Space, Earth, and Celestial Objects
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Astronomy
the study of the motions and properties of objects in space
Celestial sphere
an imaginary sphere encircling the Earth on which all objects in the night sky appear
Celestial Object
An object in the sky outside the Earth’s atmosphere. Includes the sun, moon, stars, and planets.
Constellations
The patterns people have observed that mark the positions of stars in the sky.
North Star
Polaris. (Located almost directly above the North Pole. Often used for navigation or finding local latitude).
Geocentric model
An early model of the solar system and universe in which the Earth is a stationary object located at the center of the universe around which all celestial objects revolved.
Heliocentric model
The modern model of the solar system and universe in which the planets revolve around the sun and the Earth undergoes daily rotation.
Axis
The imaginary line around which an object rotates.
Rotation
The spinning of a body around an internal axis. (Caused day&night for the Earth).
Year
The time required for a planet (Earth) to complete one orbit around its primary (sun).
Orbit
The path (usually an ellipse) of any satellite around its primary.
Orbital Speed
The measure of a satellite’s orbital motion.
Revolution
The orbital motion of a satellite around its primary. (Earth revolves around the sun yearly).
Foucault Pendulum
A swinging weight that is free to rotate as it swings back and forth. The slow change in direction proves that Earth rotates.
Apparent motion
The way celestial objects appear to move through the sky.
Apparent Solar Day
The time required for the sun to go from its highest point in the sky on one day to its highest point the next day.
Seasons
The annual cycle of weather conditions as the Earth orbits the sun.
Zenith
The point on a celestial sphere that is directly overhead with respect to an observer.
Equinox
The time at which the sun is directly above the equator, and all places on the Earth have 12 hours of daylight and 12 hours of darkness. (One at the end of March, one at the end of September).
Solstice
The longest or shortest day (hours of daylight) of the year when the sun is directly above the Tropic of Cancer or of Capricorn. One on Dec. 21, one on June 21.
Direct rays
sunlight that strikes the Earth from straight overhead. (Vertical)
Vertical rays
sunlight that strikes the Earth from directly overhead (the zenith). Strikes the Earth at a single location.