Astronomical Dictionary Page #7

Radiant

The point on the Celestial Sphere where the meteors of a meteor shower appear to originate. Meteor s...

Red Giant

A type of star with high luminosity and low temperature, eg Betelgeuse in Orion... ...

Red Shift

The lengthening of the wavelength of electromagnetic radiation caused by relative motion between sou...

Resolution

The amount of small detail visible in an image (usually telescopic); low resolution shows only large...

Retrograde

Rotation of a planet, or orbit, opposite to that normally seen. ...

Right Ascension

The angular distance, measured Eastwards, from the Vernal Equinox. It is one of the ordinates used t...

Relief

The maximum regional difference in elevation....

Rhea

In Greek mythology, mother of Zeus and wife of Cronos the Titan ....

Rift

A valley formed at a divergence zone or other area of extension....

Rille

One of several trenchlike, or cracklike valleys up to several hundred km long and 1-2 km wide common...

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...