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OBERON Oberon is one of the larger of the 18 moons of Uranus. Oberon is covered by many craters (indicating an old surface) and ice. Oberon has a diameter of 1,523 km and a mass of 3.03x1021 kg. It orbits Uranus at an average of 582,600 km. Oberon was discovered by Wm. Herschel in 1787.

OBJECT-GLASS An object-glass is the main lens of a refracting telescope.

OBJECTIVE The objective is the light-gathering lens (or mirror) of a telescope.

OBLATE An oblate sphere is one that is flattened at its poles. Saturn is the most oblate planet in our Solar System; the difference in its equatorial and polar diameters is almost 10%.

Planet Obliquity to the Ecliptic (Axial Tilt) Mercury 2° Venus 2° Earth 23.45° Mars 24° Jupiter 3.1° Saturn 26.7° Uranus 97.9° Neptune 28.8° Pluto 57.5° OBLIQUITY Obliquity is the angle between the plane of a planet's orbit and that of the planet's equator.

OBSERVATORY An observatory is a place set up with a device (or devices) for observing astronomical or meteorological phenomenon, like stars, planets, nebula, etc. Observatories often have powerful telescopes (visual, radio, or other types).

OCCULTATION Occultation is when a smaller astronomical body passes behind a larger astronomical body (wholly obscuring its view). One example of occultation is when a planet passes behind the Sun (from our perspective) and it is hidden from our view. (see transit)

OCEANIC PLATES The crust of the Earth is broken into plates. The plates are enormous chunks of rock that float atop the soft mantle. The plates are moving at a speed that has been estimated at 1 to 10 cm per year. Oceanic plates (those that are under the ocean) are thinner, younger, and denser than continental plates. These underwater plates are about 75 kilometers thick and are made of basalt rock. They are relatively young since plate formation (seafloor spreading) occurs at the margins of oceanic plates.

OLBERS, HEINRICH Heinrich Wilhelm Matthäus Olbers (1758-1840) was a German astronomer and physician who published Olbers' paradox (Why is the sky dark at night? or Why doesn't starlight make the night sky bright?) (1823), determined that Uranus is a planet, not a comet (1781), discovered Olbers's comet (1815), the asteroids #2 Pallas (1802) and #4 Vesta (1807), and formulated a method for calculating comet orbits.

OLBERS' PARADOX The Olbers' Paradox is the seemingly simple question "Why is the sky dark at night? or Why doesn't starlight make the night sky bright?" If the universe is infinitely old and it is filled with stars, then there would be a star in any direction you look in, making the sky bright (day and night). This paradox was originally formulated by Wm. Halley and later published by H. Olbers. The solution lies both in the expansion of the universe (which red-shifts the incoming starlight, reducing the visible light) and the non-infinite age of the universe.

OMEGA Omega (the last letter in the Greek alphabet) refers to ratio of the observed density of the Universe (how much mass there is per unit volume) to the critical density of the Universe (the density that would be necessary to stop the expansion of the Universe). If omega is greater than one, then the Universe will eventually stop expanding and begin to contract, and the Universe will end in a "Big Crunch." If omega is equal to one, the Universe will eventually stop expanding but will not collapse. If omega is less than one, the Universe will continue expanding.

ONIZUKA, ELLISON S. Ellison Shoji Onizuka (June 24, 1946 - January 28, 1986) was a Lieutenant Colonel in the United States Air Force, an aerospace engineer, and a NASA astronaut. Onizuka flew on two Space Shuttle missions; he died in the Challenger explosion on January 28, 1986.

OORT CLOUD The Oort Cloud is a cloud of rocks and dust that may surround our solar system. This cloud may be where long-period comets originate. The Oort Cloud was named for Jan H. Oort, who proposed its existence in 1950. It has been hypothesized that the Oort Cloud is responsible for the periodic mass extinctions on Earth.

