AstroGlossary - O
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
objective lens
The primary light gathering optic of a refraction telescope, located opposite of the eyepiece. observable universe
All that the largest telescope can see. The cosmological theory of inflation suggests that very early on in the history of the universe, just fractions of a second after the Big Bang, the universe went through a period when it was effectively expanding faster than the speed of light. As a result, there may be regions which are within our universe, but are so far away that light emitted there just hasn't had enough time to reach the Earth yet. observatory
The entire complex of buildings, telescopes, equipment, and staff involving scientific astronomical observations. Small observatories might consist of just one telescope situated in a dome, one instrument and computer, and one observer. McDonald Observatory is an example of a major observatory, and consists of five major telescope in five different domes, numerous instruments, computers, living quarters, outbuildings, astronomers, and engineers. occultation
An eclipse of a celestial object by the Moon or another solar system body. Olber's paradox
An apparent disagreement between cosmological assumptions and observation first noted by German astronomer Heinrich Olbers in 1826: if the universe is infinite and static, as Newton had declared, every line of sight leading from the Earth should intercept the surface of a star. Therefore, the sky should always be bright. However, we observe that the night sky is dark. This paradox is resolved if the universe is expanding and evolving. Oort Cloud
A halo of debris left over from the formation of the Sun and planets that surrounds our solar sSystem. The Oort Cloud has a diameter of about three or four light-years and contains 100 billion to 1 trillion fossils of solar system formation. It is likely that some comets come from this cloud. opposition
A planet's position when it is 180 degrees east or west (i.e., directly opposite) from the Sun in the sky. orbit
The motion of a massive body around another body, governed by the force of gravity. Planets in our solar system follow an orbit around the Sun, as first noted by Johannes Kepler, in the shape of an ellipse. An orbit is actually composed of two motions: one directly toward the other body (planet or star...) and the another that points away. In the case of a circular orbit, these two component motions are orthogonal (90 degrees apart, or at right angles). Einstein would say that an orbit is actually a geodisc in space -- time, formed by the interaction between space and the mass of the two bodies. orbital period
The duration of one orbit. For instance, Earth's orbital period around the Sun is one year, and the Moon orbits Earth in 27.3 days. orbital plane
A plane containing at least two masses orbiting around a common center of gravity. Earth and Sun orbit each other in an orbital plane, as do Earth and Moon. However, the Earth-Moon orbital plane crosses the Earth-Sun orbital plane at a five-degree angle. See also: orbital periodouter planets
The planets in the solar system that orbit the Sun beyond the orbit of Mars: Jupiter, Saturn, Uranus, and Neptune.
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