celestial sphere - ANSimaginary sphere surrounding Earth to which all objects in the sky were
once considered to be attached
celestial equator - ANSthe projection of Earth's equator onto the celestial sphere
celestial pole - ANSprojection of Earth's North or South pole onto the celestial sphere
ecliptic - ANSthe apparent path of the Sun, relative to the stars on the celestial sphere, over the
course of a year
polaris - ANSthe north star
astronomical unit - ANSthe average distance of Earth from the Sun. Precise radar
measurements yield a value for the AU of 149,603,500km
winter solstice - ANSpoint on the ecliptic where the Sun is at its southernmost point below the
celestial equator, occurring on or near December 21
summer solstice - ANSpoint on the ecliptic where the Sun is at its northernmost point above the
celestial equator, occurring on or near June 21
vernal equinox - ANSdate on which the Sun crosses the celestial equator moving northward,
occurring on or near March 21
autumnal equinox - ANSdate on which the Sun crosses the celestial equator moving southward,
occurring on or near September 21
sidereal day - ANSthe time needed between successive risings of a given star
solar day - ANSthe period of time between the instant when the Sun is directly overhead to the
next time it is directly overhead
degree - ANSunit of angular measure
arc minute - ANSAn angular measurement equal to 1/60th of a degree
arc second - ANSAn angular measurement equal to 1/3600th of a degree
retrograde motion - ANSbackward, westward loop traced out by a planet with respect to the
fixed stars
, epicycle - ANSa construct of the geocentric model of the solar system that was necessary to
explain observed planetary motions
ellipse - ANSgeometric figure resembling an elongated circle
perihelion - ANSthe closest approach to the Sun of any object in orbit about it
aphelion - ANSthe point on the elliptical path of an object in orbit about the Sun that is most
distant from the Sun
semi-major axis - ANSone-half of the major axis of an ellipse
geocentric model - ANSa model of the solar system that holds that Earth is at the center of the
universe and all other bodies are in orbit around it
heliocentric model - ANSa model of the solar system that is centered on the Sun, with Earth in
motion about the Sun
ptolemaic model - ANSgeocentric solar system model, developed by the second century
astronomer Claudius Ptolemy. it predicted with great accuracy the positions of the ten known
planets
inertia - ANSthe tendency of an object to continue moving at the same speed and in the same
direction, unless acted upon by a force
mass - ANSa measure of the total amount of matter contained within an object
force - ANSaction on object that causes its momentum to change. the rate at which the
momentum changes is numerically equal to this
the newton - ANSSI unit of force
the pound - ANSImperical unit of force
the kilogram - ANSSI unit of mass
the meter - ANSSI unit of distance
kilometer - ANS1000 meters
m/s - ANSSI unit of speed
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