Astronomical Dictionary on Letter #S

Semi major Axis

Half an ellipse's major axis. The distance from an ellipse's center to its furthest edge. The semi-m...

Shepherd Moon

Is a moon which constrains the extent of a planetary ring by means of gravitational forces. It is be...

Sidereal Time

Star time; the hour angle of the vernal equinox. Time measured with respect to the fixed stars rathe...

Sidereal Month

The 27.32166 day period of the Moon's orbit. Because of the many disturbances in the Moon's motion, ...

Solar Cycle

1. The 11-year variation in sunspot activity. 2. A 28-year period applicable to the Julian calenda...

Solar Flare

A sudden, short lived, burst of energy on the Sun's surface, lasting from minutes to hours. Solar fl...

Solar Wind

A stream of charged particles emitted from the Sun which travel into space along lines of magnetic f...

Solstice

Literally, the `Sun standing still'. This is the time when the Sun reaches its most northerly or sou...

Spectral Classification

A method of classifying stars which is based upon the appearance of the absorption lines in their sp...

Star Cluster

A loose association of stars within the the Milky Way. Examples are the Pleiades (Seven Sisters) Hya...

Sunspot

Sunspots are relatively cool regions of the Sun's photosphere(which, thus, appears dark), and are la...

Superior Conjunction

This is when Mercury, or Venus, are behind the Sun. ...

Superior Planets

Also known as the outer planets. These are the planets beyond the Earth's orbit. They are, in order:...

Supernova

An exploding star. ...

Sacajawea

Sacajawea (Bird Woman) (1786?-1812) Native American who accompanied and guided the Lewis and Clark e...

Sapas

In Phoenician mythology, goddess of commerce and travel, and messenger of the gods....

Saturn

Planet sixth in order from the sun. In Roman mythology, god of agriculture, and father of Jupiter (G...

Scarp

A cliff or steep slope of some extent that may form a marked topographic boundary....

Schiaparelli

Giovanni Virginio Schiaparelli (1835-1910). Italian astronomer at the Milan Observatory who reported...

Shearing

The motion resulting from stresses that cause or tend to cause contiguous parts of a body to slide r...

Shield volcano

A broad volcanic cone with gentle slopes constructed of successive nonviscous, mostly basaltic, lava...

Sif

In Scandinavian mythology, the grain goddess renowned for her long golden hair. Mate of the thunder...

SIR C X SAR

Spaceborne Imaging Radar-C and X- Band Synthetic Aperture Radar. An instrument that performs detaile...

Slumping

A landslide that results from the downward sliding of rock debris as a single mass, usually with a b...

Stickney

Angeline Stickney (1830-1892) The wife of Asaph Hall, known for her persistent encouragement of her ...

Stratosphere

An upper portion of a planetary atmosphere, above the troposphere and below the ionosphere, characte...