We know that the Earth travels around the Sun in a 26,000 year elliptical orbit. My question is, where are we now, at the start of 2011? Are we close to the Sun or are we far away? Does anyone know exactly where we are right now in this orbit?
– Lonnie J. Parrett, St. Louis, Missouri
In answer to your immediate question: Earth has just passed out of the perihelion, or closest, point of its orbit. It arrived there on Monday, January 3, at 2 p.m. Eastern Standard Time. Earth’s distance at that time was 91,407,282 miles from the Sun.
The Earth will reach aphelion, its farthest point from the Sun during the year, on July 4, at 10 a.m., Eastern Daylight Time. At that time, the Earth will be 94,509,130 miles from the Sun, a difference of about 3,000,000 miles.
In speaking of a 26,000 year elliptical orbit, you are no doubt referring to the motion known as precession. Precession is a change in the orientation of the rotation axis of a rotating body. Here is an illustration using a gyroscope.
Earth’s axis takes 26,800 years to undergo one precessional “wobble.”
In any case, what I believe you are referring to is called “apsidal precession,” and has a period closer to 21,000 years. Basically, at this moment in time the occurrence of the winter solstice in the Northern Hemisphere, very nearly coincides with that point in the Earth’s orbit where we are closest to the Sun (perihelion). At this point in time, perihelion usually happens about two weeks after the solstice.
In about 5,000 years, perihelion will closely coincide not with the winter solstice, but the March equinox. In about 10,000 years, perihelion will coincide with the June solstice, and in just over 15,000-years it will coincide with the September equinox. Finally, about 21,000 years from now, everything will return to where it is now: perihelion will again be closely aligned with the December solstice. This diagram may make it a little clearer.
It may seem strange to learn that, while the Northern hemisphere is undergoing the coldest temperatures of the year, the Earth is actually closest to the Sun than at any other time during the year. Of course, as most of us learned in school, seasons are controlled by the tilt of the Earth’s axis, rather than by its distance from the Sun. We experience summer or winter conditions based on whether our half of the Earth is pointed toward the Sun, or away from it. While a 3,000,000-mile change in relative distance may sound like a lot, the truth is that our overall distance from the Sun is so great that this otherwise large figure amounts to a drop in the vast astronomical bucket of infinite space. The effect of this slight change on distance throughout the year is virtually nil.
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