Weax 201

Tutorial on Earth/Sun Relations and Seasons [pic]

To understand why we have seasons, we need to look at the relations between the Earth and the sun.

The Plane of the Ecliptic is the plane of the Earth's orbit around the sun.

Most of the planets in the solar system also orbit on or near the plane of the ecliptic (you can see that the planets (except Pluto) are more or less lined up, instead of circling the sun at just any old angle. No, this is not drawn to scale).

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The sun's rays are pretty close to parallel to each other when they reach the Earth,

so, if the Earth's axis of rotation were perpendicular to the plane of the ecliptic,

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the direct rays of the sun would always shine on the equator (that is, the angle of incidence at the equator would be 90 degrees) and you would always see the sun just on the horizon at the North and South Poles (that is, the angle of incidence at the Poles would be zero).

If that were the case, we would have no seasons.

However, the Earth's axis is not perpendicular to the plane of the ecliptic. If you drew a line perpendicular to the plane of the ecliptic, the Earth's axis of rotation would be tilted, or inclined, at an angle of about 23 1/2 degrees with respect to the perpendicular line.  It is this inclination of the Earth's axis of rotation that is the reason for the seasons.

The earth maintains the same angle of inclination throughout its orbit.  Another way of saying this is that the earth’s axis is parallel to itself at all points in its orbit.

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Question: to which star does the Earth's axis point?  That is, what star is almost directly overhead at the North Pole? Answer

The inclination of the Earth's axis means that at different locations on its orbit around the sun, different parts of the earth are tilted toward the sun, and the part that is tilted toward the sun is the part that is having summer.   If the northern hemisphere is tilted toward the sun, the northern hemisphere receives...