AstroGlossary - U
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ultraviolet radiation
A wavelength, or frequency, of light that is more energetic than visible light, but less energetic than x-ray radiation. Ultraviolet, or UV, radiation ranges in wavelength between 10^-8 and 10^-7 meters and in frequency between 10^15 and 10^17 Hz. UV rays can be very harmful to life because they are strong enough to ionize atoms and destroy cells. Fortunately, the Earth's atmosphere shields us from most UV radiation. Astronomers who want to study UV light from celestial sources must do so from space-borne telescopes, such as the Hubble Space Telescope. umbra
The darkest part of a shadow. In astronomy, umbra can refer to the darkest areas of phenomena such as eclipses and sunspots. For example, the umbra is the darkest region of shadow cast by the Sun or Moon during an eclipse. If an observer stood within the Moon's umbra during a solar eclipse, the Sun would "disappear" behind the Moon. Likewise, the Moon disappears from view when it passes through Earth's umbra during a total lunar eclipse. The umbra is also the darkest region of a sunspot, where the sunspot temperatures are lowest. See also: universal time
Official Earth time, defined as local Greenwich Meridian time at 0 degrees longitude. Astronomers depend on a consistent time standard in order to accurately record and report their observations. universe
All matter, energy, space, and time. Uranus
The seventh planet from the Sun in the solar Ssystem. Uranus orbits the Sun at an average distance of 19.2 AU. The planet has a mass 14.5 times the mass of Earth and a radius 4.0 times the radius of Earth. Uranus is a gas planet made of hydrogen, helium, and methane and has no solid surface. The planet's axial tilt is an extreme 98 degrees, causing one day on Uranus to equal 84 years, equal to the time it takes Uranus to orbit the Sun once. It has a small ring system and 17 known moons, the largest of which are Titania and Oberon.
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