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SYFY WIRE Bad Astronomy

Perihelion 2016

By Phil Plait

Saturday at 22:49 UTC (17:49 Eastern U.S. time), the Earth will be at perihelion, the point in its orbit where it’s closest to the Sun. At that moment the center of the Earth will be 147,100,176 km (91,403,811 miles) from the center of the Sun. Give or take.

That’s still a long way. At highway speeds it would take more than 160 years to drive there in a car (I don’t even want to think about how you’d pave that), and almost 19 years to fly there in a typical commercial jet. I hope they have lots of those little bags of peanuts.

Why are we closer to the Sun now than other times? Well, if the Earth orbited the Sun in a circle, we’d always be the same distance from it (in one way, that’s the definition of a circle). But our orbit is ever so slightly elliptical, so sometimes we’re closer to the Sun, sometimes farther.

And I do mean “ever so slightly.” Here is a diagram I made comparing Earth’s orbit to a circle. Can you tell which is which?

Drawings of a circle and an ellipse representing Earth's orbit. Can you tell which is which?

I originally posted that in 2013, and to be honest I had to check that article again to make sure I knew I had it correct: The Earth’s orbit is the one on the right. It’s very close to a circle; the distortion is only a couple of percent!

When you look at it this way, it dispels the misconception that the seasons are caused by Earth’s changing distance from the Sun. If you do the math (and I have), that difference can only add up to a change in temperature of a couple of degrees. Where I live, the seasonal temperature swing can be as much as 50° Celsius (110° Fahrenheit). So there you go.

What does cause the Earth to have different seasons then? Its axial tilt! Go read that for gory details, which are fun and cool and face it you’re a science nerd and already clicked that link, haven’t you?

Anyway, here we are, at the bottom of our orbit, gravitationally speaking. From here on out, every moment in the year until July, that distance from the Sun will increase. On July 4, at 16:24 UTC, the Earth will be at the top of its orbit, as far from the Sun as it can be: aphelion. At that moment its center will be about 152,103,776 km from the Sun (94,512,905 miles), and then the Earth will slowly start to approach the Sun once again.

It’s funny that perihelion happens so close in time to the December/winter solstice, and every year I do get a few people asking me about that. It’s a coincidence! The Earth’s north polar axis is tipped most directly away the Sun at about Dec. 21 every year, which is usually only a couple of weeks before perihelion.* But the Earth’s axis isn’t fixed in space; it wobbles like a top. Called precession, the axis makes one full rotation every 26,000 years or so.

This gets complicated fast, but basically, because of precession and other factors, the date on which the Earth reaches perihelion regresses, moves later in the calendar over the years. On average that regression is one day every 58 years. Right now, perihelion is usually around Jan. 2-4, but it’s getting later all the time, moving away from the December solstice. Eight hundred or so years ago it happened at the same time as the December (northern winter) solstice, and in roughly 11,000 years it'll happen around the time of the June (northern summer) solstice.**

So perihelion can happen at any time relative to the seasons, and we happen to be at a point in history when it’s a couple of weeks off of the December solstice. Coincidence!

If you want to learn more about perihelion, I’ve written about it a few times, like in 2015, 2014, 2013, 2012 (which is also when I started doing a daily BAFact, in fact), 2011, 2010, and 2009. Say.

So happy perihelion! It’s all uphill from here.

*Correction, Jan. 2, 2015, at 15:20 UTC: I originally wrote that the Earth's North Pole is tipped toward the Sun on Dec. 21. It's tipped away on that date, and tipped toward the Sun in June.

**Correction, Jan. 2, 2015, at 20:30 UTC: Sigh. When I read the USNO page about this, I assumed the word "regress" meant the date of perihelion move backward in the calender, so perihelion was getting earlier every year. However, I was wrong; in this case it means perihelion is moving later. Figuring this stuff out can be pretty confusing, so my apologies if I made it worse for you!

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