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Moon Phase

A B C D E F & G H & I J, K & L M
N & O P Q & R S T, U & V W, X, Y & Z

Glossary: J

Jansky A unit used in radio astronomy to indicate the flux density (the rate of flow of radio waves) of electromagnetic radiation received from outer space. A typical radio source has a spectral flux density of roughly 1 Jy. The jansky was named to honor Karl Gothe Jansky who developed radio astronomy in 1932.
Jet A narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.
Jupiter 5th planet

Glossary: K

Kelvin A temperature scale used in sciences such as astronomy to measure extremely cold temperatures. The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius.
Kepler's First Law A planet orbits the Sun in an ellipse with the Sun at one focus.
Kepler's Second Law A ray directed from the Sun to a planet sweeps out equal areas in equal times.
Kepler's Third Law The square of the period of a planet's orbit is proportional to the cube of that planet's semi major axis; the constant of proportionality is the same for all planets.
Kiloparsec Distance measure. 1000 parsecs.
Kirkwood Gaps Regions in the main belt of asteroids where few or no asteroids are found. They were named after the scientist who first noticed them.
Kuiper Belt A large ring of icy, primitive objects beyond the orbit of Neptune. Kuiper Belt objects are believed to be remnants of the original material that formed the Solar System. Some astronomers believe Pluto and Charon are Kuiper Belt objects.

Glossary: L

Lagrange Point French mathematician and astronomer Joseph Louis Lagrange showed that three bodies could lie at the apexes of an equilateral triangle which rotates in its plane. If one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable. Such bodies are sometimes referred to as Trojans. The leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5.
Lenticular Galaxy A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies.
Libration An effect caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon's surface can be seen over a period of time.
Light Year  The distance light travels in a vacuum in one year.  9.461 Pm (Peta = 10^15)
Limb The outer edge or border of a planet or other celestial body.
Local Group A small group of about two dozen galaxies of which our own Milky Way galaxy is a member.
Luminosity The amount of light emitted by a star.
Lunar Eclipse A phenomenon that occurs when the Moon passes into the shadow of the Earth.  A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow.  In a total lunar eclipse, the Moon passes into the Earth's umbra, or total shadow.
Lunar Month The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a synodic month.
Lunation The interval of a complete lunar cycle, between one new Moon and the next.  A lunation is equal to 29 days, 12 hours, and 44 minutes.
Lunation number The count of New Moons since Jan 1923-01-17 02:41 UTC.  New Mon on 26-Oct-2011 is start of Lunation 1099.