Jul. 01, 2011

Science Dad on Aphelion and Perihelion

by Vince Harriman

Click to enlarge images

Positions of the Earth

Happy 4th of July weekend to everyone! Since our country achieved independence at the height of summer and we celebrate with fireworks and ice cream, we often say that no day is hotter than the 4th of July. But why is it so hot? We spent some time at the beach this week, with many days of perfect blue skies and high temperatures. Every morning we all, including Rowan, put on liberal doses of sunscreen to keep the power of the sun at bay. It sure feels like the sun is closer some days than others, and indeed the earth has an elliptical orbit. But here's a question I put to Beckett -- when is the Earth closest to the sun?

Since just last week we paused to admire the solstice and the longest day of the year, Beckett's first guess was the solstice. Many people guess that the Earth is closest to the sun on solstice and just as many guess that its closest some time in July or August, when the sun feels like it is just above the clouds and baking everything. This year, the earth will be farthest from the sun -- a situation called aphelion -- on July 4. At 3:00 pm Universal Time, the earth will be 152,097,700 kilometers (94,509,130 miles) from the sun. Perihelion -- when the Earth is closest to the sun -- won't be until January 4, 2012 at which point the distance will shrink to 147,098,070 kilometers (about 91,402,500 miles) a difference of 5 million kilometers or 3 million miles!

The real reason it is so hot this weekend!

So why is it so hot now and so cold in January? Our very first Science Dad project was about Daylight Saving Time and Beckett showed how the tilt of the earth on its axis has more to do with the length of the day and the weather than distance from the sun. This picture also shows why we have things like equinoxes and solstices -- the earth spins regularly on its axis as it travels around the sun. Even though the difference between aphelion and perihelion is pretty large -- 3 million miles or about 3.3% of the total distance, the effect that the change in distance has on the Earth's weather is small.

View from Wallops, courtesy of NASA

The other exciting thing this week here on the East Coast was the launch Wednesday night of the Minotaur rocket from the Wallops Flight Facility just across (and down a bit) the Chesapeake Bay from us. Despite several delays that took the launch from prime time viewing for Beckett to long after his bed time, the launch finally went off just after 11:00 pm. The sky was patchy with clouds so even though I yanked Beckett out of bed, all we really got here was a brief flash of red. The International Space Station had just passed by in the other direction less than an hour before, and coming from the Northwest there were fewer clouds and much greater visibility.

All this space activity makes me want to start training to be an astronaut! I may be a couple of months past eligibility, but I can't imagine the possibilities for the next generation. Hand Rowan a smart phone and watch him navigate it, while I still feel that my iPhone is alien technology. Already space flight and launches are so routine that they no longer make the 'news'. Who knows what will be considered news when Beckett and Rowan are my age?

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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