Closer to the bonfire

Earth will tonight be at its nearest to the Sun, can you believe it!

Yes, it will reach the perihelion point of its slightly elliptical orbit on January 5 at 7 Universal Time, which is 7 AM in Britain, 2 AM in Eastern Standard Time, and on the Pacific coast 11 PM in Jan. 4.

The minimum distance from the Sun is, this year (it varies slightly from year to year), 0.98328 of the average distance, or “astronomical unit.” That works out to mean that Earth is nearer in than average by about 2,500,000 kilometers. A seemingly enormous distance, 196 times the width of the Earth itself! Here is a sketch: two Earths, one of them a fictitious Earth traveling on a perfectly circular orbit, the other the real Earth traveling the perihelic part of its elliptical orbit. The Earths are 1 millimeter wide, and the distance between them is 196 millimeters.


And there is a dot (which should really be smaller) 6.5 millimeters from the Earth and almost in the anti-sunward direction. That’s the Moon, which will be Full a day later, on Jan. 5 at 5 Universal Time.

So the Earth is now nearer than average to the Sun by the seemingly enormous distance of 196 times its own width. So why don’t we feel hotter? Because this difference is a tiny fraction of the much huger (149,600,000 kilometer) distance to the Sun.

The blue circle is Earth’s average orbit, the black ellipse is its actual orbit.  The dot for the Sun is true to scale; the dot for Earth is exaggerated 100 times in size!

Perihelion happens to occur near the middle of north-hemisphere winter and the middle of south-hemisphere summer, and it has essentially no effect on either. Imagine two people standing at distances of sixty yards and sixty-one yards from a bonfire. It’s not the extra yard that makes one of them warmer than the other.

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