The ''rising and setting'' of the sun

Discussion in 'General Science & Technology' started by wegs, Jan 8, 2020.

  1. wegs Matter & Pixie Dust Valued Senior Member

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    As we know, the sun stays in its position always at the center of the solar system. It appears to rise because of the Earth's rotation on its axis.

    So, I have two questions on this. Why do parts of Alaska go dark for nearly 70 days per year? And same for Norway, but in reverse - for about a period of approximately 76 days (May to late July), the sun never seems to "set." Bright sunlight can appear for nearly 20 hours, daily, during those months! How does the Earth make this ''shift'' then to Alaska and Norway experiencing ''even'' amounts of sunlight, as other parts of the world?
     
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  3. DaveC426913 Valued Senior Member

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    Because the Earth is tilted on its axis by 23.5 degrees.

    In the Northern hemisphere's winter, the North pole is pointed away from the Sun (see December Solstice in diagram). As the Earth rotates, Alaska and Norway stay in shadow - even at their local noon. (Imagine Earth rotating around the arrow marked N every day.)

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    In the Northern Hemisphere's summer (June Solstice in diagram) Alaska and Norway are still in sunshine, even at local midnight.

    In spring and fall (Equinoxes), Alaska and Norway spend equal parts of the day in sunlight and in shadow.
     
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  5. wegs Matter & Pixie Dust Valued Senior Member

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    Ah! I see, now. Thank you so much! I find this fascinating. So Alaska and Norway ''come out of the dark'' so to speak, at the March Equinox?
     
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  7. DaveC426913 Valued Senior Member

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    Note that the only place on Earth that experiences even amounts of day and night throughout the year are on the equator. Everywhere else has longer days in the summer and longer nights in the winter.
     
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  8. DaveC426913 Valued Senior Member

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    No. It's gradual.
     
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  9. wegs Matter & Pixie Dust Valued Senior Member

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    Okay. This might be a silly question but when I see the sun rise in the morning, about an hour later, it’s “high” in the sky. Overhead, so to speak. I realize that the sun itself isn’t moving but why don’t we “feel” the Earth shifting? I ask because it seems like a quick shift from sun “rise” to the sun appearing overhead.
     
  10. exchemist Valued Senior Member

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    Because the Earth spins at a constant rate on its axis and we rotate along with it.
     
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  11. wegs Matter & Pixie Dust Valued Senior Member

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    Okay, thanks. So, does this mean that the sun appears to be ''rising and setting'' at the same ''rate'' ...everywhere in the world? (different timezones, of course)
     
  12. exchemist Valued Senior Member

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    Everywhere at the same latitude, yes.

    But if you are at the equator, you are moving faster than if you are half way to the pole. So the sun appears to rise and set more quickly.
     
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  13. wegs Matter & Pixie Dust Valued Senior Member

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    Okay, I get it, now. Thank you

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  14. Janus58 Valued Senior Member

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    The number of degrees per minute the sun travels across the sky remains the same. However, the angle its path has relative to the horizon differs. Below is a diagram comparing the position of the Sun relative to its sunrise point (where the lines meet) as seen from different latitudes and at the same amount of time after sunrise. The vertical line would be at the Equator during the Equinox, and the slanted line a higher latitude.

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    If you measure along the lines, the Sun has traveled the same distance, but when traveling along the diagonal, this equals less height above the horizon. The sun rises slower in terms of height above the horizon,while still traveling at the same speed across the sky.
     
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  15. DaveC426913 Valued Senior Member

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    Yes. This is quite noticeable when you travel between the temperate zones and the tropical zones.

    Up here, the sun always sets at an angle, making the sunsets gradual. Nearer the equator, there's very little sunset - it's day and then it's night. Catches a lot of tourists off-guard.
     
  16. wegs Matter & Pixie Dust Valued Senior Member

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    Thanks Janus, this is very helpful. So, the sun always ''rises'' at an angle?
     
  17. exchemist Valued Senior Member

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    Yes I recall a couple of related things from my time living in Dubai in the 1980s. One was the speed of sunset. The other was the angle of the crescent moon, which was quite different from how it looks in Britain.
     
  18. RainbowSingularity Valued Senior Member

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    wobble
     
  19. DaveC426913 Valued Senior Member

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    I assume by "wobble", you're referring to precession - the wobble of Earth's axis. That occurs on a 26,000 year cycle and is unrelated to day or yearly cycles.

    In fact, as evidenced by the Pole Star being in the same position of the sky day-after-day for millennia, one can easily see that the Earth's axis is quite stable, and does not wobble in human timeframes.
     
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  20. James R Just this guy, you know? Staff Member

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    Yes.

    The tilt of the Earth's axis also explains why the seasons are the opposite in the Northern and Southern hemispheres. When it's constant daylight in Norway, it's constant night in Antarctica, and vice versa.

    Because the Earth rotates at a constant rate. For the same reason, you don't feel your car moving when you're driving along at a constant 100 km/hr. We don't feel constant velocity. As a rough rule, we tend to feel accelerations (speeding up or slowing down).

    In fact, since we're moving in a circle as the Earth rotates, we are accelerating towards the centre of the Earth all the time, even when we're standing still. But most of what we feel is the ground reacting against gravity. If the ground wasn't there, we'd be in free fall towards the centre of the Earth. The effect of the earth's rotation is actually to reduce the "weight" we feel as a result of the Earth's gravity by a small amount. You can think of it as a kind of "centrifugal force" that results from the rotating earth constantly "trying" to throw us off into space. Gravity wins the battle.
     
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  21. exchemist Valued Senior Member

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    In fact I did the calculation for Theorist in post 18 of this thread: http://www.sciforums.com/threads/how-does-water-not-spin-off-the-earth.162679/. The spin reduces the apparent force of gravity by ~0.3% at the equator (less at higher latitudes of course, and zero at the poles).
     
  22. wegs Matter & Pixie Dust Valued Senior Member

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    I saw the sun “rise” this morning, and this thread went through my mind. How you all explained to me, something so beautiful. It’s worth waking up early to watch.

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  23. wegs Matter & Pixie Dust Valued Senior Member

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    Hmm, that's true. I guess since the Earth isn't hitting any speed bumps or ''curves'' along the path like a car, it's impossibly to ''feel'' its movement.

    Really? I don't know why I find this such an odd thing. lol Perhaps, I take gravity for granted.

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    I see, so we are ''held down'' so to speak, to the surface of the Earth by gravity.
     

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