Day 365: We're on our way back out again
(Phil Plait is an Astronomer and Author who writes the Bad Astronomy blog for Discover Magazine. His blog is dedicated to clearing up public misconceptions about astronomy and space science in the media. After working for ten years with the Hubble Space Telescope team at NASA Goddard Space Flight Center he went on to work on astronomy education. Phil has appeared on numerous TV and online astronomy features. Here Phil celebrates Perihelion which is just a few days away, and the point when 23 Degrees started it's project a year ago. Phil can be reached @badastronomer)
Distance travelled ~ 938'107'200 km
The Earth travels 'round the Sun at the terrifying speed of 30 kilometers per second, making a gigantic circle nearly a billion kilometers in circumference once per year.
That's a fantastic distance, but in one way it's a bit disappointing. After all, a year after you started this high-velocity trip, you're back where you started.
However, the journey isn't actually a perfect circle. It's off by a tiny amount, so slight that you'd never notice if someone didn't tell you. But in fact, this minor deviation can make a big difference: it means that the Earth's distance from the Sun changes over its orbit to the tune of over 5 million kilometers!
That difference amounts to about 3.4% of the average distance to the Sun, which astronomers, for convenience, call an Astronomical Unit, or AU. In real terms, an AU is 149,597,870 km plus a bit. When the Earth is at aphelion - its farthest point from the Sun - it's about 152,141,000 kilometers from our star. At perihelion, that distance shrinks to 147,055,000 kilometers.
Aphelion occurs on one side of the Earth's orbit, and perihelion on the other. It takes the Earth half a year to travel that distance, of course, so in a sense it drops toward the Sun by 5 million kilometers in about 182.5 days - an average velocity of well over 1100 km/hr.
And my teachers told me I'd never go anywhere.
You might expect this change in distance to the Sun would have an effect on the Earth's temperature. It does, in fact, but it's pretty small, only a couple of degrees Celsius. That's swamped by the seasonal change in temperature due to the Earth's tilt, which is what really drives the seasons. Think of it this way: the Earth reaches perihelion in January, which is the dead of winter for the northern hemisphere. While this does make northern winters a tad more clement, the amount is too small to make an appreciable difference.
Oddly, for those in the antipodes of the southern hemisphere, we're closest to the Sun in their summer. You'd expect temperatures then to be higher than average, but in reality they're about the same as their boreal neighbors. Why is that? It's because the southern hemisphere is dominated by the Pacific Ocean, and water is an excellent heat sink. It absorbs the extra heat in the summer, and releases it in the winter, mitigating temperature extremes.
Weather, it turns out, is complicated.
There is one measurable effect, though: the size of the Sun changes by that same 3.4% over the course of half a year. You'd never suspect that by eye - of course, looking at the Sun is not a suggested pastime - but if you carefully take pictures of it over the course of the year you'll see it, as astronomer Anthony Ayiomamitis did in the picture here. Since the change is slow day-by-day it's beneath our notice, but the telescope doesn't lie.
Credit: Anthony Ayiomamitis
So when is the next perihelion? Why, it's January 5, 2012, and it will occur at about 01:00 GMT. At that moment, or thereabouts, the Earth will again be as close to the Sun as it will get all through 2012. The 23 Degrees project started on the day of Perihelion for 2011 and we're already on our way back out, and on July 5, at 04:00 GMT, the Earth will reach the apex of its orbit, and oh so slowly start the fall back toward the Sun.
And it will do this, over and over for the next few billion years, until the Sun swells into a red giant and cooks the Earth like a banger in a blowtorch.
So enjoy the ride while you can.