| A | B |
|---|
| perpendicular/direct solar rays | as the Earth revolves around the sun these rays move between the tropics causing points on the horizon of sunrise and sunset to vary during the year. |
| perihelion | shortest distance between the Earth and the Sun when the sun appears the largest. |
| aphelion | longest distance between the Sun and the Earth when the Sun appears the smallest. |
| local noon | when the Sun reaches its maximum altitude at the observer's longitude |
| zenith position | when the sun is directly overhead |
| equinox | the time period when daylight and dark hours are equal on Earth (usually March 21st and September 23rd) |
| Tropic of Cancer | first day of summer occurs when the sun's vertical rays are at 23 1/2 degrees North; directly over this location |
| winter solstice | when the Sun's rays are vertical at 23 1/2 degrees South, over the Tropic of Capricorn (usually December 21st) |
| orbit | the path taken by one body as it revolves around another body |
| focus | one fixed central point on a circle |
| eccentricity | "out of roundness" |
| orbital velocity | time per distance |
| Harmonic Law of Planetary Motion | the square of time of revolution of a planet divided by the cube of its mean distance from the Sun gives a number that is the same for all planets |
| astronomical unit | the mean distance of the Earth from the Sun; about 150,000,000 kilometers) |
| gravitation | the attractive force between any two objects anywhere |
| kinetic energy | energy of motion |
| inertia | the tendency of an object in motion to continue in motion in a straight path |
