Ultraviolet rays Energy emission with wavelengths shorter and frequency greater than visible light, but less than X rays; the presence of these rays can be evidence of extreme mass and temperature, and sometimes, celestial activity, such as the explosion of a star. Umbra Dark central convergent part of a shadow, surrounded by the penumbra. Umbriel Satellite of Uranus, 715 miles (1,172 km) in diameter. Orbits the planer in 4.1 days. Has a very dark surface, covered with craters. Unfavorable opposition When a superior planet (to Earth) is at its aphelion while in alignment with the Sun and Earth. Universe Everything that we know exists and that we believe may exist. Uranus The seventh planet, with a diameter of 31,700 miles (51,000 km). Formally discovered in 1781, although it has appeared by accident on astronomical charts for centuries before, since it is visible to the naked eye under ideal conditions. Uranus takes over 84 years to orbit the Sun. The planet rotates tipped over on its side, instead of almost upright, like all the other planets (except Pluto). Valles Marineris An enormous canyon on Mars, almost as long as the continental USA. Van Allen Belts Two rings of electromagnetic radiation, trapped by the Earth's magnetic field around the equator. The inner belt is about 190 miles (300 km) above the Earth's surface; the larger outer belt about 13,700 miles (22,000 km) away. Variable stars Stars that change in brightness, either periodically such as Cepheids, or erratically. Vega A main sequence star, 26 light-years away. Shines in the constellation of Lyra. Vega, the 5th brightest star in the sky, is slightly more luminous than Sirius, but because it is farther away Sirius seems brighter in the night sky. Venus The ancient Roman goddess of love and the 2nd planet out from the Sun. The planet was named after her because of its great beauty. Venus is 25 million miles (41 million km) closer to the Sun than Earth. It is 7500 miles (12,000 km) in diameter, nearly the same size of Earth. Sometimes called the Earth's twin. Heat is been trapped under the dense clouds that covers its surface, making it overall the hottest world in the Solar System. The atmosphere of Venus is so dense, it is more like water than air. The Soviet space probe Venera landed on Venus and survived only 45 minutes before melting! Vernal equinox The beginning of spring in the Northern Hemisphere. The time/day that the sun crosses the equatorial plane going from south to north.  Virgo cluster Large cluster of several hundred galaxies, about 50 million light-years away. Near its center is a giant elliptical galaxy, M87, which sends out strong X-rays. Voids Regions of space between super clusters that apparently contain few, if any, galaxies. It may hold the theoretical dark matter of the universe. Wavelength The distance between the peak of one wave of energy and the next, e.g. ocean waves, which are meters apart, or electromagnetic waves, which are only a few billionths of a meter apart. White dwarf The small, white-hot body that remains when a star like our Sun collapses. Wobble The motion of the axis of a planet due to external forces, or due to the constant redistribution of mass in the planet because of atmospheric or oceanic motion. X-rays Second most energetic form of electromagnetic radiation; Can show evidence of highly compact and fast moving objects in space. Zenith In astronomy, the point above a stargazer's head. Zodiac A band of sky about 20 degrees wide with the ecliptic passing along the center. The Zodiac passes through the 12 Zodiacal constellations. Due to various factors, the Sun now passes through 14 different constellations.