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Seasons due to Earth's elliptical orbit

  1. Feb 16, 2015 #1
    Hello there! What I have never understood is that our seasons are the result of the tilt of the earths axis, and I've always interpreted that to mean that the northern hemisphere is closer to the sun in June, July, and August, and the southern in December, January, and February. If that is the case, I don't understand why a shift in the axis results in temperature variations of ~50 degrees (around 40 degrees N), yet a variation of distance from the sun of 3 million+ miles does not affect the temperature. Or do I have this all wrong, and the elliptical orbit also plays a part in the temperatures?
  2. jcsd
  3. Feb 16, 2015 #2


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    There is an effect, it's just hard to see in the middle of everything else that is happening.

    The day length changes quite drastically summer to winter. In Toronto, where I live, a winter day can be as short as 6 hours. Plus the sun does not get very high in the sky. In summer, a day can be 14 hours. So more than double. And it gets higher in the sky so the sun light is falling closer to vertical. So this produces much more heat per square meter.

    The Earth's orbit is slightly non-circular, as you refer to. And it is slightly closer for part of the year. This amounts to a change of about 3 percent. (To one digit.) So, crank out your inverse-square law and work out how much this increases total solar radiation. (Hint 1.03 * 1.03 is just a bit more than 1.06.) So comparing that to the more-than-doubling of daylight in Toronto means that the distance effect is probably pretty small.

    There are other perturbations and effects that make the effect hard to see. For example, the southern hemisphere has a lot more ocean than the northern. Ocean currents and clouds tend to be very different in the southern hemisphere.
  4. Feb 16, 2015 #3


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    Actually, it is the tilt of the earth's axis which is the major determining factor in creating the seasons. In the northern hemisphere, the axis is tilted toward the sun during the summer months, while the southern hemisphere is tilted away at the same time, producing winter in that region. The opposite holds true in the winter season of the northern hemisphere.

    As to the influence of the earth's orbit on the seasons, it is relatively minimal and surprisingly the opposite of what is expected. The distance of the earth from the sun varies by about 5 million km out of 150 million km average during the year. However, the earth reaches perihelion, or the closest point from the sun, about January 3 every year, and aphelion, or the farthest point from the sun, about July 4.

  5. Feb 16, 2015 #4
    Take a look at these attached sketches also:
    (main sketch and enlarged views of the solstices)
    (you will have to rotate them)
    The red line is a daily path of a point at about 45 degrees latitude above the equator.

    Attached Files:

  6. Feb 16, 2015 #5

    Ken G

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    If you read between the lines of the very complete answers you have gotten above, you can see that the difference in distance to the Sun that is caused by the tilt is indeed completely negligible. So you have that part right-- the tilt has nothing to do with distance to the Sun. But what you did not understand is what is being explained above-- all the other important things that tilt does do (it changes the length of the day, and how close to overhead the Sun gets, and you know from daily experience that the temperature rises when the Sun is closer to directly overhead). In fact, when teaching the simplest understanding of the seasons, the first thing we usually say is completely forget distance to the Sun, you can understand seasons on Earth without even thinking about distance variations. If you do want to think about distance, consider Mars' seasons instead-- they combine tilt and distance variations from the elliptical orbit, so are quite a bit more complicated than Earth seasons.
  7. Feb 16, 2015 #6
    Thank you all for your answers, you cleared things up for me!
  8. Feb 16, 2015 #7
    Me too! Thanks for asking the question Mister!
  9. Feb 16, 2015 #8


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    For the northern hemisphere, that is. The Aussies don't know they are upside-down!
  10. Feb 17, 2015 #9

    D H

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    These are very good points.

    As everyone has already said, the primary driver of seasons on the Earth is the Earth's axial tilt. The Earth as a whole is cooler in January (when it is closest to the Sun) than it is in July (when it is furthest from the Sun), showing how unimportant distance is when it comes to driving the seasons. The reason the Earth as a whole is cooler in January than in July is because of the uneven distribution of ocean and land between the northern and southern hemispheres. This is a secondary cause of the seasons. Distance from the Sun? That's at best tertiary, at least for the Earth.

    Mars doesn't have oceans and its orbit about the Sun is much more eccentric than that of the Earth. Mars' seasons are a complex mix of eccentricity and axial tilt.
  11. Feb 22, 2015 #10
    There is also the 25000 earth wobble the Southern Hemisphere is close now but over time it will be the northern hemisphere. the Sahara will be wet not dry ect ect
  12. Feb 22, 2015 #11


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