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PALLAS Pallas is the second-biggest asteroid and one of the four brightest asteroids. Its dimensions are: 570 x 525 x 482 km and its mass is 3.18 x 1020 kg. Pallas is about 4.145 x 108 km from the Sun and takes 4.61 years to orbit the Sun once (Pallas' year). Pallas was discovered by Heinrich Olbers in 1802.

PALOMAR OBSERVATORY Palomar Mountain Observatory is an astronomical observatory located on Palomar Mountain in southern California, USA. It has a 200-in. (508-cm) reflecting telescope and a 48-in. (122-cm) Schmidt telescope. It was founded by the astronomer George E. Hale.

PAN Pan is one of the smaller of the many moons of Saturn. Pan is located within the Encke Gap in Saturn's rings. Pan has a radius of about 10 km. Pan orbits at 133,583 km from Saturn. Its orbital period is 0.575 earth days. Pan was discovered by Mark R. Showalter/Voyager 2 in 1990.

PANDORA Pandora is one of the smaller of the 18 moons of Saturn. Pandora is a shepherd satellite for the outer edge of Saturn's F Ring. It has a radius of about 55x44x31 km. Pandora orbits at 141,700 km from Saturn. Its orbital period is 0.62 earth days. Pandora was discovered by S. A. Collins and D. Carlson/Voyager 1 in 1980.

PANGAEA (pronounced pan-JEE-ah) Pangaea was a supercontinent consisting of all of Earth's land masses. It existed during the Permian period through the Jurassic period. It began breaking up during the Jurassic period, forming the continents Gondwanaland (South America, Africa, India, Australia, and Antarctica) and Laurasia (North America, Europe, Asia, Greenland, and Iceland).

PARABOLA A parabola is a conic section, a curve that is a set of points (P) such that the distance from a line (the directrix) to P is equal to the distance from P to focus F. Parabolas have an eccentricity of 1.

PARALLACTIC MOTION parallactic motion is the apparent motion of the stars that is caused only by the motion of the Earth as it orbits the Sun.

PARALLAX Parallax is the apparent change in the position of a star that is caused only by the motion of the Earth as it orbits the Sun. Friedrich Wilhelm Bessel first detected the parallax "motion" of a star in 1838 observing the star 61 Cygni (this was definitive proof that the Earth orbits the Sun, and not the other way around).

PARKES TELESCOPE Parkes Telescope is a 64 meter radiotelescope in New South Wales, Australia.

PARSEC A parsec is a unit of distance that is equal to 3.26 light-years or 3.085678 x1013kilometers. It is the distance at which a star would have a parallax of 1 second of arc.

PARTIAL ECLIPSE A partial eclipse happens when the moon partially blocks the sun or the Earth's shadow partially blocks our view of the moon.

PASIPHAE Pasiphae is Jupiter's fifteenth moon. Pasiphae is 22 miles (36 km) in diameter and orbits 14,600,000 miles (23,500,000 km) from Jupiter. Pasiphae has a mass of 2 x 1023kg. It orbits Jupiter in 735 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Pasiphae. Pasiphae was discovered by P. Melotte in 1908.

PATERA A patera is shallow crater with a complex, scalloped edge.

PAYLOAD The payload is the cargo carried on the Space Shuttle or other spacecraft.

PECULIAR GALAXY A peculiar galaxy is an irregular galaxy that has an abnormal shape (neither elliptical, spiral, nor lenticular) and/or has another unusual characteristic, like jets of gas spewing from the nucleus, unusual amounts of dust, low surface brightness, etc. They are probably formed as galaxies collide or have companion galaxies that influence them. Peculiar galaxies are generally smaller and optically dimmer than regular galaxies. They have the designation p or pec at the end of their name. In 1966, the astronomer Halton Arp published a catalogue of 338 of these galaxies in his "Atlas of Peculiar Galaxies." Arp suggested that peculiar galaxies create stars in intense bursts. Ring galaxies are a type of peculiar galaxy.

