The Earth, Moon, and Sun Orbital Mechanics

Seasonal Changes

Changes in season…..Summer (June 21) Result from the tilt of the axis of the earth. When the earth’s axis faces the sun, the sun rays hit most directly over our northern hemisphere and we get the most intense and longest duration of sunlight.(Insolation)

Seasons…Fall (Sept 21) As the earth continues its revolution around the sun, the sun strikes the earth most directly over the equator. At this time, all earth locations receive both hemispheres receive 12 hours of darkness and 12 hours of light.

Seasons….Winter (Dec 21) As the revolution continues, the earth’s axis ends up pointing away from the sun. At this time, the northern hemisphere receives glancing insolation resulting in fewer hours of less intense sunlight.

The earth’s orbital motion causes the apparent position of the sun to shift about 1 degree each day on the celestial sphere.

Orbital motion also results in phases of the moon….

Orbital motions

Objects remain in orbit because of……. Inertia - objects in motion tend to stay in motion in the same direction and with the same velocity unless acted upon by some outside force (such as gravity). This keeps the planet moving in a straight line into space.

and gravity... which pulls the planet back into the sun. The two forces, gravity and inertia, balance one another to cause planets and other satellites to remain in orbit around the sun or planet.

“Laws” of orbits All satellites orbit around their primaries (like the earth around the sun) in elliptical orbits. Ellipses have two “centers” called foci.

The law of equal area... states that a line connecting a planet to the sun sweeps out an area in such a manner that equal areas are swept out in equal times. Thus, the earth is moving more slowly when it is farthest away from the sun.

In order for out space shuttle to go into orbit, the 2 forces of inertia and gravity must be in balance.