Aphelion
Published on Jul 6, 2007 at 1:39 pm.
2 Comments.
Filed under Earth, astronomy, planets.
Earth is currently at aphelion (or will be just a few hours after I write this). Like all the planets, Earth’s orbit is somewhat elliptical. Johannes Kepler studied planetary motion in the early 17th Century. His first law of planetary motion, based upon his analysis of Tycho Brahe’s observations, was that planets move in elliptical orbits, with the Sun at one focus of that ellipse.
The nearest point that a planet gets to the Sun is called perihelion. Earth was last at perihelion on January 3, 2007, when it was 147,093,602 kilometers from the Sun. Now, on July 7, 2007, at about 00:00 UT (that is 8pm CDT on July 6 for my readers here in Texas or elsewhere in the US’s Central Time zone) the Earth will be at aphelion. It will be 152,097,053 kilometers from the Sun.
Notice that the difference between aphelion and perihelion isn’t really all that huge for Earth, not at all like my drawing. In fact, there is only about a 3.3% difference between the two figures. If I had drawn the figure above to scale for Earth, you’d never have been able to tell just from looking at it that the orbit was not circular. There is a measure of just how elliptical an orbit is. This is called its eccentricity. There are several ways to compute an orbit’s eccentricity, but the easiest is to simply divide the difference between aphelion and perihelion by the sum of those two figures. This yields an eccentricity for Earth of about 0.0167. The closer that the eccentricity is to zero, the more circular the orbit is. An eccentricity equal to 1 or more is an unbound orbit (something that swings by the Sun once and then heads out into interstellar space). An eccentricity of 0.0167 isn’t all that big. There are two planets that have lower eccentricities. Venus has an eccentricity of 0.0068, and Neptune has an eccentricity of about 0.008. But, most planets have higher eccentricities than Earth. Mars, for example has an eccentricity of 0.093, and Mercury has an eccentricity of a whopping 0.206. That means that Mercury’s aphelion is nearly 52% farther from the Sun than its perihelion! Compared to that, Earth’s eccentricity is really pretty tiny.
Earth’s small eccentricity is important to us. It means that the orbital distance from the Sun has little effect on our seasons. Right now, it is summer in the northern hemisphere, even though we are at our farthest point from the Sun. It is winter for us in the northern hemisphere when we are closest to the Sun. The seasons are caused by the tilt (inclination) of the Earth, not it’s distance from the Sun. All too many people mistakenly think that summer is caused by the Earth being closest to the Sun and winter by the Earth being farthest from the Sun. That simply isn’t the case. Even if the Earth had a much smaller inclination, the orbital distance would not have a major effect on the seasons.
But, that is not the case with Mars. Because Mars has a much larger eccentricity than Earth, the orbital distance does matter. Mars’ inclination still matters more, so summer occurs whenever a hemisphere is tilted towards the Sun. Like Earth, Mars has its southern hemisphere tilted towards the Sun at its perihelion and its northern hemisphere tilted towards the Sun at its aphelion. But, due to its much greater eccentricity, Mars is nearly 21% farther from the Sun at aphelion than it is at perihelion. That means that the summers and winters are made somewhat more extreme on that planet’s southern hemisphere, and a bit less extreme on its northern hemisphere. Were Mars to have a substantially lower inclination, the orbital factor might actually dominate the seasons.
But, Earth’s lower eccentricity means that our seasons are dominated completely by the planet’s inclination. One thing, though, that the elliptical orbit does for us is to change the length of the seasons. Kepler’s second law tells us that planets speed up and slow down as they orbit the Sun. They move slowest at aphelion and quickest at perihelion. Kepler observed this effect, but did not understand why it must be this way. Now, we understand that it has to do with conservation of angular momentum. But, this speeding up and slowing down of the orbit means that in the northern hemisphere summer lasts several days longer than winter, and in the southern hemisphere, winter lasts several days longer than summer. The seasons are not equal in length like so many people believe. The effect is even more pronounced on Mars, where the greater eccentricity makes for an even greater difference in orbital speeds at perihelion and aphelion.
So, there you have a bit of your astronomical trivia for the day.
-Astroprof
A Ler…-- Rastos de Luz on July 9, 2007 at 8:53 am: 1
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Paul Peters on May 31, 2008 at 9:05 pm: 2
Excellent analysis. I can’t seem to find data on when Aphelion and Perihelion last occured on Mars. My calculations place the summer solstice on May 25 2008 and the last winter’s on June 1, 2007. Would you have those dates by any chance. Excellent write-up again.