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Day 365: We're on our way back out again

Phil Plait Phil Plait | 15:00 UK time, Saturday, 31 December 2011

(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.

earth orbit


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.

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.


  • Comment number 1.

    About the relation between the sun and the earth .. I think that the influence which the influence of solar does not stop at the earth, but it has on the surroundings is great.
    It is a wonderful thing that a magnet is newly generable with the relation between to have magnetism or electric power with electricity semipermanently like an egg and a chicken about a sunspot.
    When that the sunspot of this sun has magnetism, magnetism and a magnetic field being located in north and the south also in the earth, and thunder fell on the ground, it knew that a magnetic field would occur, and a certain idea thought of me.
    Although it is related also to rotation of the earth, although called the chronic law, a magnetic field occurs by thunderbolt, and it considers rotation of the earth that rotation of the earth will continue in response to the influence of a solar magnetic field.
    Therefore, rotation of the earth changes.
    moreover, human beings are going to control nature, and it is going to control climate, and danger is also entailed to it -- I think that it is.
    I think that all the phenomenon and phenomena will have the formula of maintaining the optimal balance in the time.
    The magnetic field of the earth is subject to the influence of the magnetic field possibly also according [ that that it was warming in recent years has cooled ] to the quantity and the position of a sunspot, Or [ when air is mixed by hurricane, a typhoon, a tornado, etc., electric power is raised and inclination and rotation of the earth change under the influence of the magnetic field by it / it causes cooling ] ..
    Has not the food chain occurred on a scale of the universe?
    I say since it cooled and think that human beings should not neglect nature.
    by the way -- Supposing this magnetism is controllable (is my idea dismissed if it cannot do?) ... electromagnetism -- I use power and think that a warp in the universe is attained.
    that is, -- facing to the magnet of a sunspot in space -- temporary -- electromagnetism -- it moves using power.
    magnetism is stopped in a favorite distance (electromagnetism -- since what is necessary is just to stop electricity if it is power, it is possible!?).
    Although based on the strength of magnetism, is space unmovable at speed early possibly?
    And I want you to investigate the moon first.
    If the crack of the back side of the moon or the surface of the moon was investigated and something collided in the past, it will be what came from which direction.. dinosaurs may also find the key of why to have been ruined.

  • Comment number 2.

    The planets do not travel "'round the Sun". They travel in multiple perpetual forward wave-like motions with wave amplitudes greater than that of the Sun and greater than that of the galaxy. They never retrace their steps. Nor, after a year, are you "back where you started". You are, in relation to the Milky Way galaxy, some 5,000,000,000 miles past where you started. In relation to the rest of the Universe, you are probably some further unimaginable distance past where you started. I realize that doing calculations for planetary orbits using the assumption that the Sun is a fixed unmoving object makes the calculations and visualization much easier, but they do not reflect reality. When are astronomers and physicists going to take the bull by the horns and try to analyze their data and build their theories using realistic assumptions? Even if the calculations become so much more complex?



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