CONSEQUENCES OF THE ORBITAL MOTION OF THE EARTH


The annual variations (seasonal variations and changing constellations in the night sky) are caused by the orbital motion (revolution) of the Earth around the Sun. The Earth moves from the west-to-east around the Sun (i.e., in the CCW-sense as viewed from the NCP--the Earth rotates and revolves in the same sense, west-to-east). As a result of the orbital motion of the Earth, the Sun moves slowly through the stars on the Celestial Sphere from west-to-east. The path it traces out is referred to as the Ecliptic and the constellations through which it passes are referred to as the Zodiac Constellations. The Sun takes one sidereal year (roughly 365.2564 days) to make one complete trip around the Celestial Sphere with respect to the stars. Helpful video

Equinoxes and Solstices

To define positions of objects on the Celestial Sphere, we use a system analogous to longitude and latitude. The rotation axis of the Earth and the orbital axis of the Earth are not parallel (they form an angle = 23.5o).

As a result, the Ecliptic and the Celestial Equator are inclined with respect to each other. Because of this, the Sun is sometimes above the Celestial Equator and sometimes below the Celestial Equator. This gives us some natural checkpoints on the sky (spheres have no ears to grab). We define the following points on the Celestial Sphere;

Vernal (Mar 20, 2021 UTC) and Autumnal (Sep 22, 2021 UTC) Equinoxes and Winter (Dec 21, 2021 UTC) and Summer (June 20, 2021 UTC) Solstices

The two equinoxes occur when the Sun passes through the Celestial Equator and the two solstices occur when the Sun is at its greatest distance north and south of the Celestial Equator.


The Seasonal Variations

The seasons change on a period of 365.2422 days (the Tropical year); an interval slightly shorter than the sidereal year. The cause of the changing of the seasons is the misalignment between the rotation axis and orbital axis of the Earth coupled with the orbital motion of the Earth. Huh? The question is then

How do the axis misalignment and orbital motion lead to seasonal variations on the Earth?

There are two primary effects: