Earth's orbit and tilt

  • #1
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Main Question or Discussion Point

In the Northern Hemisphere of North America is the sun closest in the elliptical motion in the winter but the tilt is away from its intensity at the Tropic Of Capricorn? In the summer is the sun farthest in the elliptical motion but the tilt is toward the Tropic of Cancer and North America?
 

Answers and Replies

  • #2
mgb_phys
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In your winter the hemisphere you are in is tipped away from the sun.
Additionally the Earth's orbit around the sun is an ellipse, so when it is winter in the Northern Hemisphere we are also at the closest point to the sun.
 
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Why then is the Pericenter at Jan 3,2008?
 
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mgb_phys
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Sorry typo - fixed
 
  • #5
russ_watters
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Why then is the Pericenter at Jan 3,2008?
Do you mean 'why doesn't that cause it to be summer instead of winter?' It's because the earth's orbit is not very eccentric, so the difference between peri and apihelion is not very large - not large enough to cause a noticeable climate change.
 
  • #6
cepheid
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Hey maybe you (the OP) already know this, but I thought it was worth saying. Just to reinforce what russ watters said:

The seasons are not caused by a change in the distance between Earth and the sun (Earth's orbit is nearly circular)

The seasons are not caused by the fact that the hemisphere in question is closer to the sun due to the axial tilt during its warm season. The axial tilt causes a *negligible* change in the distance between that hemisphere and the sun (because the sun is so far away).

Simply put, the seasons are caused by the axial tilt, but for two very specific reasons:

The tilt means that in the summer, the sun is higher in the sky.
The tilt means that in the summer, the sun is up for a longer duration (the days are longer).

Regarding the first point, it is relevant because it means the flux being received (W/m^2) is actually larger. Draw a picture of a vertical "beam" of sunlight (that we interpret to contain a certain power) hitting the surface of the earth. Now draw a beam of the same width, hitting the surface at a very oblique angle (representing the sun being lower in the sky in winter). You will see that the same amount of power is spread over a much larger surface area, making for a lower power per unit area.

That is my understanding, anyway.
 
  • #8
Janus
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From the link it appears that the vernal equinoctial point is at the top of the image(If you enter March 22, the date of the vernal equinox, into the simulator, the Earth is straight down from the Sun). Since the longitude of perihelion is measured in the direction of the orbital motion and from a line pointing from the Sun to the vernal equinoctial point, and the Earth looks about 104 degrees counter-clockwise from this line, I see no problem.
 
  • #9
D H
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On the perihelion (Jan 3,2008) where the earth is closest to the sun, why do my calculations say the longitude of the sun equals 103.8359563deg and clearly from the JPL site it is approximately 190deg.
Janus is correct: The x axis points to the top of the screen, y to the left.

You're value of 103.8359563 degrees is not correct. Your Julian date algorithm as implemented in the spreadsheet in https://www.physicsforums.com/showthread.php?t=262916" appears to be the problem. You must use integer division when converting calendar date to Julian date. You should get a Julian date of 2454468.5 for 2008 January 3 00:00:00.0 UT1.
 
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