Astronomical Dictionary


Dictionary of Astronomical Definitions and Terms.
Astronomical [1] [2] [3] [4] [5] [6] [7] [8] [9]

 

Definition


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Radiant
The point on the Celestial Sphere where the meteors of a meteor shower appear to originate. Meteor showers are usually named after the constellation in which the radiant originates.
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Red Giant
A type of star with high luminosity and low temperature, eg Betelgeuse in Orion...
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Red Shift
The lengthening of the wavelength of electromagnetic radiation caused by relative motion between source and observer. Spectral lines are red-shifted from distant galaxies, indicating that the galaxies are moving away from us due to the expansion of the Universe. This is one of the central facts in the Expanding Universe Theory.
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Resolution
The amount of small detail visible in an image (usually telescopic); low resolution shows only large features, high resolution shows many small details.
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Retrograde
Rotation of a planet, or orbit, opposite to that normally seen.
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Right Ascension
The angular distance, measured Eastwards, from the Vernal Equinox. It is one of the ordinates used to reference objects on the celestial sphere. It is the equivalent to a longitude reference on the Earth. There are 24 hours of right ascension within 360 degrees, so one hour is equivalent to 15 degrees. Together with declination, it represents the most commonly used co-ordinate system in modern astronomy.
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Relief
The maximum regional difference in elevation.
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Rhea
In Greek mythology, mother of Zeus and wife of Cronos the Titan .
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Rift
A valley formed at a divergence zone or other area of extension.
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Rille
One of several trenchlike, or cracklike valleys up to several hundred km long and 1-2 km wide commonly occurring on the Moon's surface.
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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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Astronomical [1] [2] [3] [4] [5] [6] [7] [8] [9]

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