Sunday, July 10, 2016

Tonight's Sky for July 10: Venus at Perihelion

Today,Venus is as close as it will ever get to the Sun thanks to the fact that it is at perihelion, a point in its orbit that is closest to the Sun.

What many people may not realize is the fact that Venus (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 the point of an orbit farthest from the Sun, it's called aphelion. Lastly, those planetary distances taught in schools? They're the planet's average distance to the Sun.

Additionally, Venus is as fast 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.

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