Unit 6 - Astronomy Vocabulary
Barycenter – center of mass between two or more celestial bodies that orbit each
other
Precession – the wobble in Earth’s rotational axis caused by gravitational pull from
the Moon and the Sun
Retrograde motion – when a planet appears to move in the opposite direction
(East to West)
Nutation – swaying or nodding motion in the axis of rotation
Heliocentric model – theory that the Earth, Moon and planets revolve around the
Sun
Geocentric model – early theory that the Sun, Moon, planets and starts revolve
around the Earth
Perigee – closest point to the Earth in the Moon’s orbit
Apogee – farthest point from the Earth in the Moon’s orbit
Synchronous rotation – state where the Moon’s orbital and rotational periods are
equal
Refracting telescope – telescopes that use lenses to focus light
Reflecting telescope – telescopes that use mirrors to focus light
Aphelion – when a planet is farthest in its orbit from the Sun
Perihelion – when a planet is closest in its orbit to the Sun
Nebula – a cloud of gases from which stars are formed
Spring tide – occurs when the Moon, Earth, and Sun are in a straight line (new and full
moon); maximum difference in high and low tide
Neap tide – occurs when the Moon, Earth, and Sun are at right angles (1st & 3rd
quarter phases); has a small difference between high and low tide