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earth's_orbit_and_the_seasons

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					Earth's orbit and the seasons

Question:       I understand that the Earth orbits the Sun in an elliptical path but on what date
is the Earth is closest to the Sun, and it this has any bearing on the Seasons?

Answer:

The seasons do not depend on the elliptical nature of the Earth's orbit but on the 23.5 degree
inclination of its axis to the perpendicular to that orbit that orbit. In summer in the Northern
Hemisphere the north pole of the Earth is tilts towards the Sun and in the winter the South
Pole tilts towards the Sun.

If the axis was at right angles to the orbit, in other words the Earth rotated round the Sun in
an "upright" position there would be no seasons.

This is not exactly true because the distance effect would then have a tiny bearing on the
energy received. Actually the Earth is closest to the Sun around the second of January and
furthest from it around the first or second of July. However since the orbit of the Earth is very
nearly circular (the eccentricity being only 0.017) the difference in the energy received per
square metre at the Earth's surface due to this change in distance is only 7%.

Other planets have orbits with a much greater eccentricity – that of the orbit of Pluto being
0.25.

The time of year at which the Earth is closest to the Sun does alter very slowly due to
precession but this is imperceptible over a lifetime. The Earth's perihelion precesses in the
direction of the Earth's orbital motion. 50" is due to the Earth's bulge and the other 0.3" is due
to the other planets.

				
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