Sun's path in the sky during Equinox if watching from equator?


by Lebombo
Tags: equator, equinox, path, watching
Lebombo
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#1
Dec25-12, 09:09 PM
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Suppose it's the equinox, when the earth's closest point to the sun is the equator. Would people who live somewhere along the equator see the sun rise directly from the east, move directly above so, and set directly to the west?

For instance, if someone, living on the equator, were to put a stick into the ground on the equinox, would the sun create a shadow on the stick that directly follows in an exact east and west path?
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Drakkith
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Dec25-12, 09:33 PM
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That is correct.
Lebombo
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Dec26-12, 12:05 AM
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Thanks,

Suppose a person is somewhere along the tropic of cancer and watches the sky during an equinox. Since the tropic of cancer is 23.5 degrees above the equator, if a person, at noon, looked into the sky, they would, I am guessing, see the sun in the south. However, I am trying to imagine at what direction the sun would rise from and set in.

Would the sun rise 23.5 degrees south of east, then at noon be 23.5 degrees south of directly upwards, and then set 23.5 degrees south of west? Or, would the sun continue to rise directly in the east, then at noon be some bit south of directly upward, and then set directly in the west? Or would the sun take some path other than that which I've tried to imagine?

Drakkith
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Dec26-12, 12:51 AM
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Sun's path in the sky during Equinox if watching from equator?


The Sun would rise further south than directly east. I don't know how far exactly. At noon the Sun would be 23.5 degrees south from zenith, which is straight up in the air. At sunset the Sun would then set a little further south of directly west.
Lebombo
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#5
Dec26-12, 01:38 AM
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If a person at the tropic of cancer put a stick in the ground and tracked the path of the shadow created by the sun throughout the day on an equinox, would the tip of the shadow create a perfect east/west line (so as to successfully determine the directions via the sun)?

I've seen a couple videos on youtube of people accurately determining the east and west directions by placing a stick in the ground and plotting points at the edge of the shadow as it moves over a period of time throughout the day. I recall a person said this always works, regardless of your location and day of the year. So should this then be true if a stick is placed in the tropic of cancer during an equinox and its shadow plotted?
Drakkith
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Dec26-12, 01:59 AM
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Not sure about that one. Without testing it I'd guess that it would though.
Damo ET
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#7
Dec26-12, 05:27 AM
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Quote Quote by Lebombo View Post
If a person at the tropic of cancer put a stick in the ground and tracked the path of the shadow created by the sun throughout the day on an equinox, would the tip of the shadow create a perfect east/west line (so as to successfully determine the directions via the sun)?

I've seen a couple videos on youtube of people accurately determining the east and west directions by placing a stick in the ground and plotting points at the edge of the shadow as it moves over a period of time throughout the day. I recall a person said this always works, regardless of your location and day of the year. So should this then be true if a stick is placed in the tropic of cancer during an equinox and its shadow plotted?
For the first part of the post, the practical answer would be yes. BUT, there would be a very very small curve to the line (from just south of east, to just south of west), which would be produced by the angle between the distance from the equator, and the distance to the sun in a right angle triangle (neglecting the change in distance the curve of the earth produces). But you won't reproduce it marking a shadow of a stick.

For the second part, your latitude and time of the year are absolutely essential to determine anything close to East or West. In summer at the north pole the sun doesn't set, so the whole 'sun sets in the west' is rubbish. Besides, when you are at the north pole, every other direction is due south.


Damo
256bits
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#8
Dec26-12, 08:48 AM
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Quote Quote by Lebombo View Post
Thanks,

Suppose a person is somewhere along the tropic of cancer and watches the sky during an equinox. Since the tropic of cancer is 23.5 degrees above the equator, if a person, at noon, looked into the sky, they would, I am guessing, see the sun in the south. However, I am trying to imagine at what direction the sun would rise from and set in.

Would the sun rise 23.5 degrees south of east, then at noon be 23.5 degrees south of directly upwards, and then set 23.5 degrees south of west? Or, would the sun continue to rise directly in the east, then at noon be some bit south of directly upward, and then set directly in the west? Or would the sun take some path other than that which I've tried to imagine?
At equinox, at all latitudes the sun rises eat and sets west. At noon, the sun would be overhead at an angle 90- (your laitude) . The angle of path the sun makes with the horizon at sunrise or sunset would also be 90 - (your latitude)
Drakkith
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Dec26-12, 11:58 AM
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Quote Quote by 256bits View Post
At equinox, at all latitudes the sun rises eat and sets west. At noon, the sun would be overhead at an angle 90- (your laitude) . The angle of path the sun makes with the horizon at sunrise or sunset would also be 90 - (your latitude)
At latitudes other than the equator the Sun rises and sets further south or north of due east/west. The closer you get to the poles the more extreme it is.
cepheid
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Dec26-12, 04:38 PM
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I don't know if it's necessary, but in case this thread can benefit from a visual aid, if you think about the celestial sphere as rotating around the Earth, and the circular paths around the north celestial pole that every object on that sphere would trace out, you can more easily understand how the sun's path through the sky depends on the declination (celestial latitude) of the sun, and the latitude (on Earth) of the observer:





I like the second diagram, because it explains why the days are longer or shorter as the sun's declination varies with the season.
Drakkith
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Dec26-12, 09:31 PM
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Thanks Cepheid. Where do you guys find these awesome visual aids?
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Dec26-12, 09:52 PM
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Quote Quote by Drakkith View Post
Thanks Cepheid. Where do you guys find these awesome visual aids?
Well, I remembered seeing diagrams like this in The Cosmic Perspective, which was the textbook for a course I TA'd once, but there don't seem to be any free online previews of that book, so in the end I just Googled something like "suns path at different latitudes" or something, and found diagrams like the ones I was looking for on some person's blog:

http://notesfromnoosphere.blogspot.c...sun-paths.html

(I hope this link is ok for PF)
Drakkith
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Dec26-12, 10:03 PM
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Ah ok. Thanks.
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Dec26-12, 10:06 PM
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Also, is it just me, or does that diagram of 90 degrees North look odd. I'm pretty sure that the Sun wouldn't move in a perfect circle like that. At the summer solstice a person at the north pole would be pointing 23 degrees towards the Sun, so it would make some kind of ellipse in the sky, would it not? I'm pretty sure I've read descriptions of the Sun at the north pole dipping low towards the horizon but never going under it before starting its rise.
Damo ET
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Dec27-12, 12:24 AM
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Quote Quote by Drakkith View Post
Also, is it just me, or does that diagram of 90 degrees North look odd. I'm pretty sure that the Sun wouldn't move in a perfect circle like that. At the summer solstice a person at the north pole would be pointing 23 degrees towards the Sun, so it would make some kind of ellipse in the sky, would it not? I'm pretty sure I've read descriptions of the Sun at the north pole dipping low towards the horizon but never going under it before starting its rise.
At the summer solstice in the northern hemisphere, if you were to stand at the north pole the sun would rotate at 23deg above the horizon for the entire day, and everyday after that till the equinox it will rotate lower and lower in the sky till it passes the horizon for the 6months of no sun.
The further you are away from the poles, the more arc the sun tracks across the sky, except for the equinox where the sun tracks a straight line across the sky at any latitude.

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Drakkith
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Dec27-12, 12:36 AM
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Perhaps you are right. I'm just having a hard time imagining it. I think all the videos I've seen in the past aren't taken from the pole itself, but just a high latitude where the Sun does indeed make an ellipse in the sky.


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