Earth will reach its farthest point from the Sun for the entire year around midday tomorrow. We’ll be about 3.1 million miles farther than we were at closet approach, in early January.

That far point is known as aphelion or ap-helion. The term comes from a mash-up of Latin and Greek. “Ap” is from a Latin word that means “far away,” while “helion” is a Greek word for the Sun.

The changing distance is a result of the shape of Earth’s orbit. Instead of a nice, round circle, it’s an ellipse – like a lopsided circle.

Over tens of thousands of years, the shape changes – the result of the gravitational influence of the Sun, Moon, and planets. The orbit seesaws between being a little more circular and a little more lopsided. Right now, we’re toward the more-circular end of the seesaw.

Today, aphelion always comes about two weeks after the summer solstice. But that won’t always be the case. Thanks to a slow wobble in Earth’s rotation, aphelion shifts an average of one day later every 58 years. So it occurred on the solstice about 900 years ago. And about 4600 years from now, it’ll occur on the fall equinox, in September.

Incidentally, the lopsided orbit has a big impact on the length of the seasons. Our planet moves slowest when it’s farthest from the Sun. That makes summer in the northern hemisphere almost five days longer than winter.

Script by Damond Benningfield