Aphelion

The earth’s orbit around the sun is nearly circular. It’s actually elliptical, but a circle is just a special case of the ellipse; we say that a circle has an eccentricity of zero. The eccentricity of an ellipse is greater than zero but less than one. The orbital path of the earth has an eccentricity of about 0.0167 which is pretty small and means the path is damn close to a circle. Halley’s Comet has an eccentricity 0.9671, that’s highly elliptical.

Another way to think about it is that the mean or average distance between the earth and the sun is about 93 million miles (150 million km). At aphelion, the furthest point in the orbit, the distance is about 94.5 million miles (152 million km). At perihelion, the closest point, that distance is about 91.4 million miles (147 million km). Now a few million kilometers might seem like a lot, and it is, but it’s not a lot compared to the total distance, only about one-and-a-half percent.

Aphelion is today, July 3rd. Perihelion will be on January 3rd, 2018. So we are furthest from the sun during our (northern hemisphere) summer and closest in our winter. It’s just the opposite ‘down under’ as they are experiencing winter right now. The seasons are due to the earth’s axial tilt of 23.5 degrees. At this time of year the earth’s northern hemisphere is leaning towards the sun and so we have summer. At the opposite end of the orbit, the perihelion in January, the northern hemisphere will be leaning away from the sun and thus we’ll have winter.

Interestingly the earth is moving most slowly in its path at aphelion and so summer in the north is longer by about five days than winter in the south. Which means our winter is about five days shorter than winter in the south. I don’t like that. I think it should be flipped around. I like winter way better than summer and I don’t like having to endure five extra days of hellish heat. And I feel cheated out of five days of blissful cold. But since I’m not moving to Chile or New Zealand I’ll just have to put up with it.

The idea that celestial bodies had elliptical orbits is due to Kepler. Ancient peoples knew the sun’s movement along its path through the sky (called the ecliptic) was not uniform. Lots of schemes were created to explain why this was so but nothing stuck until Kepler. Newton confirmed this work later by showing elliptical orbits to be a consequence of his own law of gravitation.

Standing on the equator the movement of the earth’s surface relative to its center is about 1000 miles per hour. In its nearly-circular orbit around the sun the earth is racing along at about 30 kilometers per second or 67,000 miles per hour. That’s quite fast—humankind’s fastest spacecraft (Helios 2) hit 70 km/sec or 157,000 mph. Escape velocity for an earth-launched vehicle is about 11 km/sec or 25,000 mph.

The next time you think you aren’t going anywhere in life just sit back and enjoy the ride that comes for free. It’s a pretty good one.

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