Exactly 1 AU!

Hey, hey, everyone. Today’s kind of a nifty day; it’s Exactly 1 Astronomical Unit Day (Spring Edition)!

Oy. Lemme write some science at you.

Remember a couple of months ago, when we were talking about the Earth reaching aphelion, its furthest point from the Sun? This is part of the conversation that always comes with the changing seasons. We were all taught in school that the Earth is 93 million miles from the Sun. Since distances in space are so big it very quickly becomes impractical to talk in miles and kilometers, just as, here on the ground, going around saying “The distance from San Francisco to Albuquerque is 175 million centimeters” would get you some funny stares, especially here in the US with the metric system and all. So, we have all of these other units to talk about, for instance, the light year and the parsec. The astronomical unit (AU) is another. It represents the average Earth-Sun distance, so 92.96 million miles (149.6 million km).

The Earth’s orbit around the Sun isn’t round; it’s a slightly elongated ellipse, as Johannes Kepler told us. The Sun is one of the two foci of that ellipse, which means the Earth’s orbit isn’t quite centered around the Sun. This means there are times when they’re closer together, other times when they’re father apart, and only two points in the orbit when they’re actually at that handy grade school, 93 million mile distance. The distance between the extremes —perihelion (nearest) and aphelion (farthest) — is about three million miles, about three percent overall. Since January, when the earth was at perihelion, it’s been chugging away from the sun.

Well, average distance fans, today’s the day you’ve been waiting for! At about 3:00 US Eastern time (2 Central, 1 Mountain, noon pacific), the Earth will reach one of those two magical 1 AU points. The other is in October. The rest of the time, the Earth-Sun distance is either more or less than the Earth-Sun distance (I love saying that). From here, we’ll keep moving away until we reach aphelion, in July, right after the start of water fight season here in the northern hemisphere. Hot dog!


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