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Table of Contents Enchanted Learning
All About Astronomy
Site Index
Our Solar System Stars Glossary Printables, Worksheets, and Activities
The Sun The Planets The Moon Asteroids Kuiper Belt Comets Meteors Astronomers

Astronomy Dictionary
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

Click on an underlined word for more information on that subject.

U


ULTRAVIOLET RAYS

Ultraviolet rays are a type of electromagnetic radiation with very short wavelengths (below those of the color blue). Ultraviolet rays are invisible to us. The ozone layer traps much of the Sun's ultraviolet energy coming through Earth's atmosphere.

ULTRAVIOLET TELESCOPE

An ultraviolet telescope recieves UV rays (a type of electromagnetic radiation with very short wavelengths) from space. Since the ozone layer traps much of the Sun's ultraviolet energy coming through Earth's atmosphere, orbiting UV telescopes like the Hopkins Ultraviolet Telescope (HUT) are now used.

ULYSSES

The Ulysses spacecraft, a joint mission of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA), was launched from the space shuttle on October, 1990 to explore the Sun. It will study the sun's magnetism, solar wind plasma, solar prominences, gamma ray emissions, coronal mass ejections, and other solar phenomena.


UMBRA

The umbra is the inner, dark, cool (3700 K = 6600 °F = 3400 °C) region of a sunspot. The umbra of a sunspot can be up to 12,000 miles (20,000 km) wide. In the umbra, the Sun's magnetic field is very strong. (Compare to penumbra)


UMBRA

The umbra is the area of total shadow (compare to penumbra).


UMBRIEL

Umbriel is one of the larger of the 18 moons of Uranus, and the darkest. It has a heavily-cratered surface, indicating an old surface. It has a bright ring at its top (probably a crater floor) called a fluorescent cheerio. Umbriel was discovered by Wm. Lassell in 1851. Umbriel has a diameter of 1,170 km and orbits, on average, 265,970 km from Uranus. It has a mass of 1.27x10+21

UNDAE

Undae are dunes. There are undae on Mars.

UNIVERSE

The universe is everything, all matter and energy that is in existence.

UNIVERSAL GRAVITATIONAL CONSTANT

The universal gravitational constant (abbreviated G) is the constant of proportionality in Newton's equation (formulated in 1666) that describes the gravitational attraction between objects; their gravitational attraction (F) depends only on their masses and the distance between them, according to the formula F = Gm1m2 / r2. Henry Cavendish, in 1798, determined the numerical value of G to be 6.668 x 10-8 dynes cm2/g2.

UNIVERSAL TIME

Universal time (abbreviated UT) is the same as Greenwich Mean Time (abbreviated GMT); it is the time zone of Greenwich, England (longitude zero).
Planet Distance from U.A.
(Astronomical Units)
Orbital period
Planetary Year
Mass
#1 0.06 AU 4.6 Earth days 3/4 the mass of Jupiter
#2 0.83 AU 242 Earth days 2 times the mass of Jupiter
#3 2.5 AU 3.5-4 Earth years 4 times the mass of Jupiter

UPSILON ANDROMEDAE

Upsilon Andromedae is a star in the constellation Andromeda. Astronomers Geoffrey W. Marcy and R. Paul Butler discovered a massive planet orbiting this star (with each orbit taking only 4.6 days.) in 1996. Recently, 2 even more massive planets have been discovered orbiting this star. The three planets orbit within 2.5 Astronomical Units of the star.

URANIA

Urania was the Greek muse of astronomy.

URANOMETRIA

Uranometria was a detailed star chart/catalog made by Johannes Bayer in 1603. It contained 51 star charts (one for each of the 48 Ptolemaic constellations, a chart of the newly-discovered southern skies, and two planispheres), using his new method of classifying stars. The current edition of Uranometria plots every star in the sky down to magnitude 9.5 (each star is given a Uranometria Chart Number).

URANUS

Uranus is the seventh planet from the sun. This huge, icy planet is covered with clouds and is encircled by a belt of 11 rings and 18 moons. This gas giant is the third-largest planet in our Solar System (after Jupiter and Saturn), and is about 4 times the diameter of Earth and 14 times as massive.


URSA MAJOR

Ursa Major (The Great Bear) is a well-known constellation in the Northern Hemisphere that contains the 7 stars of the Big Dipper. The two brightest stars in Ursa Major (Dubhe and Merak) "point" to the current North Star, Polaris.


URSA MINOR

Ursa minor (The Little Bear) is a Northern Hemisphere constellation that is also known as the Little Dipper. This group of stars starts at Polaris, the pole star of the Northern Hemisphere


URSID METEOR SHOWER

The Ursids are a meteor shower that occur each year from Dec. 17-25, with a maximum on Dec. 22. This meteor shower occurs each year as the Earth passes through the orbit of comet Tuttle 1790, and icy remnants of the comet burn up as they enter the Earth's atmosphere. The meteors in this shower seem to emanate from the constellation Ursa Minor, (but they do not).

UV

UV stands for ultraviolet radiation.
Astronomy Dictionary
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

Click on an underlined word for more information on that subject.
If the astronomy term you are looking for is not in the dictionary, please e-mail us.

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