Does the Sun seem far away today? Probably not, especially if you’re in a part of the U.S. that is experiencing a heat wave this week.

So, probably no one reading this post today is marking that today is the aphelion.

It is the point in the year when the Earth is at its farthest distance from the Sun. All the planets of our solar system have eccentric orbits that are more elliptical than circular.

The opposite point is the perihelion — the point in the orbit when we’re closest to the Sun which occurs in January. I know that your first thought might be that it seems like we should be cooler in July and warmer in January when we are about 5 million kilometers closer than we are at aphelion in July, but that’s not the case.

1. Planet at aphelion 2. Planet at perihelion 3. Sun

Actually, I discovered two things in researching this event. First off, on the astronomy side, the term apsis is the term actually used to describe the point of greatest or least distance of a body from one of the foci of its elliptical orbit. The point of closest approach is generally called the periapsis or pericentre. A straight line drawn through the periapsis and apoapsis is the line of apsides.

There are some derivative terms are are somewhat more common to our conversations. Terms that are used to to identify the body being orbited include perigee and apogee referring to orbits around the Earth, and perihelion and aphelion refer to orbits around the Sun. A more recent term was used during the NASA Apollo program, when the terms pericynthion and apocynthion (referencing Cynthia, an alternate name for the Greek moon goddess Artemis) were used when referring to the Moon.

The second thing I discovered in doing a search on one of these terms in Bing was that the top result was not Wikipedia (as I expected), but it was the Simple English Wikipedia at http://simple.wikipedia.org. Even though people often bad mouth Wikipedia as a poor place to do research (I disagree), apparently the information there is too complex for some users.

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