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The Sciences

# At the bottom of Earth's orbit

[Update: My apologies: due to a cut-and-paste error, I had mistakenly listed the perihelion distance as the average distance of the Earth to the Sun (147 versus 149 million km). To avoid confusion, I simply replaced the error with the correct value. The rest of the post is correct since this wasn't a math error but a typographical one, and I used the right value when doing my calculations below.] Since last July, the Earth has been falling ever closer to the Sun. Every moment since then, our planet has edged closer to the nearest star in the Universe, approaching it at over 1100 kilometers per hour, 27,500 km/day, 800,000 km every month. But don't panic! We do this every year. And that part of it ends today anyway. The Earth's orbit around the Sun is not a perfect circle. It's actually an ellipse, so sometimes we're closer to the Sun, and sometimes farther away. Various factors change the exact date and time every year -- you can get the numbers at the Naval Observatory site -- but aphelion (when we're farthest from the Sun) happens in July, and perihelion (when we're closest) in January. And we're at perihelion now! Today, January 3, 2011, around 19:00 GMT (2:00 p.m. Eastern US time), the Earth reaches perihelion. At that time, we'll be about 147,099,587 kilometers (91,245,873 miles) from the Sun. To give you an idea of how far that is, a jet traveling at a cruising speed of 800 km/hr would take over 20 years to reach the Sun. Of course, since today is when we're closest to the Sun this year, every day for the next six months after we'll be a bit farther away. That reaches its peak when we're at aphelion this year on July 4th, when we'll be 152,096,155 km (94,507,988 miles) from the Sun. Not that you'd notice without a telescope, but that means the Sun is slightly bigger in the sky today than it is in July. The difference is only about 3%, which would take a telescope to notice. Frequent BA Blog astrophotograph contributor Anthony Ayiomamitis took these images of the Sun at perihelion and aphelion in 2005:

^* Relative to the stars it's moving faster, that is, since if you were to measure its speed across the sky as it rises and sets, the Sun would actually appear to be moving more slowly, because as you stand on the Earth its spin moves the Sun left to right relative to you (if you're in the northern hemisphere facing south, or standing on your head in Australia facing north) making a single circuit across the sky once per day, while the Earth's orbital motion moves the Sun right to left relative to the stars making a circuit once per year, with that motion fastest at perihelion, therefore subtracting from or slowing the diurnal (daily motion) of the Sun left to right, so the Sun appears to move in the sky most slowly at perihelion. Got it?

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