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We’re all more enlightened today

The Sun by NASA's SoHo


Do you feel closer to the light today? Dare we say a bit more enlightened? You should! Because late last night or early this morning US time was perihelion, earth’s closest annual approach to the sun:

Bad Astronomy — At 04:37 UTC today (11:37 p.m. Eastern US time last night), the Earth reached perihelion, the point in its orbit when it’s closest to the Sun for the year. At that time, the center of the Earth was 147,098,161 kilometers from the center of the Sun.

To hell with those commie metric units, you know that 93 million mile distance between the earth and the sun you long ago memorized? It’s more like 91 million today. Interestingly, the exact time and date varies not by the year but by the lunar month. Can you figure out why?

Calculating the exact time of perihelion turns out to be a bit complicated; it’s different every year (last year it was on January 5). I was surprised initially to find out the biggest effect changing the time of perihelion is from the Moon! Upon reflection it makes sense, though. As the Moon orbits the Earth in a big circle 770,000 km (475,000 miles) across, the Earth makes a smaller circle too. 

A couple of million miles might not sound like much compared to 93 million. And it’s not enough that you could tell by looking at the apparent size of the sun on perihelion vs the farthest point, called aphelion. But over time that change in sunlight could make a huge difference. In fact, indirectly, it has made a huge difference.

The average amount of solar energy reaching the earth, called insolation, is about 1366 watts/square meter. But that’s based on our average distance. If the earth is 1% closer or 1% farther away that number varies by maybe a dozen or more watts either way. You wouldn’t think that would amount to much and over the course of a year you’d be right. But over the course of centuries it can matter, a lot.

The point where the earth reaches perihelion changes over time, the earth’s orbit itself, the points of the ellipse, rotate and the same is true for other planets. In our case it takes over a 100,000 years for the orbit to rotate once. In addition the earth precesses on its axis once per 26,000 years and the angle of its axis varies a couple of degrees every 41,000 years. It gets even more complicated! Now add in that the nearest planets, Mars and Venus, not to mention mighty Jupiter, also have their various cycles, and they can perturb the earth a little bit depending where they fall into our cycles. These things can all add up and affect the earth’s perihelion and aphelion, plus they change what pole is tilted to the sun and by how much when closest and farthest approaches happen.

Now add in that these days one pole is covered in water and the other by land, with different seasonal melting, reflection, and solar absorption factors, and viola, we have the Milankovitch Cycle! Also known as the cause of recent periodic glaciations, the Ice Age phenomena that dominated human development from the time we hung out in trees to the era of hanging out in caffeinated wi-fi hotpsots.

It’s a sobering conclusion: just a dozen so watts out of 1366 add up to ice ages or warm periods. Our climate is hair-trigger sensitive to tiny changes, provided those changes accumulate in the same direction over many years. A dozen watts, more or less, can mean a different planet, with different sea levels and coastlines. Given millennia to work with, a dozen watts can drive the evolution of plants and animals in unexpected and beautiful ways!

There’s another lesson here: the same hyper sensitivity holds true if we absorb or reflect a few more watts of that sunlight regardless of why the absorption or reflection is changing. Thanks to greenhouse gases liberated from the ground into the atmosphere by human activity, right now we are definitely absorbing a few more and that absorption is increasing.

Remember that the next time your crazy Uncle JoeBob, proud Teaparty member, goes off on an ignorant tirade about how puny lil ole humanity cannot change the climate. Or better yet, try to get out and enjoy your local life giving star this week, even if perihelion happens to occur in the dead of winter in the northern hemisphere. It’s way better for your blood pressure.


  1. sheila says

    I used to write telescope control software for the Royal Greenwich Observatory. Stuff like Milankovitch cycles matter when your telescope is on a moving planet and is supposed to point to an accuracy of 3 arc seconds. (Quick recap for those who want it: 360 degrees in a circle, 60 minutes in a degree and 60 seconds in a minute. So there are 1,296,000 arc seconds in a circle.)

  2. says

    If it is the hottest day of the year, why does it feel cold around here?

    (I grew up down under, where perihelion does happen at the hottest time of the year)

  3. left0ver1under says

    The pro-ignorance crowd can’t grasp how a 1% difference can cause drastic changes.

    And yet, some of those same people could probably explain why a gun can’t fire if it’s a few microns off, or why a cylinder in a NASCAR engine needs to be bored to within a thousandth of an inch.

  4. StevoR, fallible human being says

    @2. Neil Rickert :

    If it is the hottest day of the year, why does it feel cold around here?

    Because you’ve got the air conditioner working flat out, have your feet in a bucket of ice and are drinking a coldie or three (beer) maybe? ;-)

  5. StevoR, fallible human being says

    PS. Incidentally just heard on the radio that today is Adelaide’s fourth hottest day on record – temperature now 44.5* and all time record hottest here is 45.7* Celsius

    * 112.10 F & 114.26 F respectively according to same converter as used in #4 here.


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