PEGASUS Pegasus (the flying horse) is a large northern hemisphere constellation. Delta Pegasi is shared with the constellation Andromeda. 51 Pegasi (51 Peg), off the Great Square of Pegasus, is a nearby, Sun-like star that has been found to have an orbiting planet.

PENUMBRA The penumbra is the outer, relatively light region of a sunspot shaped like an annulus (ring) surrounding the darker, cooler umbra.

PENUMBRA The penumbra is the area of partial shadow (compare to umbra).

PERIGEE For an object orbiting the Earth, the perigee is the point in each orbit which is closest to the Earth. The perigee varies a small amount from orbit to orbit. The closest perigee is called the proxigee.

PERIHELION The perihelion is a planet or comet's closest approach to the Sun. The Earth is at perihelion (the Earth is closest to the Sun) in January.

PERIOD OF REVOLUTION The period of revolution of an orbiting body is the time it takes for it to make one revolution around its primary body. The period of revolution for the Earth is one year.

PERIOD OF ROTATION The period of rotation of an orbiting body is the time it takes for it to make one rotation around its axis. The period of rotation for the Earth is one day.

PERMIAN PERIOD The Permian period preceded the Triassic period, when dinosaurs and mammals evolved. It lasted from 280 to 248 million years ago.

PERSEID METEOR SHOWER The Perseids are a meteor shower that occur each year from July 23-Aug. 22, with a maximum on Aug. 12. This meteor shower occurs each year as the Earth passes through the orbit of the comet Swift-Tuttle, and icy remnants of the comet burn up as they enter the Earth's atmosphere. This meteor shower has am average magnitude of 2.3. The meteors in this shower seem to emanate from the constellation Perseus (but they do not).

PERSEUS Perseus is a constellation in the Milky Way in the Northen Hemisphere. It is abbreviated Per. Perseus' major star is alpha Per, called Marfak or Algenib; it is an F5 supergiant. Beta Per is the eclipsing binary Algol. Perseus was a hero in Greek mythology who killed the monstrous Medusa. Each year in August, the Perseid meteor shower radiates from this constellation's northern part.

PERTURBATION A perturbation of the orbit of a planet or a satellite causes it to deviate from a simple, theoretically regular elliptical orbit. Orbital perturbations are caused by other celestial bodies (like nearby planets) exerting gravitational forces on the orbiter. Orbital perturbations can lead to exciting discoveries; Pluto was discovered because Uranus and Neptune had orbits that were known to be perturbed by some unknown object.

PHASES OF MATTER Matter can exist in four phases (solid, liquid, gas, and plasma) and a few other extreme phases, like critical fluids and degenerate gases. The phase diagram of water (above) shows its phase at various temperatures and pressures.

PHASES OF THE MOON As the moon circles the Earth, the shape of the moon appears to change; this is because different amounts of the illuminated part of the moon are facing us. The shape varies from a full moon (when the Earth is between the Sun and the moon) to a new moon (when the moon is between the sun and the Earth). The phases of the Moon include: the new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent, and new moon again.

PHOBOS Phobos (meaning "fear") is the larger of the two tiny moons of Mars. It is only 13.8 miles (22.2 km) across and has a mass of 1.08x1016. It orbits Mars at a mean distance of 5,600 miles (9,000 km). Its major feature is a large crater, named Stickney (Hall's wife's maiden name), which is 6.2 miles (10 km) wide. Phobos may be a captured asteroid. Phobos was discovered by A. Hall in 1877.

PHOEBE Phoebe is one of the smaller of the 18 moons of Saturn. Phoebe has an almost circular shape and a reddish color. It rotates on its axis every 9 hours, unlike the other moons of Saturn (except Hyperion) which always show the same face to Saturn. It has a radius of about 110 km and mass of 4 x 10 18kg. Phoebe orbits at 12,952,000 km from Saturn. Its orbital period is 550.48 earth days. Phoebe was discovered by W. Pickering in 1898.

PHOTOELECTRIC EFFECT The photoelectric effect is a phenomenon in which electrons are ejected from a the surface of a metal when light shines on the metal. Albert Einstein explined the photoelectric effect in 1921.

