S

Second, Système International (SI)

The duration of 9 192 631 770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.

 

Selenocentric

With reference to, or pertaining to, the center of the Moon.

 

Semidiameter

The angle at the observer subtended by the equatorial radius of the Sun, Moon or a planet.

 

Semimajor axis

1. Half the length of the major axis of an ellipse. 2. A standard element used to describe an elliptical orbit. (See orbital elements.)

 

SI second

See second, Système International (SI).

 

Sidereal day

The period between successive transits of the equinox. The mean sidereal day is approximately 23 hours, 56 minutes, 4 seconds. (See sidereal time.)

 

Sidereal hour angle

Angular distance on the celestial sphere measured westward along the celestial equator from the equinox to the hour circle passing through the celestial object. It is equal to 360° minus right ascension in degrees.

 

Sidereal month

See month, sidereal.

 

Sidereal time

The hour angle of the equinox. If the mean equinox is used, the result is mean sidereal time; if the true equinox is used, the result is apparent sidereal time. The hour angle can be measured with respect to the local meridian or the Greenwich meridian, yielding, respectively, local or Greenwich (mean or apparent) sidereal times.

 

Solstice

Either of the two points on the ecliptic at which the apparent longitude of the Sun is 90° or 270°; also the time at which the Sun is at either point. (See longitude, ecliptic.)

 

Spectral types or classes

Categorization of stars according to their spectra, primarily due to differing temperatures of the stellar atmosphere. From hottest to coolest, the commonly used Morgan-Keenan spectral types are O, B, A, F, G, K and M. Some other extended spectral types include W, L, T, S, D and C.

 

Standard epoch

A date and time that specifies the reference system to which celestial coordinates are referred. (See mean equator and equinox.)

 

Stationary point

The time or position at which the rate of change of the apparent right ascension of a planet is momentarily zero. (See apparent place.)

 

Sunrise, sunset

The times at which the apparent upper limb of the Sun is on the astronomical horizon. In The Astronomical Almanac they are computed as the times when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°50′, based on adopted values of 34′ for horizontal refraction and 16′ for the Sun’s semidiameter.

 

Surface brightness

The visual magnitude of an average square arcsecond area of the illuminated portion of the apparent disk of the Moon or a planet.

 

Synodic month

See month, synodic.

 

Synodic period

The mean interval of time between successive conjunctions of a pair of planets, as observed from the Sun; or the mean interval between successive conjunctions of a satellite with the Sun, as observed from the satellite’s primary.

 

Synodic time

Pertaining to successive conjunctions; successive returns of a planet to the same aspect as determined by Earth.

 

Syzygy

1. A configuration where three or more celestial bodies are positioned approximately in a straight line in space. Often the bodies involved are the Earth, Sun and either the Moon or a planet. 2. The times of the New Moon and Full Moon.