us with the sun on the same plane as the center of the Earth directly over the equator. Or, in other words, it leaves the sun directly over the equator — an equal distance between the Earth’s northern and southern tilts. Today is the Autumnal Equinox. This positioning happens twice per year; once today and once six months hence.

George Harrison's tribute to the sun is an ode to spring and the fading winter. Image: Metrolyrics.com

That’s not the only thing equal about today’s equinox. Not only is the sun an equal distance between its range of northern and southern tilt, there is also an equal amount of daylight and night, called the equilux — 12 hours of daylight, 12 hours of night (or a very close approximation).

Solstice and Equinox table. Source: Wikipedia

For the vernal or spring equinox (March 21), Earth is in the process of tilting the Northern Hemisphere towards the sun, with summer on the near horizon. Six months later, on the autumnal equinox (today), Earth is tilting on its axis so the Northern Hemisphere is titled away from the sun as the  first day of winter draws near. Of course, this tilting is a never ending process: each day of the year, with each passing tick of the clock, Earth is either tilting towards or away from the sun — except for a split second twice a year.

Next on the calendar is the first day of winter — the winter solstice — and if you guessed that’s three months from now — halfway to the vernal equinox — you would be correct. If you also guessed that’s the day of the least amount of daylight for the Northern Hemisphere, you would also be correct. On the first day of winter in the Northern Hemisphere, the Earth has reached it’s maximum southern tilt with maximum sunshine and warmth for the Southern Hemisphere and the least amount of sunshine and warmth (in the form of incoming longwave solar radiation) for the Northern Hemisphere. At this time, the sun is directly over the Tropic of Capricorn. Exactly six months from now the sun will once again be precisely over the equator, but this time the Earth will be tilting so that the Northern Hemisphere will be more exposed to the sun. Longer days and more incoming solar radiation will be the result. Or in other words, “Here comes the sun, and I say, it’s all right.” (Although I hope next summer isn’t as broiling hot as this summer was!)

This image highlights the Tropic of Cancer, which lies 23° 26′ 16″ north of the Equator. The Tropic of Capricorn lies 23° 26′ 16″ south of the Equator. Image: Wikipedia

 

 

 

 

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