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Due to Earth's elliptical orbit, its distance from Sun varies by almost 5 million Kilometers (147 million Kilometers at closest point & 152 million Kilometers at farthest point, i.e. almost 3% of the average distance).

As evident from the fact that that Venus has hotter environment than Mars due to their respective distances from the sun.

Why then Earth does not observe two winters (at farthest points) and two summers (at closest points)?

Additional Note: I know that Earth's seasonal climate change is caused by its 23 degrees tilt that causes the sunlight density variations for the hemispheres.

But to me this 5 million Km distance seems more relevant than the 23 degrees tilt.

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One problem with the question is that the closest and farthest points only happen once per orbit. See the Wikipedia article: en.wikipedia.org/wiki/Elliptical_orbit –  Donald.McLean Dec 16 '13 at 15:03
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"But to me this 5 million Km distance seems more relevant than the 23 degrees tilt." -- It may seem that way to you, but our planet disagrees with you; the tilt has a much stronger effect. (I don't have the time or math to prove it.) –  Keith Thompson Dec 16 '13 at 21:17
    
Are you thinking that we'd have one summer at when the northern (for example) hemisphere is tilted toward the Sun, and a second summer at perihelion? One of several problems with that is that the perihelion is in early January, quite close to the northern midwinter. –  Keith Thompson Dec 16 '13 at 21:25
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The sun is at one focus of the elliptical orbit, not at its center. Perihelion is when the earth is at the end of the major axis nearest the sun, not (both times) the earth is at the end of the minor axis. There is only one per year. The 3% change in distance makes a 6% change in solar energy received. At San Francisco, the day is 14:45 long in June and 9:33 in December, an increase of 54% and we haven't accounted for the higher angle of the sun in the sky. –  Ross Millikan Dec 16 '13 at 23:11
    
The extreme temperatures of Venus and Mars are only partly explained by their distances from the Sun. Venus is affected by a runaway greenhouse effect; it might be a lot cooler if its atmosphere were thinner. And Mars might be substantially warmer if its atmosphere were thicker (it would probably need to be larger to hold onto a thicker atmosphere). –  Keith Thompson Apr 26 at 0:47

5 Answers 5

up vote 26 down vote accepted

There are a few incorrect assumptions in your post, so it is difficult to answer as asked. But I can address the misconceptions.

1. The seasons are not caused by the distance from the sun
The seasons are caused by the 23.5° tilt in Earth's axis. When the Northern Hemisphere is tilted towards the sun (summer), the Southern Hemisphere is simultaneously tilted away from the sun (winter). So the seasonal temperature difference has little to do with the Earth's position in its elliptical orbit. Without this tilt, there would be no seasons and the temperature day to day across the globe would be relatively uniform.

2. Even the GLOBAL temperature is NOT consistent with our change in distance
As a matter of fact, the average temperature of the Earth globally is hottest when it is the furthest from the sun — hotter by about 2.3°C (ref). That's because there is a lot more landmass in the Northern Hemisphere facing the sun (when Earth is farthest away in its orbit). So even though there is less intensity of sunlight, the land is able to be heated up much faster than the vast oceans which have to be heated at perihelion.

This distance-temperature inconsistency isn't unique to the Earth. Look at the average temperature of the other inner planets as we move away from the sun:

  1. Mercury (167°C)
  2. Venus (460°C) farther, but hotter than Mercury?
  3. Earth (14.0°C)
  4. Mars (-60°C)

Venus is actually warmer than Mercury because of the thick carbon dioxide atmosphere causing runaway global warming. So it isn't simply the distance from the sun that determines the average temperature of a planet.

3. There's only ONE aphelion/perihelion
The closest point of the Earth's orbit (perihelion) and the farthest (aphelion) only happens once per year; not twice. That is because the elliptical orbit of the Earth is such so the sun is at one of the foci, not the center (as illustrated below).

Earth's Elliptical Orbit](http://i.imgur.com/hxAbz8y.png)

Note that the size of the bodies and the eccentricity of the orbits are greatly exagerated here.

