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If the Earth's rotational axis stood upright as our planet revolved around the Sun, the Sun would always rise due east and set due west everyday of the year. Everyday would feature an equinox, with everyone worldwide receiving about 12 hours of sunlight and 12 hours of darkness day after day, all year long. It'd be the land of eternal spring, with no summer or winter solstices.
As it is, the Earth's rotational axis tilts about 23.5 degrees out of vertical. That's why we have the seasons. That's why we have two equinoxes and two solstices in the course of one year. On the Northern (June) solstice, the Northern Hemisphere tilts most toward the Sun, and the high-arcing summer solstice Sun brings the longest day and the shortest night of the year. (In the Southern Hemsiphere, the June solstice is their winter solstice, giving them their shortest day and longest night of the year.) This wonderful animation beautifully displays the seasonal interplay between the Earth and Sun.
The June 21st Northern Solstice
This year's 2007 June 21st Northern solstice falls at 6:06 p.m. Universal Time (2:02 p.m. Eastern Daylight Time), at which instant the Earth's North Pole points maximumly toward the Sun. The Sun will be shining at zenith (straight overhead) at the tropic of Cancer in the Gulf of Mexico, pretty much due south of St. Louis, Missouri.
According to the computational wizard Jean Meeus*, the Earth's rotational axis is inclined at 23 degrees 26' 26" on the 2007 June solstice. Yet, next year - 2008 - the Earth's axis is forecasted to tilt 2" less, at 23 degrees 26' 24". Converting this 2" angular figure into distance, the June 2008 solstice Sun will reside some 200 feet south of this year's Northern (June) solstice Sun. And in 2009, the June solstice Sun is forecasted to move another 2" (some 200 feet) south of where it was in 2008. The tropic of Cancer isn't quite the static place the globe would lead us to believe!
Earth's Oscillating Axial Tilt
It's said the Earth's rotational axis oscillates from about 22 to 24.5 degrees, going from minimum to minumum (or maximum to maximum) in a period of about 41,000 years. This graph illustrates this variation for the last 750,000 years. At some time in Earth's storied past, the tilt apparently got as small as 21.5 degrees. How astronomers even begin to fathom this escapes my understanding; but for more on the subject, I include this Wikipedia article.
We're not far from the middle range at present, but are on a downhill slide. The last maximum took place roughly 10,000 years ago, and the next mimimum will be forthcoming in roughly 10,000 years.
*page 38 of the 2nd edition of Astronomical Tables of the Sun, Moon and Planets by Jean Meeus
|Solstice dates and times from 1992 to 2020|
copyright 2007 by Bruce McClure
May 2007 Feature * July 2007 Feature