AstroGlossary - T
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telescope
An optical instrument that gathers and focuses light into a camera, CCD, spectrograph, or an astronomer's eye. Two major types of telescopes dominate astronomy: reflectors and refractors. temperature
A measure of the average kinetic energy of the atoms or molecules that make up an object. Temperature can be measured in degrees Fahrenheit (oF), degrees Celsius (oC), or in Kelvin (K). The respective freezing and boiling points of water are 32 degrees and 212 degrees F, 0 degrees and 100 degrees C, and 273.16 and 373.16 K. One degree Celsius and one Kelvin measure the same amount of change in temperature.
When referring to the temperature of stars, astronomers usually refer to the effective temperature, or temperature of the photosphere, of a star. In the core however, the temperature is highest around 15 million degrees. In between the core and the surface, the temperature is about 5 million degrees, and at the photosphere, where almost all photons escape, the temperature is 6000 degrees. terminator
A distinct shadow edge that marks the boundary between the night and day side of a moon or planet. Because of shadows cast along this jagged boundary on the Moon, it is easy to see the three-dimensional structure of lunar mountains and craters. Sometimes, towering crater rings still catch a bit of sunlight as they slip behind the terminator. terrestrial planets
The four inner planets of our Solar System: Mercury, Venus, Earth and Mars. They consist mainly of rocky material, including iron, sulfur, magnesium, silicon, and nickel. Unlike the outer planets, no hydrogen nor helium exists in the atmospheres of the inner planets. The inner planets are also much smaller and more dense than the outer planets. Daytime temperatures range from 600 K (621 degrees F) on Mercury to 300 K (81 degrees F) on Mars. thermal radiation
Energy that is output from a source as a result of its temperature. tidal force
A stretching force that is caused by the difference between gravitational forces on opposite sides of an object, such as a planet or moon. For example, because the Moon pulls on opposite sides of the Earth with different strengths, water on the Earth is pulled either toward or away from the Moon, resulting in the ocean tides. Elsewhere in the Solar System, the magnitude of gravitational force on Jupiter's moon Io is greatest on the side of Io that faces Jupiter and least on the back side. The difference between the two forces compresses and stretches Io, which in turn heats up Io's interior. As a result, Io is the most geologically active body in the solar system. transit
1) The act of one celestial body passing in front of another. For example, when astronomers observe Mercury to be moving in front of the Sun, they say that Mercury is transiting the Sun. 2) The time at which a celestial object is highest in the sky. The time at which a celestial object crosses the meridian is called the transit time. At the transit time, the light from the object travels through the thinnest possible layer of Earth's atmosphere, making it the best time to view the object.
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