OORT, JAN H. Jan Hendrik Oort (1900-1992) was a Dutch astronomer who calculated the distance to the middle of the Milky Way galaxy, mapped our galaxy, proved that the areas around the center of a galaxy revolves, and proposed the existence of the Oort Cloud in the 1950's. The Oort Cloud is a cloud of rocks and dust that may surround our solar system. This cloud may be where long-period comets originate. It has been hypothesized that the Oort Cloud is responsible for the periodic mass extinctions on Earth.

OPEN CLUSTER An open cluster is a loose collection of up to about 1,000 relatively young stars that formed around the same time. An open cluster is about 10 parsecs across. Examples include the Pleiades and Hyades.

OPEN UNIVERSE An open universe is a model of the universe in which it will expand forever. In this model, there isn't enough matter (and its accompanying gravitational forces) to stop the current expansion. Consequently, space and time are infinite in this universe. Contrast with a closed universe.

OPPOSITION A planet is in opposition when the Earth is exactly between that planet and the sun. Mercury and Venus can not be in opposition.

OPTICAL TUBE The optical tube is the main body or tube of a telescope. This optical tube holds the objective.

ORBIT An orbit is a closed path that an object takes as it revolves around another body. Orbits are generally elliptical, but may be perturbed by the presence of yet other bodies and may even form unusual figures.

Planet Orbital Eccentricity Mercury 0.206 Venus 0.007 Earth 0.017 Mars 0.093 Jupiter 0.048 Saturn 0.056 Uranus 0.047 Neptune 0.009 Pluto 0.248 ORBITAL ECCENTRICITY Eccentricity is a measure of how an orbit deviates from circular. A perfectly circular orbit has an eccentricity of zero; higher numbers indicate more elliptical orbits. Parabolas have an eccentricity of 1. Neptune, Venus, and Earth are the planets with the least eccentric orbits in our solar system. Pluto and Mercury are the planets with the most eccentric orbits in our solar system.

ORBITAL INCLINATION Orbital inclination is the angle between the plane of an orbit and the plane of the ecliptic. Orbital inclination is abbreviated as "i".

ORBITAL SPEED As the planets orbit the Sun, they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun (this is Kepler's Second Law of Planetary Motion).

ORION Orion, also known as "The Hunter," is a constellation. The brightest stars in Orion are Rigel. Betelgeuse, and Bellatrix. The Horsehead Nebula and the nebulae M42 and M43 (called the Orion nebula) are also in this constellation.

ORION ARM The Orion Arm (also called the Local Arm) is the arm of the Milky Way Galaxy where our solar system is located.

ORIONID METEOR SHOWER The Orionids are a meteor shower that occur each year from Oct. 15-29, with a maximum on Oct. 21-22. This meteor shower occurs each year as the Earth passes through the orbit of Halley's comet, 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 Orion, (but they do not).

ORION NEBULA The Orion Nebula (M42 and M43) is a huge, nearby, turbulent gas cloud (mostly hydrogen) that is lit up by bright, young hot stars (including the asterism called Trapezium) that are developing within the nebula. This nebula is located about 1,500 light-years away from us towards the constellation of Orion. The Orion Nebula is roughly 30 light-years in diameter.

OUTER PLANETS The outer planets are those planets that orbit far from the Sun. They are: Jupiter, Saturn, Uranus, Neptune, and Pluto. They are mostly huge, mostly gaseous, ringed, and have many moons ( the exception is Pluto which is small, rocky, and has one moon, Charon).

OZONE Ozone is a form of molecular oxygen (O3); it consists of three connected oxygen atoms.

OZONE LAYER The ozone layer is a region of the stratosphere which contains most (about 90%) of the Earth's atmospheric ozone. It is about 10-25 miles (15-40 km) above the Earth's surface. The ozone layer shields the Earth from Ultraviolet B rays that come from the Sun. The ozone layer is becoming depleted, and there is an "ozone hole" over Antarctica.