I couldn’t find the answer with a Straight Dope search, so I thought I’d launch a new thread.
The reason why I’m wondering is that it seems to me that the Sun’s light is actually stronger in the winter than it is in the summer. It could be my imagination, it could be because of the higher reflection/lower absorption of sunlight because of snow and there being little vegetation. It could be because there’s much less humidity. Or the Earth could be closer to the Sun.
Related question: Does the Earth’s variance in distance from the Sun have a measurable affect on ambient light, weather, tides, what have you?
paging Bad Astronomer. If we can attract his attention, he’ll know for sure! He’s my hero! I wish I was an astronomer, instead of having to deal with all this slimy stuff at work.
Anyway, I do believe we are actually closest to the sun sometime during the northern hemisphere winter. But I have no cites to back that up.
However, there is a complication. The Earth’s orbit is very close to being a perfect circle, but not quite. It is somewhat elliptical, which means that the distance between the Earth and the Sun varies over the course of the year. This effect is too weak to cause the seasons, but it might have some influence over their severity. The remainder of this page explains this possibilty.
The Earth reaches perihelion - the point in its orbit closest to the Sun - in early January, only about two weeks after the December solstice. Thus winter begins in the northern hemisphere at about the time that the Earth is nearest the Sun. Is this important? Is there a reason why the times of solstice and perihelion are so close? It turns out that the proximity of the two dates is a coincidence of the particular century we live in. The date of perihelion does not remain fixed, but, over very long periods of time, slowly regresses (moves later) within the year. There is some evidence that this long-term change in the date of perihelion influences the Earth’s climate.
JoltSucker, it’s unlikely that you’re able to perceive any difference in intensity of sunlight. The Sun’s apparent diameter is only 3 percent greater in January than July, and the disk is only 7 percent larger.
Tides, no, because the phase of the moon and even the distance of the moon overwhelm the variation in solar distance. Light and weather, yes, but it’s hard to measure. The fact that we experience perihelion in winter does tend to moderate Northern hemisphere seasons and exacerbate Southern hemisphere seasons, but other factors like the distribution of land and water in each hemisphere affect climate to a greater degree.
As has been mentioned, perihelion isn’t always in January. It cycles through the seasons over a period of about 21,000 years.
To add to the above responses, the sunlight in the northern hemisphere during winter is actually “weaker” because the earth’s poles are tilted. We are tilted away from the sun and the sunlight falls on us at something of an angle instead of more straight on. The fact that we are slightly closer to the sun during this time has very little effect on the season–it is the tilt that makes the difference. I don’t know why winter sunlight seems stronger or brighter to you.
The next perihelion occurs on 4 Jan 2004 at about 6PM UTC. Here’s a handy table. It’s definitely measurable, or else we wouldn’t know about it. Although I suspect that you can’t measure it with your eyes, I could be easily convinced otherwise with some evidence.
It’s only “weaker” if measured in terms of light per unit are of ground. If you had a piece of paper and held it at right angle to the sunlight, then the light received by that piece of paper is the same regardless of season. But if you hold the paper horizontally, then it receives less light in winter than in summer.
I think sunlight does appear brighter in winter, but only because the atmosphere tends to be more hazy in the summer. Or maybe because people tend to spend more time indoors in winter. When they do go out, their eyes are not adjusted to the bright light.
Well, I ask the question based on a very simple (and admittedly stupid) observation: If I try to set paper on fire with a magnifying glass, it’s much easier in the winter. Maybe it’s just the level of humidity, both in air (dampening the infrared) and in the paper (making it harder to ignite). Or maybe it’s that being near perihelion makes enough of a difference to make the trick work.
It’s a matter of lower air humidity. The increase due to perihelion is insignificant in comparison to the decrease due to passing through more atmosphere.
For example, according to the US Department of Energy, the incident solar radiation (energy from the sun that makes it to the ground) for Indianapolis in January is 900 BTU/Sq.Ft./day, only counting “clear” days. The distance from earth to sun is about 92 million miles (NASA). In July, the incident solar radiation is around 2540 BTU/Sq.Ft./day, while distance from earth to sun is around 95 million miles (NASA).
“BTU/Sq.Ft/day” sounds like it’s the sunlight integrated over a whole day per square feet of ground. If so, the larger number for the summer is mostly caused by the longer day (more hours of sunlight) and the more vertical angle of the sunlight. Neither factor affects how easily you can ignite a piece of paper with a magnifying glass.
I’d bet that JoltSucker’s assesment is correct - paper is easier to ignite in winter because it contains less moisture.
Although I suspect that you can’t measure it with your eyes, I could be easily convinced otherwise with some evidence.