Friday, November 20, 2015

Tonight's Sky for November 20: Mars at Aphelion

Today, Mars is as far as it will ever get from the Sun thanks to the fact that it is at aphelion, a point in its orbit that is farthest from Sun.

What many people may not realize is the fact that Mars (and all other celestial bodies) do not orbit their parent bodies in circles, but ellipses, which are slightly elongated circles. Result: any given day of an orbital period, any orbiting body will be at a slightly different distance from its parent body. As for Mars, this variance in orbital distance to the Sun amounts to about 26 million miles, a difference of roughly 13 million miles from its average distance from the Sun, 143 million miles.

Additionally, Mars is as slow as it will get today, too. Why is this? As a planet locked in an orbit gets farther from the Sun, the Sun's gravitational force on the planet lessens and the planet will slow down. Then, as the planet round the aphelion point and begins to move closer to the Sun, its speed will increase as the distance to the Sun decreases, culminating at maximum speed at, you guessed it, perihelion. This fact was first discovered in the early 1600s by Johannes Kepler and serves as his 2nd law of planetary motion, the law of equal areas.  

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