Calendars

KEY TERMS

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Eccentricity

—Measurement of the earth's deviation from a circular orbit around the Sun, based on its actual elliptical orbit.

Ecliptic

—The plane of Earth's orbit about the Sun as projected on the sky. The Sun always appears to lie directly on the ecliptic; the Moon and planets lie near it but not necessarily on it, as their orbital planes are all oriented slightly differently from Earth's.

Equinox

—The days of the year when the Sun appears to lie directly on the celestial equator, meaning it appears to rise due east and set due west. This happens twice per year, on or about March 21 (the spring or vernal equinox) and September 22 (the fall equinox), and on these dates the day and night are each 12 hours long. The word equinox is derived from Latin words meaning "equal" and "night."

Precession

—The wobbling motion of Earth's rotational axis, much like a spinning top wobbles about its axis of rotation.

Sidereal year

—The time interval needed for the earth to make a complete 360° orbital revolution around the Sun, 365.25636 days.

Synodic month

—The time interval in which the phases of the Moon repeat (from one Full Moon to the next), and averages 29.53 days.

Tropical year

—The time interval between successive crossings of the Vernal Equinox by the Sun, 365.2422 days.

Zodiac

—The zone 9° on each side of the ecliptic where a geocentric observer always finds the Sun, Moon, and all the planets except Pluto.