Earth Slows Down 2,250 Mph To Enjoy The Summer

The hot weather in our part of the world of late has been good for firefly watching. Their numbers grow as the nights warm. This picture was taken Sunday night. Photo: Bob King

If anything, the sun felt 3.2 million miles closer yesterday. With temperatures in the upper 80s and high humidity, it was one hot day here in Duluth. I know, I know. Southerners must laugh at what we consider summer heat. But if you’re looking for the reason why summer is hotter than winter, it has all to do with the sun’s steep path across the sky and not Earth’s changing distance from the sun over the year.

The sun's gravity keeps Earth in orbit, but its path, like all the other planets, is a gently squashed circle called an ellipse with the sun off slightly to one side. This causes our distance from the sun and orbital speed to vary during the year. When closest, we feel the 'tug' of the sun more strongly and move faster. Illustration: Bob King

Because we orbit the sun in an elliptical rather than circular orbit, our distance and speed subtly changes day to day. When Earth is closest to the sun around January 4 every year, we’re 91.3 million miles away and moving fastest at 67,777 mph. Yesterday Earth reached its farthest point from the sun at 94.5 million miles with the speedometer barely pushing 65,520 mph. That’s a difference of over 2,000 mph. Could that be why the day seemed to drag on forever?

Sunlight falling on Earth is about 7% more intense in January than in July, but it’s winter then in the northern hemisphere, so we really can’t feel the slight difference it might make. You might wonder if Australians feel the extra heat, since it’s summer for them. As it turns out, they don’t, because of the moderating effect of the oceans that dominate the southern half of the globe.

Annular eclipse of October 3, 2005. Credit: Sancho Panza/Wiki

Closest approach to the sun is called perihelion, while yesterday Earth was at aphelion (AP-hee-lee-un).  Similarly, when the moon is closest to Earth in its elliptical orbit, we say it’s at perigee. Farthest point is apogee. Perigee happens this Thursday July 7; apogee on the 21st with distances of 229,637 miles vs. 251, 255 miles.

While these terms and numbers might sound obscure, they make a big difference when it comes to solar eclipses. When the moon is farthest from Earth, it appears slightly smaller than normal. If apogee happens around the time of perihelion (sun closer and bigger), the moon’s disk isn’t big enough to completely cover the sun’s. You can picture what happens – a ring of unblocked sunlight surrounds the moon in what astronomer’s call an annular or ring eclipse. The next one visible in the U.S. will occur on May 20, 2012 for those living in the desert Southwest.

Just a final note. Solar activity ticked up in the past few days with a favorably positioned coronal hole and the expected arrival of a gusty cloud of electrons and protons from a recent CME (coronal mass ejection). Observers across southern Canada and the U.S. should watch for possible auroras tomorrow through Thursday nights. Keep an eye open as well for the curious, blue-tinted noctilucent clouds visible during late twilight as the stars come out in the northern sky. High summer is the best time to see them.

2 Responses

  1. thomas s

    hi bob, scrolled thru the comments re comet elenin, brown dwarfs and all that. you are to be congratulated for your patience and tact in dealing with such a verbal deluge. incidentally, much of it reminds me of the speculation that accompanied the “visit” of Halley’s comet in 1910, to say nothing about 1066.

    1. astrobob

      Thanks Tom, I try but it can be frustrating at times. Despite my best efforts, I don’t I’m going to change the minds of those who view Comet Elenin, etc. as signs of the apocalypse. Going forward, I will be monitoring the Comments area more closely so it doesn’t become a dumping ground for doomsday propaganda. Readers also won’t have to hear me repeat myself so many times either!

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