PHOTOEVAPORATION Photoevaporation is a phenomenon in which the intense light from hot stars causes gases to evaporate (boil away) into interstellar space. In the Eagle Nebula (pictured above), ultraviolet light boils off some of the hydrogen gas (H2), continually re-shaping the columns of the nebula.

PHOTOMETRY Photometry is the measurement of apparent magnitudes of astronomical objects, like stars.

PHOTON A photon is a quantum of electromagnetic energy. For example, light is composed of light.

PHOTOSPHERE The photosphere is the layer of the Sun which is visible to us. It consists of a plasma and has a temperature of about 10,800°F (6,000°C).

PIAZZI, GIUSEPPE Giuseppe Piazzi (1746-1826) was an Italian astronomer who discovered and named the first (and largest) asteroid, Ceres, on January 1, 1801. Asteroids are large, rocky objects that orbit the Sun in a belt between Mars and Jupiter.

PIONEER 10 Pioneer 10/11 were early NASA missions to explore the Solar System and beyond. Pioneer 10 was launched in 1972 and went to Jupiter. 29 years later, in 2001, signals were received from Pioneer 10 - it was over 7 billion miles from Earth heading towards the star Aldebaran (in the constellation Taurus).

PIONEER 11 Pioneer 10/11 were early NASA missions to explore the Solar System and beyond. Pioneer 11 launched in 1973, travelling to Jupiter, heading toward the constellation of Aquila. The last transmission from Pioneer 11 was received in 1995.

PISCES [Abbreviation: Psc] Pisces (the fish) is a faint equatorial constellation of the zodiac. Pisces is seen along the ecliptic between Aries and Aquarius. The brightest star in Pisces (Alpha Psc) is Al Rischa (or Alrisha), the second-brightest-star (Beta Psc) is Fum al Samakah. The spiral galaxy M74 is in Pisces.

PISTOL STAR The Pistol star is the largest-known star; it is the most massive and the brightest star. It is 10 million times brighter than our Sun and 100 times more massive. This star is at the center of the Pistol Nebula and created it by expelling tremendous amounts of gas in violent eruptions.

PLANE OF THE ECLIPTIC The plane of the ecliptic is the plane of our solar system. In the course of a year, the sun traces a path in the sky along the ecliptic. The Earth's axis is tilted at a 23.5° from the ecliptic (which causes the seasons).

PLANET A planet is a large celestial body that orbits a star and does not shine on its own. There are nine planets orbiting the sun in our solar system.

PLANETARIUM A planetarium is a room in which images of the stars, planets, and other celestial bodies are projected.

PLANETARY ALIGNMENT Planetary alignment is when many of the planets in the solar system are roughly in a line. A large planetary alignment occured on May 5, 2000, when Mercury, Venus, Earth, Mars, Jupiter, and Saturn will be (very roughly) lined up, with all the planets on the other side of the Earth from us. Also, the Moon was almost between the Earth and Sun. This type of alignment happens every few hundred years and has little or no significance.

PLANETARY NEBULA A planetary nebula is a nebula formed from by a shell of gas which was ejected from a certain kind of extremely hot star. As the giant star explodes, the core of the star is exposed. Planetary nebulae have nothing to do with planets. The Hourglass Nebula is a planetary nebula.

PLANETESIMAL A planetesimal is a small object that orbits the Sun. Planetesimals are thought to have formed when the Solar System itself formed, and they were perhaps the building blocks from which the planets were built.

PLANETESIMAL HYPOTHESIS The planetesimal hypothesis is a theory about the formation fo the Solar System. It was proposed by Thomas Chrowder Chamberlain (Sept. 25, 1843 - Nov. 15, 1928), an American geologist and teacher. In this theory, a star is supposed to have passed near the Sun, pulling matter away from the Sun. Later, this matter is to have condensed into larger masses, forming the planets.

PLANETOID A planetoid is a very small planet. The larger asteroids are sometimes called planetoids.

PLANETOLOGIST A planetologist is an astronomer who studies the physical features of planets.

PLANETOLOGY Planetology is a branch of astronomy that studies planets.

