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oceanus literally "ocean"; really a large circular plain old a planetary surface that has been modified little since its formation typically featuring large numbers of impact craters (compare young ). Oort, Jan Hendrik 1900-1992 Dutch astronomer made major contributions to knowledge of the structure and rotation of our galaxy. More or less as a sideline, Oort studied comets as well. The result of this work was a theory, now widely accepted, that the Sun is surrounded by a distant cloud of comet-stuff, now called the Oort cloud. bits of which are occasionally hurled into the solar system as comets. ( more ) opposition A superior planet is said to be "in opposition" when it is directly on the opposite side of the Earth from the Sun. This is generally the closest it comes to the Earth and the time at which it is most easily visible. (nice diagram ) ovoid shaped like an egg

palus literally "swamp"; really a small plain parsec = 206265 AU = 3.26 light year patera shallow crater; scalloped, complex edge. penumbra literally, "dim light"; the outer filamentary region of a sunspot. perihelion the point in its orbit where a planet is closest to the Sun. when referring to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies. (opposite of aphelion ) Perrine, Charles Dillon 1867-1951 Argentine-American astronomer who discovered Himalia and Elara. perturb to cause a planet or satellite to deviate from a theoretically regular orbital motion. photosphere the visible surface of the Sun ; sunspots and faculae are observed in the photosphere. plage bright regions seen in the solar chromosphere. Piazzi, Giuseppe 1746-1826 Astronomer, born in Ponte di Valtellina, Italy. He became a Theatine monk, professor of theology in Rome (1779), and professor of mathematics at the Academy of Palermo (1780). He set up an observatory at Palermo in 1789, published a star catalog (1803, 1814) and discovered and named the first minor planet, Ceres. (more ) Pickering, William Henry 1858-1938 American astronomer. His photographs of Mars, among the earliest obtained, provided a basis for his opposition to Lowell 's observations of supposed canals on Mars. Discovered Phoebe. planet The recently adopted IAU resolution states that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:
  1. A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
  2. A "dwarf planet"

    is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2. (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

  3. All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
So by this official definition there are exactly eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto, Ceres and Eris (aka 2003 UB313) are "dwarf planets" with a potentially large number of additional objects falling into this category in the near future. planitia low plain. planum plateau or high plain. Pope, Alexander 1688-1744 English writer best remembered for his satirical mock-epic poems The Rape of the Lock and The Dunciad. prominence a strand of relatively cool gas in the solar corona which appears bright when seen at the edge of the Sun against the blackness of space. promontorium cape; headland Ptolemy 87-150 (aka Claudius Ptolemaeus) Alexandrian astronomer, mathematician, and geographer who based his astronomy on the belief that all heavenly bodies revolve around the Earth. (10k gif ; more )

red giant a star that has low surface temperature and a diameter that is large relative to the Sun. regio region. Relativity . Theory of more accurately describes the motions of bodies in strong gravitational fields or near the speed of light than newton ian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not specifically for Relativity but rather for his 1905 work on the photoelectric effect and "services to Theoretical Physics".) (see Spacetime Wrinkles. an excellent WWW site from NCSA) resolution the amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details resonance A state in which one orbiting object is subject to periodic gravitational perturbations by another. reticulum reticular (net-like) pattern retrograde rotation or orbital motion in a clockwise direction when viewed from above the north pole of the primary (i.e. in the opposite sense to most satellites); the opposite of direct. The north pole is the one on the same side of the ecliptic as the Earth's north pole. rift valley an elongated valley formed by the depression of a block of the planet's crust between two faults or groups of faults of approximately parallel strike. rima fissure. Roche limit the closest a fluid body can orbit to its primary without being pulled apart by tidal forces. A solid body may survive within the Roche limit if the tidal forces do not exceed its structural strength. The Roche limit is calculated with the equation

RL = 2.456*R*(p'/p)^(1/3)

Source: nineplanets.org

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