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Sun angle by season

  1. Oct 6, 2011 #1

    DaveC426913

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    This seems like a naive question that I ought to know the answer to, but autumn is upon us and it seems like a good time to ask.

    The sun sets much earlier in autumn than in the height of summer, and it follows a higher or lower angle (shortening dusk) to the horizon, resulting in a change in the duration of dusk. But ultimately at any given minute of the day, it is simply a matter of geometry wrt the height of the sun above the horizon.

    Let's say that today's 5:30 sun is at 20 degrees high in the sky - same as as the summer's of, say, 8:30. So why does everything look so different now? I'm coming home at 5:30 today and the sun is glaring so much I can barely drive, its angle so low the shadows are dramatically long. Yet, last time I headed west in the summer at 8:30 it didn't seem nearly so stark.

    What else is making this so obviously uniquely an autumnal sunset distinct from a summer sunset?
     
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  3. Oct 7, 2011 #2

    Drakkith

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    Well, we are getting closer to the sun. But I think it's just that you noticed it more today than otherwise. Not sure though.
     
  4. Oct 7, 2011 #3
    It could be a different angle you're thinking of. As the day progresses, the angle of the sun as it climbs and descends the sky does change. But there is also another angle, the declination angle. This doesn't really change with the time of the day. At spring and autumn, the declination is about the same. Summer and winter are opposites, though. Summer is more perpendicular to the sun, enabling more sunlight. Winter is more slanted, giving less sunlight. Think of a triangle. If you shine a straight beam of light perpendicular to the hypotenuse, you get all the light energy. But if you were to shine it perpendicular to the base, the hypotenuse has more length in the area of the beam, giving less energy for the same length.
     
  5. Oct 7, 2011 #4

    jtbell

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    Different atmospheric conditions, maybe? Around here, the air tends to be hazy in the summer, but clearer in the fall, especially after a cold front moves through. During the past week the air has been very clear and everything looks startlingly crisp.
     
  6. Oct 7, 2011 #5

    turbo

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    It's just the angle of direct sunlight, Dave. One place where my wife and I lived, the main road out of our neighborhood faced us directly into the sun on spring mornings and directly into the sun on fall afternoons when heading to work. If your windshield (and eyeglasses) weren't nice and clean, trouble could ensue.
     
  7. Oct 7, 2011 #6

    russ_watters

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    The azmuth is the same every day at a different time, so setting later means setting further north, so a certain road will have two optimal days when the sun will set directly in front of you. In addition, the steeper angle in the summer means it sets faster, spending less time giving you glare.

    And, of course, glare isn't worse at 5:30pm, but if more people are driving, more people see it.
     
  8. Oct 7, 2011 #7

    D H

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    Traffic.

    When you drove home at 8:30 or so at night during the summer you had a relatively small amount of traffic to contend with. Now when you drive home at 5:30 in the evening you are driving on a road packed with rush hour traffic, with every driver going the same way you are confronted with these same nasty conditions.

    The same applies to other conditions. Driving through a rainstorm is a lot easier when nobody else is on the road compared to driving through a rush hour rainstorm. Rush hour is bad enough as is without nasty driving conditions.
     
  9. Oct 7, 2011 #8

    turbo

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    Traffic could definitely contribute to the problem. Where I live, though, traffic is not that much of a problem most of the time. Our latitude (45 deg N) contributes to those very long, grazing sunrises and sunsets. They can be quite a problem in the winter when there is a lot of snow-cover. The sun is a problem even around mid-day when there is a lot of snow. Your eyes will "stop down" and you will find it quite hard to discern potential problems that are low-contrast.
     
  10. Oct 7, 2011 #9

    D H

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    That might be a problem in wintertime at very high latitudes, but it is not a problem now, and it isn't really an issue even for 45 latitude.

    Why it isn't a problem now: For one thing, there's a slight lack of snow covered ground. Much more importantly, the equinoxes mark the times at which sunset and sunrise are the shortest in duration. The duration of sunrise and sunset peaks at the summer solstice, with a lesser peak at the winter solstice. Sunrise and sunset occur quickly at the equinoxes. (And right now we are fairly close to the autumnal equinox.)

    Why it isn't an issue even at 45 degrees: Sunrise and sunset vary little in duration over the course of a year for latitudes between 55 south and 55 north. At 45 degrees, about 4 minutes pass between the time the bottom and top of the sun cross the horizon at the summer solstice. This time is about 40 seconds less at the equinoxes. At 60 degrees latitude, the duration of sunrise/sunset varies from under 5 minutes to over 8. The seasonal variability becomes even more stark at even higher latitudes.
     
  11. Oct 7, 2011 #10
    It's because the more the sun travels in elevation (sun goes higher in the sky in summer), the less time it spends in any 5 degree swath of elevation. As a result, sunsets last a much longer time in the autumn than in the summer. When they last longer, you have more time to notice all the stages in lighting as the sunset progresses.
     
  12. Oct 7, 2011 #11

    xts

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    From my (Europe/Poland 53° N) perspective - on summer evenings the road stays usually dry. But now the asphalt gets moist with evening dew - making it glossy.

    I do also second jtbell - in my area Summer evenings are often hazy, and high clouds (cirrostratus) are common, while Autumn evenings are either dark-cloudy, or perfectly clear.
     
  13. Oct 7, 2011 #12

    D H

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    That's backwards. Sunsets are shortest right about now.
     
  14. Oct 7, 2011 #13

    turbo

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    That's wrong, IMO. Sunsets are getting very long and grazing right now. Sunsets are shortest when you are at low latitutudes and the sun seems to dive into the horizon, when you are near summer's peak. It can get dark really fast then.
     
  15. Oct 7, 2011 #14

    DaveC426913

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    I think you guys may be on to something about the streets and the traffic.

    The road I travel is aligned with sunset only at this time of year. Any other time of year, the combination of traffic, road alignment and sun height doesn't line up.

    But I'm not sure that's all though. It twigged my attention when I was out walking too. So, no traffic, no streets aligned (or not as big a factor, anyway).

    I should be able to set my watch ahead 3 hours or so, step outside my office at 5:30 and not be able to tell that it isn't 8:30.

    I'd do an experiment with a camera but that would drag out for six months (or a year to be thorough).
     
  16. Oct 7, 2011 #15

    D H

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    Your opinion is wrong. The grazing happens at the solstices, not the equinoxes.

    Think about what happens at the Arctic circle. Sunrise/sunset is one long interval at the summer solstice. There is no real "nighttime" to separate sunset from sunrise. Instead the sun hovers along the horizon all "night" long, eventually rising above the horizon to make for a very, very long day. The opposite occurs at the winter solstice, where it never quite turns into day. The sun instead hovers at the horizon all day long. Sunrise/sunset again blur into one. Sunrise and sunset occur much more quickly at the equinoxes. They are still longer in duration that our sunrise/sunset, but they are not daylong events.

    Moving south of the Arctic circle, we do have day and night even on the solstices. Sunrise and sunset still take a long, long time at the solstices. They are shorter yet at the equinoxes than they are are the Arctic circle. At 45 degrees latitude, the difference between the duration of sunrise/sunset is much less marked. In the southern US, sunrise and sunset are, for all practical purposes, the same duration year round.
     
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