In our science class in high school, we were taught that there are four seasons in an annual global cycle that are very distinct in the northern and southern hemispheres but not in the equatorial regions. This is because the axis of the earth is tilted from the vertical at 23½o and this creates the seasonal variations as the earth completes its annual orbit around the sun.

The inclination of the earth’s axis gives four significant seasonal points along the earth’s orbit. We have two “solstices” when the axis of the earth is inclined either away from or towards the sun in the annual orbit: (1) the winter solstice on December 22, and (2) the summer solstice on June 21. In the northern hemisphere, December 22 has the longest night and June 21, the longest day. In the southern hemisphere, all the conditions are reversed. The longest day and summer conditions occur on December 22, the longest night and winter conditions on June 21.

We also have two “equinoxes”: (1) the vernal equinox on March 21 (the start of spring) and (2) the autumnal equinox on September 22, when the sun is directly overhead or vertical at noon at the equator. Days and nights are equal (12 hours each) over the entire globe.

The nine planets of the solar system move in a counter clockwise direction and at different speeds around the sun. Planets move about in their respective elliptical orbits and are tilted differently. The earth’s axis has a fixed inclination of 66½o from the ecliptic or plane of its orbit or angled 23½o from the vertical position. Most of the other planets have a tilt of less than 11o.

The earth’s inclination is caused by the sun but more forcefully by the moon’s gravitational force because of its proximity and relative size. Of the ten moons as satellites of the seven smaller planets (excluding Jupiter and Saturn) in the solar system, our moon is exceptionally large in comparison with the parent planet. The moon is considered as a planetary partner of the earth rather than a satellite.

The moon orbits perpendicularly to the earth’s axis. Likewise, the moon exerts a gravitational influence on the earth’s daily tidal deformations.

(To be continued)