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satellite
a small body that orbits a planet or asteroid
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secondary mirror
a relatively small mirror used in a telescope to redirect the light gathered by the primary mirror
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seeing
the quality of observing conditions induced by turbulence in Earth’s atmosphere, which blurs the images of astronomical objects
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semimajor axis
the average distance of an orbiting body from its parent body
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setting circles
circular scales on the two axes of an equatorial mount that help an observer point a telescope to a specific right ascension and declination
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shock wave
a powerful wave caused by a sudden change in density, pressure, or temperature that travels though a medium faster than sound travels through that same medium
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short-period comet
Comets that have orbital periods of less than 200 years.
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sidereal
relating to or measured with respect to the stars
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sidereal year
the amount of time it takes one body to revolve about another with respect to the stars
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siderostat
a flat mirror that can be moved to reflect light from a celestial object to a specific spot
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singularity
a point at which space and time are infinitely distorted, such as the central point of a black hole where matter is concentrated into an area of zero volume and infinite density
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solar eclipse
an eclipse of the sun caused by the moon passing between Earth and the sun
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solar filter
a filter used to block almost all of the sun’s light so our star can be viewed safely and comfortably
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solar irradiance
the radiant energy emitted by the sun over all wavelengths that falls each second on one square meter of Earth's atmosphere
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solar mass
the amount of mass contained in the sun, about 330,000 times that of Earth’s mass
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solar system
the system containing the sun and all the smaller bodies in orbit around it
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solar wind
the stream of charged subatomic particles emanating from the sun
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solstice
either of the two points on the celestial sphere where the sun is farthest north or south of the celestial equator; when the sun is at a solstice, the amount of daylight hours is greatest for summer and least for winter
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space weathering
the process of altering the surface of an object in space by such phenomena as micrometeoroid impacts, cosmic rays, and the solar wind
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space-time
the intertwining of the three dimensions of space with one dimension of time within which events can be specified exactly
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special relativity
the theory of relativity governing uniform motion; it states the equivalence of mass and energy and differs from Newtonian physics only when speeds approach that of light
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spectra
plural of "spectrum" (the energy emitted by a radiant source)
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spectral class
the designation of a star based on its spectrum, which is determined by its surface temperature
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spectral line
a particular wavelength of light corresponding to the energy transition of a specific atom or molecule
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spectral type
the designation of a star based on its spectrum, which is determined by its surface temperature
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spectrograph
an instrument attached to a telescope to record the spectrum of an astronomical object
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spectroheliograph
a device for photographing the sun in a single wavelength of light
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spectrometer
an instrument attached to a telescope to record the spectrum of an astronomical object
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spectroscope
an instrument for examining spectra
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spectroscopy
the study of spectra from astronomical objects
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spectrum
1. the energy emitted by a radiant source 2. the entire range of electromagnetic radiation (light)
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speed of light
the fastest possible speed in a vacuum, equivalent to 186,000 miles per second (300,000 km per second)
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spiral arm
a concentration of gas, dust, and young stars that winds its way out from the nuclear region of a spiral galaxy
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spiral galaxy
a spiral-shaped system of billions of stars, gas clouds, and dust
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standard candle
an astronomical object of known luminosity; can be used to determine distances
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star
a self-luminous sphere of hot gas held together by gravity; ordinary stars generate energy by nuclear fusion in their cores
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star atlas
an collection of maps that marks the positions of stars, nebulae, galaxies, and other astronomical objects on a coordinate system
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star hopping
the technique of using recognizable patterns of stars to “hop” from one part of the sky to another; useful in observing both with the naked eye and a telescope
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star party
a gathering of people to observe the night sky
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starburst galaxy
a galaxy undergoing an extremely high rate of star formation
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stellar evolution
the life cycle of stars
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stellar wind
a stream of electrically charged subatomic particles given off by stars
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stereocomparator
An instrument that allows astronomers to view two images of the same region of sky simultaneously. Objects that have changed their brightness or position appear to stand out of the plane of the picture.
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sublimate
the transition of a solid substance evaporating into a gas without passing through a liquid phase
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sublimated
the transition of a solid substance evaporating into a gas without passing through a liquid phase
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sublimation
the process by which a solid substance evaporates into a gas without passing through a liquid phase
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summer
a season that begins around June 21 in the Northern Hemisphere
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sunspot
a dark, temporary, relatively cool spot on the surface of the sun
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sunspot cycle
a cycle averaging 11 years in which the number of sunspots increases and decreases
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supercluster
an enormous congregation of galaxy clusters that stretches across hundreds of millions of light-years
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superfluid
an unusual state of matter characterized by apparently frictionless flow, found only in liquid helium cooled to near absolute zero
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superior conjunction
the configuration of an inferior planet when it lies on the far side of the sun
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superior planet
a planet farther from the sun than Earth; includes Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto
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superluminal motion
motion that appears to be faster than the speed of light
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supermassive black hole
a black hole at the core of a galaxy that contains millions or billions of solar masses
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supernova
the cataclysmic explosion of a star
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supernova remnant
an expanding cloud of gas that represents the outer layers of an exploded star
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synchronous rotation
when a satellite rotates at the same rate at which it revolves around a more massive object; a body with synchronous rotation shows only one hemisphere to the object it orbits
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synchrotron emission
electromagnetic radiation from high-energy electrons moving in a magnetic field
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