Key Ideas
Ideas preceded by an asterisk (*) are discussed in the Boxes.
Constellations and the Celestial Sphere: It is convenient to imagine the stars fixed to the celestial sphere with Earth at its center.
- The surface of the celestial sphere is divided into 88 regions called constellations.
Diurnal (Daily) Motion of the Celestial Sphere: The celestial sphere appears to rotate around Earth once in each 24-hour period. In fact, it is actually Earth that is rotating.
- The poles and equator of the celestial sphere are determined by extending the axis of rotation and the equatorial plane of Earth out to the celestial sphere.
- *The positions of objects on the celestial sphere are described by specifying their right ascension (in time units) and declination (in angular measure).
Seasons and the Tilt of Earth’s Axis: Earth’s axis of rotation is tilted at an angle of about 23½° from the perpendicular to the plane of Earth’s orbit.
- The seasons are caused by the tilt of Earth’s axis.
- Over the course of a year, the Sun appears to move around the celestial sphere along a path called the ecliptic. The ecliptic is inclined to the celestial equator by about 23½°.
- The ecliptic crosses the celestial equator at two points in the sky, the vernal and autumnal equinoxes. The northernmost point that the Sun reaches on the celestial sphere is the summer solstice, and the southernmost point is the winter solstice.
Precession: The orientation of Earth’s axis of rotation changes slowly, a phenomenon called precession.
- Precession is caused by the gravitational pull of the Sun and Moon on Earth’s equatorial bulge.
- Precession of Earth’s axis causes the positions of the equinoxes and celestial poles to shift slowly.
- *Because the system of right ascension and declination is tied to the position of the vernal equinox, the date (or epoch) of observation must be specified when giving the position of an object in the sky.
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Timekeeping: Astronomers use several different means of keeping time.
- Apparent solar time is based on the apparent motion of the Sun across the celestial sphere, which varies over the course of the year.
- Mean solar time is based on the motion of an imaginary mean sun along the celestial equator, which produces a uniform mean solar day of 24 hours. Ordinary watches and clocks measure mean solar time.
- *Sidereal time is based on the apparent motion of the celestial sphere.
The Calendar: The tropical year is the period between two passages of the Sun across the vernal equinox. Leap year corrections are needed because the tropical year is not exactly 365 days. The sidereal year is the actual orbital period of Earth.