Wednesday, July 30, 2014

Tonight's Sky for July 30: Mercury at Perihelion


Tonight, the planet
Mercury, first rock from the Sun, is at perihelion, a point in its orbit that is closest to the Sun.


What many people may not realize is the fact that the Mercury (and all other celestial bodies) do not orbit their parent bodies in circles, but ellipses, which are slightly elongated circles with the Sun slightly offset from the center. Result: any given day of an orbital period, any orbiting body will be at a slightly different distance from its parent body. As for Mercury, it is about 28.5 million miles from the Sun right now as opposed to its average distance of about 36 million miles, meaning that its orbit varies by about 7.5 million miles either side of the average, meaning that Mercury's orbit takes it out to about 43.5 million miles at aphelion (farthest point from the Sun).

For the record, Mercury has the greatest eccentricity (deviation from a circle) of any of the 8 planets relative to its orbital size. 

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