Astronomical Dictionary
Letter: S


Dictionary of Astronomical Definitions and Terms.
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Semi major Axis
Half an ellipse's major axis. The distance from an ellipse's center to its furthest edge. The semi-major axis of a planetary orbit is also the average distance from the planet to its primary.

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Shepherd Moon
Is a moon which constrains the extent of a planetary ring by means of gravitational forces. It is believed that shepherd moons are responsible for Saturn's rings.
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Sidereal Time
Star time; the hour angle of the vernal equinox. Time measured with respect to the fixed stars rather than the Sun or body orbiter. Sidereal Time is measured by the diurnal motion of the mean Vernal Point ("mean equinox of date"), which is affected the secular quantities of Precession; Apparent Sidereal Time is measured by the position of the true Vernal Point ("true equinox of date"), which is affected by Nutation. The difference (Apparent minus Mean) is called the Equation of the Equinoxes (called Nutation in Right Ascension, before 1960). Apparent Sidereal Time on the observer's meridian is called LST (Local Sidereal Time); on the Greenwich meridian, it is called GST (Greenwich Sidereal Time). The difference (LST-GST) is equal to the geographic longitude of the observer. The value of GST is tabulated in an ephemeris under the heading "ST" or "Sidereal Time." The argument used to compute the GST is the UT (Universal Time), which differs from the ET (Ephemeris Time) used to compute the planets. Sidereal Time is a direct measure of the diurnal rotation of the Earth, and is, therefore, independent of the value of Delta-T.
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Sidereal Month
The 27.32166 day period of the Moon's orbit. Because of the many disturbances in the Moon's motion, this interval can vary as much as 7 hours.
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Solar Cycle
1. The 11-year variation in sunspot activity.
2. A 28-year period applicable to the Julian calendar, in which the first day of the year is restored to the same day of the week. As there are seven days in a week, and four years in an intercalary period, their product, 28, must include all possible combinations.
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Solar Flare
A sudden, short lived, burst of energy on the Sun's surface, lasting from minutes to hours. Solar flares can cause a disturbance of the Earth's Ionosphere, which in turn disrupts radio communication.
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Solar Wind
A stream of charged particles emitted from the Sun which travel into space along lines of magnetic flux. solar wind "sweeps" up all kinds of gases, particles, meteor fragments out of the solar system. It also pushes back cosmic rays during years of high solar activity. During periods of low solar activity, more cosmic rays reach the Earth.
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Solstice
Literally, the `Sun standing still'. This is the time when the Sun reaches its most northerly or southerly point (around June 21st & December 22nd. respectively.). It marks the beginning of Summer and Winter in the Northern Hemisphere, and the opposite in the Southern Hemisphere.
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Spectral Classification
A method of classifying stars which is based upon the appearance of the absorption lines in their spectra. Various classification schemes have been proposed, but the one most generally accepted evolved from work done at the Harvard Observatory. It is based on the appearance of the photographic region of a star's spectrum, from green to ultra-violet. Each class is denoted by a capital letter (O, B, A, F, G, K, and M), and are intended to represent stages of stellar evolution. Each class is subdivided by placing a numeral after the capital leter (e.g., B8 and K2). Over 90% of all stars fall into four classes: A, F, G, and K.
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Star Cluster
A loose association of stars within the the Milky Way. Examples are the Pleiades (Seven Sisters) Hyades clusters. These are divided into Open Clusters and Globular Clusters.
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Sunspot
Sunspots are relatively cool regions of the Sun's photosphere(which, thus, appears dark), and are large enough to be visible to the naked eye. They are regions of high magnetic fields with a temperature of 4,000?K, some 2,000?K cooler than the surrounding area. They provide a visible indicator of the solar magnetic cycle, which peaks every eleven years. The solar magnetic field has been shown to affect radio reception on Earth, and also, to affect living organisms.
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Superior Conjunction
This is when Mercury, or Venus, are behind the Sun.
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Superior Planets
Also known as the outer planets. These are the planets beyond the Earth's orbit. They are, in order: Mars; Jupiter; Saturn; Uranus; Neptune; Pluto.
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Supernova
An exploding star.
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Sacajawea
Sacajawea (Bird Woman) (1786?-1812) Native American who accompanied and guided the Lewis and Clark expedition from the Missouri River to the Pacific Ocean and back.
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Sapas
In Phoenician mythology, goddess of commerce and travel, and messenger of the gods.
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Saturn
Planet sixth in order from the sun. In Roman mythology, god of agriculture, and father of Jupiter (Greek name, Cronos).
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Scarp
A cliff or steep slope of some extent that may form a marked topographic boundary.
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Schiaparelli
Giovanni Virginio Schiaparelli (1835-1910). Italian astronomer at the Milan Observatory who reported markings on Mars which he called "canali".
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Shearing
The motion resulting from stresses that cause or tend to cause contiguous parts of a body to slide relatively to each other.
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Shield volcano
A broad volcanic cone with gentle slopes constructed of successive nonviscous, mostly basaltic, lava flows.
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Sif
In Scandinavian mythology, the grain goddess renowned for her long golden hair. Mate of the thunder god Thor.
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SIR C X SAR
Spaceborne Imaging Radar-C and X- Band Synthetic Aperture Radar. An instrument that performs detailed observations of Earth at any time, regardless of weather or sunlight conditions.
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Slumping
A landslide that results from the downward sliding of rock debris as a single mass, usually with a backward rotation relative to the slope along which the movement takes place.
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Stickney
Angeline Stickney (1830-1892) The wife of Asaph Hall, known for her persistent encouragement of her husband as he strove to and eventually succeeded in the discovery of the satellites of Mars.
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