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Solar System

UFO
1. Acronym for Unidentified Flying Object. Usually described as a large "dish" with "alien" occupants. Often related to the claims of abductions of humans. It's said the abductee (a.k.a Psycho) endures the most severe painful of tests by these "aliens".
2. A term conjured up for a conspiracy plotted by the U.S. government during the early 1950's.

Ultraviolet

Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.

Umbra

The dark central region of a sunspot.

Uncertainty Principle

One of Quantum theories insisted by Heisenberg in 1927. If you want two information such as, the place coordinates and momentum of one particle, you can not accurately gain both information at once, it is impossible theoretically. This rule only applies in the quantum world, meaning the particles in a microscopic world.

Let's consider measuring the place of a small particle and its momentum. Observation is an action when you expose light on to a matter, and see its condition by the reflected light. Usually with matter in this world, this given light doesn't affect them. But if you expose light on to particles in microscopic world, the particle's place and momentum changes due to the shock of light. If you send a light with a short wavelength, it crashes with the particles so you can measure its place. But when it moves when it crashes so you wouldn't know its momentum. Oppositely, if you send a light with longer wavelength, it will not effect the particle's momentum due to not enough energy, but because it doesn't crash, you will no be able to measure its place.

Heisenberg uncertainty principle is, in formula, (amount increased:X)*(amount increased:P) x=h (Planck constant). This means if you multiply uncertainty of place and uncertainty of momentum, it always gives a constant value. If amount increased:X (uncertainty of place) increases, amount increased:Px (uncertainty of momentum) decreases, and if amount increased:Px increases, amount increased:X decreases.

Undae

Dunes (literally 'waves').

Universal Constant of Gravitation (G)
The constant of proportionality in Newton's law of universal gravitation and which plays an analogous role in A. Einstein's general relativity. It is equal to 6.664 x 10^-11 newtons per square meter per kilogram squared (see scientific notation).

Universal Time

Official Earth time. Universal time is local Greenwich Meridian time, 0 degrees longitude. Astronomers depend on a consistent time standard in order to accurately record and report their observations.

Universe

The whole body of things and phenomena observed or postulated.

Unmanned

Having no humans aboard.

Vallis

Sinuous valley (plural: valles).

Van Allen Belts

Regions of radiation around Earth, where Earth's magnetic field traps particles from the solar wind.

Variable Star

A star whose brightness changes periodically. These changes may relate to several different situations, from a luminosity change to an eclipsing dark companion. Luminosity changes relate to internal structure and stellar processes. For instance, the period of luminosity change of a Cepheid variable star is directly related to the Cepheid's luminosity. Thus, all the astronomer has to do is record the Cepheid's light curve, measure the period, match the period to the luminosity, then calculate the distance to the Cepheid. When Cepheids are located inside another galaxy, the astronomer can measure the distance to the galaxy.

Vastitas

Widespread lowlands.

Velocity
The speed and the direction a body is moving.

Vernal Equinox
The point on the celestial sphere where the Sun crosses the celestial equator passing from south to north; a time in the course of the year when the day and night are roughly equal.

Virtual Particle

In quantum mechanics, a particle that can never be directly detected, but whose existence does have measurable effects.

Visible
Electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors ranging from red (longer wavelengths; ~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers.)

Voids

Immense empty regions of space, separating the filaments of galaxies.

Volatile

As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes hydrogen, helium, water, ammonia, carbon dioxide and methane.

Vortex

A spiral motion of fluid within a limited area, especially a whirling mass of water or air that sucks everything near it toward its center.

Wavelength

The distance traveled by wave such as electromagnetic wave, when it passes a certain height and returns to that height.

Wave-Particle Duality
The principle of quantum mechanics which implies that light (and, indeed, all other subatomic particles) sometimes act like a wave, and sometimes act like a particle, depending on the experiment you are performing. For instance, low frequency electromagnetic radiation tends to act more like a wave than a particle; high frequency electromagnetic radiation tends to act more like a particle than a wave.

Weak Force

The second weakest of the four fundamental forces, with a very short range; it affects all matter particles, but not force-carrying particles.

Weight
A measure of the force due to gravitational attraction.

White Dwarf

A whitish star of low luminosity, small size, and very great density.

White Hole

The exact opposite of a black hole; and object that spews out matter and energy.

Wien's Law
Formula that relates the temperatures of a blackbody to the wavelength at which it emits the greatest intensity of radiation.

WIMPs

Short for Weakly Interacting Massive Particles, which are theorized particles 10 to 100 times the size of protons, could account for lots of the dark matter.

Wolf-Rayet Star
One of a class of very hot stars that eject shells of gas at very high velocity.

Wormhole

An object with two mouths in different parts of our universe connected by a tunnel that allows two-way traffic; they may be safe shortcuts through space.