PLANET SYMBOLS Each of the planets in our Solar System has been given a symbol; the symbols are pictured above.

PLANET "X" Planet "X" was the temporary name given to the then-unknown planet beyond Neptune that disturbed the orbits of Uranus and Neptune. Percival Lowell calculated the rough location of Planet "X's" orbit, but died in 1916 before it was found. This planet was eventually found by C. Tombaugh in 1930 and named Pluto). Planet "X" is also the name given to any as-yet-undiscovered tenth planet. There may be another Planet "X" beyond Pluto (see Nemesis).

PLANITA A planita is a low plain on a planet's surface.

PLANUM A planum is a high plain (a plateau) on a planet's surface.

PLASMA A plasma is an extremely hot gas that is composed of free-floating ions (atomic nuclei stripped of some electrons - making the ions positively charged) and free electrons (negatively charged). A plasma behaves much differently than a neutral gas, and is considered the fourth state of matter. A plasma conducts electrical currents. Stars are composed of plasma.

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 and denser than continental plates.

Forward Backward PLATE TECTONICS Plate tectonics is the now-established theory that chunks of the Earth's crust (plates) float on the surface and change both position and size over time.

PLEIADES The Pleiades (M45), also known as the "Seven Sisters" (and "Subaru" in Japan), is the brightest open cluster of stars in the sky. It is located in the constellation Taurus. Six of the stars in the Pleiades are visible without a telescope. Roughly 500 stars belong to the Pleiades star cluster; most of these are faint. A reflection nebula surrounds the stars.

PLUTINO A Plutino (meaning "little Pluto") is a main Kuiper belt objects, ones that have a 2:3 resonance with Neptune (as Pluto is); a 2:3 resonace means that the Plutino completes 2 orbits around the Sun in the same time that it takes Neptune to complete 3 orbits around the Sun. About one-quarter of the Kuiper Belt objects (also called transneptunian objects) are Plutinos. Compare with cubewano.

PLUTO Pluto is the ninth and (usually) the farthest planet from the Sun. It has a highly inclined orbit. This small, cold, rocky planet has one moon, Charon.

POLARIS Polaris (alpha UMi) is the current pole star for the Northern Hemisphere; it is 1 degree from the exact Northern celestial pole. In 1780, Sir William Herschel discovered that Polaris was a double star with a faint companion star. Polaris is a blue-green Cepheid variable star (its size brightness changes periodically, with period of 3.969778 days; it varyies between mag 1.92 and 2.07). Polaris has a relatively dim companion star (9th magnitude). Polaris' distance from Earth has been estimated to be from 360 to 820 light years. At its brightest, Polairs is about 6,000 to 10,000 times brighter than our Sun. It is the larger star at the end of the handle of the Little Dipper (Ursa Minor). Polaris is also called the Lodestar or the Cynosure.

POLAR SCOPE A polar scope is a small telescope that is used for aligning a larger telescope with the North or South pole star.

POLE STAR A pole star is a star that is located almost due north or due south and is useful for navigation. Polaris is the pole star of the Northern Hemisphere.

POLIAKOV, VALERI Valeri V. Poliakov is a Russian physician and cosmonaut who broke many space endurance records, and holds the second-longest record for being in space: 678 days over two flights (Sergei Avdeyev beat his record in 1999, having been in space for 758 days). Dr. Poliakov holds the record for the single longest stay in space, 437 days, 17 hours, 58 minutes. Poliakov was on the Russian space station MIR missions Soyuz TM6 and TM7 (240 days, 22 hours, 26 minutes in 1988), and Soyuz TM18 (437 days, 17 hours, 58 minutes in 1994-1995).

POPULATION 1 STARS Population 1 stars are relatively young stars. They are found mainly in the disk of galaxies.

POPULATION 2 STARS Population 2 stars are relatively old stars. They are found mainly in the halo of galaxies.

PORE A pore is a small sunspot that doesn't have a penumbra. Pores are up to about 1,500 miles (2,500 km) across and are lighter than a sunspot's umbra.