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The 5 million kilometer variation in distance to the Sun may seem like a lot, 5 million km is a large distance, but it doesn't affect Earth's weather as much as the axial tilt. A 3% variation in distance leads to an ~7% variation in the intensity of solar radiation reaching Earth, give or take.

But consider the variations from Earth's axial tilt, and here I will use the example of a location at 45 deg. latitude, similar to the latitudes of cities like New York, London, Berlin, Paris, etc. The first factor is day length. Axial tilt means that during Summer the day will be longer and during Winter the day will be shorter. At 45 deg. latitude this translates into the Sun being above the horizon for a full 15.5 hours on the Summer solstice, and only 8.7 hours during the Winter solstice. That's nearly a 2:1 difference in the number of daylight hours, which totally swamps a small 7% variation.

Additionally, the angle of the Sun in the winter is very low, which means that the amount of Sun falling on the ground is lower per area than if it were at a higher angle. On the Winter solstice at 45 deg. latitude the Sun's rays are diminished to 37% of their strength per square meter of ground compared to how strong they would shine directly above some location nearer the equator. But during the Summer solstice they are 97% as strong as they would be if they were directly overhead. Which is a 2.5:1 difference.

So here you have a combined total of roughly a 400% difference in the amount of light and heat shining down on a given patch of ground every day between the Winter and the Summer.

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I suspect you're thinking that we'd have a summer when the northern hemisphere, for example, is tilted toward the Sun, and a second summer during the perihelion, when the Earth is closest to the Sun. For one thing, the timing doesn't work; the perihelion takes place in early January, close to the northern midwinter. That probably moderates the effects of axial tilt for the northern hemisphere (and amplifies them for the southern hemisphere), but it's not enough to override them.

The other answers have said that the axial tilt is a more significant factor than the variation in distance from the Sun, but they haven't explained why.

The following is a rough back-of-the-envelope guesstimate.

The difference in illumination caused by the varying distance from the Sun can be computed from the ratio between the perihelion and aphelion distance, which is about a factor of 0.967. Applying the inverse square law indicates that amount of sunlight at aphelion is about 93.5% of what it is at perihelion. Reference: http://en.wikipedia.org/wiki/Perihelion#Planetary_perihelion_and_aphelion

At my current location (about 33° north latitude), at this time of year (close to the northern winter solstice), we're getting about 10 hours of sunlight and 14 hours of darkness each day. (Reference: the weather app on my phone.) That's about 83% of what we'd get with 12 hours of daylight during either equinox, and about 71% of what we'd get with 14 hours of daylight and 10 hours of darkness per day during the summer solstice. The effect is greater at higher latitudes.

In addition to that, the sun is lower in the sky during the winter than it is during the summer, meaning that a given amount of sunlight is spread over a larger area of the Earth's surface, which makes the ratio even larger.

I don't have the numbers for that, but it's enough to show that the effect of the axial tilt is substantially greater than the effect of the varying distance between the Earth and the Sun.

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There are two factors here. One is that the 23 degrees tilt is way more important than the tiny 5 million Km (remember that that's only 0.033 AU). Proof for that is that when it is Summer in the north it is winter in the south, regardless of the distance.

The other is that if there were no tilt, we would have one very light summer and one very light winter (very very light, both) because the Sun is not at the center of the ellipse (two nearest points and two farthest points) but on one focus (one nearest point and one farthest point).

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It is because the 23 degrees tilt of the earth and the ellipse orbit of the earth around the sun.

If you make an equation with all the variables we have here

  1. 23 degrees tilt of earth
  2. Ellipse orbit of the earth around the sun
  3. Earth speed around itself
  4. Earth orbital speed around the sun
  5. Distance between the earth and the sun

So you will note a variation in the weather conditions regarding this variables, so we have 2 winters and 2 summers, but 2 of them are just a transition state.
A proof of that, at the earth's poles, we can only have summer and winter because the limitation of variable (1).

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