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S - T - U

Select the letter that (probably) leads off the word in question. Please suggest new words or corrections.



  • Saturn: The sixth planet from the Sun, and probably best known for its complex ring system which is visible through a small telescope. It is a gas giant.

  • seeing: See "atmospheric seeing".

  • silicon carbide: Compound commonly used in the grinding of telescope mirrors and other optical surfaces. The grains can be very large or very small depending on the level of grinding being done.

  • Sol: See "the Sun".

  • solar system: The collection of planets, asteroids, comets, debris, etc. that are in orbit around our Sun.

  • spectrum: The distribution of energy emitted by a radiant source, as by an incandescent body, arranged in order of wavelengths such as radio, x-ray, light and gamma ray waves.

  • star: A large mass of gas held together by its own gravity which emits energy that is fueled by nuclear reactions. Ex. the Sun.

  • star cluster: A group of stars held together by their mutual gravitational influences. Also see "open clusters" and "globular clusters". Ex. M44 - the Beehive Cluster.

  • Sun: The star at the center of our solar system about which the Earth (and all other planets) orbit. Also called "Sol".

  • supernova: An immense explosion of a star in its later stages of life that may form a black hole or neutron star.



  • telescope (astronomical): An instrument used by astronomers to explore space. They can be designed to detect light, radio or any other waves of the spectrum and can be built by professionals or amateurs.

    terminator: The (imaginary) line separating the lighted and dark portions of a celestial body that is being lit by the Sun. Ex. The terminator line between illuminated and dark sides of the Moon as it proceeds through its monthly phases.

  • terrestrial planet: also known as the "rocky planets"; one of the four inner planets: Mercury, Venus, Earth and Mars

  • Titan: The largest moon of Saturn discovered by Christian Huygens in 1655. It is larger than both Mercury and Pluto.

  • tool: The term for the opposing surface which is used to grind against a mirror blank while shaping the blank's surface.

  • transit: The apparent movement of a smaller object across a larger object. Ex. A Mercury transit across the Sun.



  • ultraviolet rays: The range of invisible radiation wavelengths on the border of the x-ray region, to just beyond the violet in the visible spectrum. It is from 4 nanometers to 380 nanometers in the spectrum.

  • universe: The entire conglomeration of matter in which we live.

  • Universal Coordinated Time: The 24 hour time system on Earth in which the Prime Meridian is the 0 hour and all other time zones are computed plus (to the east of the Prime Meridian) or minus (to the west of the Prime Meridian) from this time based on the individual zone's location. Also called "Greenwich Mean Time" or "Zulu Time". Astronomical times are given in Universal Coordinated Abbr. UTC, UT.

  • Uranus: The seventh planet from the Sun. It is a gas giant that also has a ring system, but it is not as complex as Saturn's.


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