ch.21

Axial Precession

In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis.

Plane of the Ecliptic

The ecliptic plane is defined as the imaginary plane containing the Earth's orbit around the sun. In the course of a year, the sun's apparent path through the sky lies in this plane.

Equatorial System

The equatorial system is the system most used in astronomy for describing the position of objects outside the solar system. Compare altazimuth coordinate system, ecliptic coordinate system

Phases of the Moon

The lunar phase or phase of the moon is the shape of the illuminated (sunlit) portion of the Moon as seen by an observer on Earth

Ecliptic

a great circle on the celestial sphere representing the sun's apparent path during the year, so called because lunar and solar eclipses can occur only when the moon crosses it.

Constellations

a group of stars forming a recognizable pattern that is traditionally named after its apparent form or identified with a mythological figure. Modern astronomers divide the sky into eighty-eight constellations with defined boundaries.

Lunar Eclipse

an eclipse in which the moon appears darkened as it passes into the earth's shadow.

Solar Eclipse

an eclipse in which the sun is obscured by the moon.

Revolution

an instance of revolving.

Synodic Month

another term for lunar month.

Rotation

the action of rotating around an axis or center.

Declination

the angular distance of a point north or south of the celestial equator.

Right Ascension

the distance of a point east of the First Point of Aries, measured along the celestial equator and expressed in hours, minutes, and seconds.

Perihelion

the point in the orbit of a planet, asteroid, or comet at which it is closest to the sun.

Aphelion

the point in the orbit of a planet, asteroid, or comet at which it is furthest from the sun.

Mean Solar Day

the time between successive passages of the mean sun across the meridian.

Sidereal Day

the time between two consecutive transits of the First Point of Aries. It represents the time taken by the earth to rotate on its axis relative to the stars, and is almost four minutes shorter than the solar day because of the earth's orbital motion.

Sidereal Month

the time it takes the moon to orbit once around the earth with respect to the stars (approximately 27 1/4 days).