Last week it was hot. Really hot, especially in the eastern part of the country. Here at the Farmers’ Almanac, we kept hearing people say “why is it so hot?” Many of you may be wondering what the catalyst is for such extremely hot temperatures. One reason we’ve heard people discuss is the location of the Earth in relation to the Sun. After all this is summer, and we are probably closest to the Sun, right?
Wrong. It’s actually quite the opposite. On Tuesday, July 6, 2010, at 7:00 a.m. EDT, the Earth was at aphelion, its farthest point from the Sun for this year.
Conversely, back on January 2, we were physically closest to the Sun, at perihelion. The difference between the closest and farthest points is equal to 3,105,866 miles, which sounds like a lot, but amounts to an overall change of only 3.3-percent.
In fact, this relatively small difference in the Earth’s distance from the Sun has very little effect on what astronomers refer to as our “insolation,” (the amount of sunlight we receive per unit of surface area).
The reason for the temperature difference lies in the tilt of the axis of the Earth itself, not the proximity of the Sun. In the summer, the Northern Hemisphere is tipped toward the Sun so that its light falls more directly and is more concentrated on each unit of the surface. This, along with the fact that the Sun shines for a longer time each day, makes summer a warmer season than winter.
Incidentally, in about 13,000 years, we northerners will tilt toward the Sun at the same time we’re closest to it. I guess things will really heat up by then!
Saturday, July 3rd, also marked the beginning of the “Dog Days of Summer,” which also closely coincided with the start of the searing heat for the Eastern U.S. Perhaps this has something to do with the recent siege of hot weather?
Everyone talks about “dog days” but few know what the expression means. Some say that it signifies hot sultry days “not fit for a dog.” Others say it’s the weather in which dogs go mad. But the dog days are defined as the period from July 3 through August 11 when the Dog Star, Sirius, rises in conjunction (or nearly so) with the Sun.
As a result, some people felt that the combination of the brightest luminary of the day (the Sun) and the brightest star of night (Sirius) was responsible for the extreme heat that is experienced during the middle of summer. Other effects, according to the ancients, were droughts, plagues, and madness.
Sirius, being so near to the Sun in our current sky, is invisible now, but beginning around August 11, just before sunrise, it might again be glimpsed rising just above the southeast horizon.
In ancient Egypt, the new year began in August with the return of Sirius. Its annual reappearance was a warning to people who lived along the Nile River. The star always returned just before the river rose, and so announced the coming of floodwater. People then opened the gates of canals that irrigated their fields.
We know today, of course, that Sirius has nothing to do with hot temperatures or floods, but after all of these years the term “Dog Days” just stuck!