EARTH at Aphelion


At 01:41 a.m. today (in local New Delhi time), the Earth reached Aphelion, the point in its orbit when it is farthest to the Sun for the whole year. The Earth is farthest from the Sun at its Aphelion – about 2 weeks after June Solstice and nearest to the sun at its Perihelion – about 2 weeks after December Solstice. At that time the center of Earth was 152 million km from the center of Sun.

You probably didn’t notice. There were no fireworks, no ball dropping from a spire, no breathless celebrities celebrating it. But it happened just the same, as it did last year, and the year before that, and all the years stretching back to the dawn of our planet 4.5 billion years ago.

On average, Earth lies 149 million km away from the Sun, a distance that has been designated 1 astronomical unit (AU). But the actual separation between the duo varies with time, because Earth orbits the Sun in an ellipse not a circle. This fact wasn’t discovered until the early 1600s, when astronomer Johannes Kepler published the first two of his three laws of planetary motion. Until that time, everyone thought the planets orbited the Sun moved along perfectly circular paths. That’s why the planet goes through aphelion in every July every year and its counterpart, perihelion, in early January. The disparity between aphelion and perihelion is slight, about 5million km. It’s a change of only about 3%, which to the eye would make it look pretty much like a perfect circle.

Some people think that the distance to the Sun is the reason we
have seasons. But as you can see, this difference is so small it hardly has any affect at all. Although we’re a little bit cooler on average when we’re farther away, but the tilt of the Earth’s axis is a far bigger influence on temperature than our distance from the Sun. Note that we reach perihelion in January, in the dead of winter for the Northern Hemisphere! That’s the opposite of what you would expect if distance to the Sun alone were the cause of the seasons.

Well, if you very carefully measured the size of the Sun, it would appear a bit smaller today than any other day of the year. The difference is pretty small as shown in the picture below:

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That’s the Sun at perihelion (left) aphelion (right), and clearly it’s not a huge deal. You’d never notice. But it also means that every day, from here on out until we reach perihelion, we’ll be heading downhill in our orbit. Then, on January 4th 2018 we’ll reach perihelion, and start to head downhill once again.

A lot of people like to make New Year’s resolutions, some way to improve their lives starting with the beginning of the year. Have you? If you did, aphelion makes a good benchmark for a checkup. Between now and then the Earth will have moved 468 million km around the Sun, and will be 5 million km further from it. What will you accomplish in that same time?

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