Aphelion, 2009
Published on Jul 4, 2009 at 1:17 pm.
1 Comment.
Filed under astronomy, physics.
Last night, Earth reached the farthest point in its orbit from the Sun. This distance is called the aphelion of Earth’s orbit. The exact distance varies a bit from year to year due to perturbations in Earth’s orbit from the other planets, but it doesn’t vary by very much. We were just under 152,100,000 kilometers from the Sun at about 02:00 UT. That was about 9pm last night, according to the clock here in Texas. This is in the heat of summer for us in Texas. It is hot today. The high the last few days had been about 103° F (39.4° C). Factoring in the humidity, and it is pretty uncomfortable out.
In school, most people hear that the Earth’s orbit is a bit elliptical. Our educational system does a pretty good job of explaining that. However, for whatever reason, people tend not to learn why we have seasons. When I ask my students why we have seasons, a great many seem to feel that we are hot in the summer because we are closer to the Sun then. That isn’t so for us in the Northern Hemisphere. In fact, we were closest to the Sun this year on January 4, when we were just under 147, 100,000 kilometers from the Sun. We call the closest distance to the Sun perihelion. That was when we were near our coldest around here! You can then show the students that the tilt of the Earth is why we have seasons. The Earth’s axis tilts about 23.5° from the axis of its orbit. During part of the year, one hemisphere is tilted more towards the Sun than the other. That hemisphere gets more direct sunlight, so it is warmer. All too often, though, students don’t completely give up their misconceptions about why we have seasons. They suppose that the tilt brings one hemisphere closer to the Sun than the other. While the tilt does make one hemisphere slightly closer to the Sun than the other, on average, the effect is rather small. Most people live in the temperate zones. Putting a few numbers into a back-of-the-envelope type calculation, that means that the tilt of the Earth has no more than about 1500 kilometers of motion for most people. While that sounds like a lot compared with the size of many familiar distances, it is vary tiny compared with the 2,500,000 back and forth from the average that the Earth’s orbital motion makes. So, it is not the distance that makes for the seasons, but rather the solar illumination. The more direct and the longer the duration of sunlight, the warmer that it gets.
But, wouldn’t the difference in distance between Earth and the Sun in January and July have any effect? Shouldn’t that make the Southern Hemisphere’s summer hotter and winter colder? It would seem that way at first glance. But, let’s put some numbers into the calculator and see what happens. When you look at the math, Earth’s orbit is actually pretty circular. The aphelion distance is, in fact, farther from the Sun. But, at Earth’s distance, that is only about 3.4% farther at apehelion than at perihelion. But light intensity from the Sun drops off as the distance squared, so that extra distance does result in a bit less solar power reaching Earth, about 6.5% less, when you do more back-of-the-envelope math. So, it looks like it really should be warmer in the southern summer and colder in the southern winter compared with those of us in the northern hemisphere. However, things are a bit more complicated than that.
One of the most important differences in the northern and southern hemispheres is the distribution of land mass. Earth’s continents and oceans are not equally distributed around the planet. The northern hemisphere has a disproportionate percentage of land compared with the southern hemisphere. Likewise, the southern hemisphere has a disproportionate percentage of ocean. This is particularly apparent at the mid temperate latitudes, where the effect of the seasons is most pronounced. So, how does this have any effect on things?
Water tends to reflect more light than land. In fact, when light strikes water at an angle of about 53° or more from perpendicular, then at least half of the light is reflected. Even when light strikes the surface at less than this angle from perpendicular some light is reflected. Smooth water can act almost like a mirror. Now, some light is also reflected from land, but normally not nearly so much. That means that more sunlight can be absorbed in the Northern Hemisphere than in the Southern Hemisphere. That warms causes the temperature of the ground to rise. Furthermore, water has a very high specific heat — much higher than the minerals that make up rocks. The temperature increase due to a certain amount of heat energy absorbed is inversely proportional to the specific heat of the body absorbing the heat. There is a lot of water in the Southern Hemisphere, so all that water takes far more heat to make it rise in temperature than does the land in the Northern Hemisphere. It also means that the water in the Southern Hemisphere can give up more heat at night than the land and so it doesn’t cool as fast. Putting all this together, it means that the Southern Hemisphere summer and winter temperatures are moderated somewhat compared with those of the Northern Hemisphere, even though the Southern Hemisphere gets slightly more solar heating in the summer and slightly less solar heating in the winter compared with the Northern Hemisphere. And, if that isn’t enough, the elliptical nature of Earth’s orbit makes it speed up and slow down as it goes around the Sun. That makes it take longer to go through the part of the orbit in July than in January. The cumulative effect of all this is so extensive, in fact, that the Earth’s average surface temperature is actually warmer in July and August than in January and February, even though Earth is farther from the Sun in July and August! You can see this effect in the following graphic produced using a tool at NASA’s Earth Science Office’s web site:
Click on the image to make it bigger and easier to see. Each color represents data from a particular year. Note that in all years, the peak appears typically in late July (give or take a little).
So, that is the astronomy lesson for the day, the way I’d explain it to my students.
-Astroprof
Sili on July 4, 2009 at 1:52 pm: 1
Presumably once the precession leads to Southern Winter coönciding with aphelion, it might have an effect.
I thought the interplay of the two was part of the cause of iceages.