Aphelion
Published on Jun 30, 2006 at 4:41 pm.
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Filed under Earth.
Earth’s orbit is not perfectly circular. Rather, it is slightly elliptical. July 3, at about 23:00UT (That will be about 8pm, Central Daylight Time here in Texas), Earth will be the farthest it gets from the Sun all year: 152,095,700 kilometers. The point in a planet’s orbit where it is farthest from the Sun we call aphelion. So, with the Sun farther away from us, that would make it cooler, right? Anyone who has ever been outdoors here in Texas can attest to the fact that NO, the distance from the Sun does not determine the seasons. Instead, it is the tilt of the Earth as it orbits the Sun that causes the seasons. Earth was tilted most towards the Sun on June 21. We call that the Summer Solstice. Being tilted towards the Sun means that you get more intense sunlight, and that the days are longer. Combine the two effects, and it is hot. We are still heating, in fact. Even though we are now tilting more and more away from the Sun each day, the days are still longer than the nights, so the heating continues. We actually hit peak heating a month or two after the maximum tilt. That also works on a smaller scale in daytime heating. The Sun is highest in the sky at mid-day, so that is when you get the strongest sunlight and fastest heating, but it continues to heat and you reach the highest temperature sometime mid to late afternoon.
But what about this distance thing? Why doesn’t the Sun’s distance matter? Well, the Earth’s orbit is only slightly elliptical. At farthest from the Sun, we are only a bit over 3% farther than we are at our closest distance. That is not a huge effect. Now, if it were the only effect. then it might matter. However, the Earth is also tilted by about 23.5 degrees. That produces a much bigger effect than the orbital distance. Now, things are quite different on Mars. The Martian orbit is far more eccentric than Earth’s orbit. At it’s farthest distance from the Sun, Mars is close to 20% farther than it is at its closest distance. That is enough to matter. So the position around the orbit has an impact on the seasons. However, Mars also it tilted much like Earth. As it turns out, the tilt still dominates, but the orbital distance has a major factor as well. Like Earth, Mars’ northern hemisphere has summer when it is farthest from the Sun, and winter when it is closest to the Sun, and vice versa in the southern hemisphere. This has the effect of augmenting the extremes of temperature in the southern hemisphere. As you’d expect, the winters in Mars’ northern hemisphere seem less severe than the southern winters. However, the summers don’t seem to have the corresponding mildness. Northern hemisphere summers seem a bit warmer than we’d expect. The reason is that there is more dust blowing around in the northern hemisphere in the summer than in the southern hemisphere’s summer. This dust absorbs more solar energy, heating the planet more than you’d expect for it being 20% farther from the Sun. Yeah, climate is very complicated — on any world.
But, as I said, Earth’s orbital changes are much less extreme, so we can basically ignore the effect.  It is the tilt that determines things. In fact, you’d hardly be able to notice that the Sun appears smaller in the sky (again by only 3%) than it did in January. I mention January, because that was when we were closest to the Sun, what we call perihelion. That was January 4, at about 15:00UT (About 9am Central Standard Time here in Texas). At that time, we were 147,103,600 kilometers from the Sun.
So, this weekend, go out and look at the Sun (CAREFULLY!!!!!!). It will be the smallest that you’ll see in in the sky for another year.
-Astroprof