POSITRON A positron (also called the positive electron or the antielectron) is an elementary particle; it is the antiparticle of the electron. It has the same mass and spin as an electron, but has a positive charge (equal in magnitude to the charge of an electron).

PRECESSION Precession is the small wobbling motion around the Earth's axis that the Earth makes as it spins (just like the wobblin.g motion of a spinning top). This causes the Earth's axis to point in a slightly different direction over time; Polaris is our pole star now, but this is not permanent because of precession. Precession is caused by the gravitational influences of the Sun and the Moon and has a period of 25,800 years.

PRECIPITATION Precipitation is rain, sleet, hail, snow, or other condensation products falling from the atmosphere. Precipitation on Earth is is key in the water cycle. Precipitation on Venus is sulphuric acid rain.

PRIMARY A primary is a planetary body around which a moon or satellite orbits.

PRIME VERTICAL The prime vertical is a great circle on the celestial sphere that passes through the zenith. It is a vertical circle that is bisected by the meridian. Degrees run from east to west.

PROBE A probe is an unmanned spacecraft that is sent to collect data from space. Some probes return to Earth, others are not meant to.

PROCYON Procyon (also called Alpha Canis Minoris, Antecanis, and the Little Dog Star) is the brightest star in the constellation Canis Minor and the eighth brightest star in the sky. This yellowish star is also part of the Winter Triangle. Procyon is a relatively close star; it is only about 11.4 light years from us. Procyon is about as seven times more luminous than our Sun.This sub-giant star is near the end of its life; it's hydrogen core is almost spent (it has been converted into helium). Procyon has a companion, a bersly-visible white dwarf called Procyon B (it is only about the size of the Earth and is about 14.9 astronomical units from Proycon A), and this small star makes Procyon wobble from our perspective. The existence of Procyon B was predicted in 1840 by Arthur Julius Georg Friedrich von Auswers (1838-1915) but wasn't seen until 1896 (by John M. Schaeberle (1853-1924)).

PROMETHEUS Prometheus is one of the smaller of the 18 moons of Saturn. Prometheus is a shepherd satellite for the inner edge of Saturn's F Ring. It has a radius of about 74x50x34 km. Prometheus orbits at 139,353 km from Saturn. Its orbital period is 0.613 earth days. Prometheus was discovered by S. A. Collins and D. Carlson/Voyager 1 in 1980.

PROMINENCE A solar prominence is an cloud-like arc of gas that erupts from the surface of the Sun. Prominences can loop hundreds of thousands of miles into space.

PROPER MOTION Proper motion is the actual motion of a star across the sky (not toward or away from the Earth). This motion is due to the orbit of the star in the Milky Way Galaxy. The proper motions of a star is the distance that it moves across the sky each year. This distance is so tiny, that it is measured in arcseconds per year. The star with the most proper motion is Barnard's Star (the star system second-closest to us), which moves only10 arcseconds per year (1/180 of the apparent width of the full moon).

PROPLYDS Proplyds (short for protoplanetary disks) are disks of dust and gas in space that surround newborn stars. They appear as fuzzy blobs, even when seen through the Hubble Space Telescope. Proplyds may be very young solar system, consisting of a rotating disk of gas that will eventually coalesce into a solar system.

PROTEUS Proteus is the second-largest moon of Neptune. It was discovered using NASA's Voyager 2 mission in 1989. It orbits 117,650 km from the center of Neptune and is about 218 x 208 x 201 km in size. This dark moon has a circular orbit just over the cloud-tops of Neptune. Proteus is dotted with impact craters and is irregularly shaped

PROTOPLANETARY DISK A protoplanetary disk is a rotating disk of dust that surrounds the central core of a developing solar system. This disk eventually coalesces into planets that orbit the star (which forms from the central core).

PROTON Protons are small, positively-charged atomic particles. In an atom, protons are in the nucleus, surrounded by electrons. Protons have a mass similar to the neutron. The proton was discovered by Rutherford in 1919 (after the discovery of the electron and before the discovery of the neutron).

PROTON-PROTON CHAIN The proton-proton chain is a hypothetical fusion reaction thought to occur deep inside stars. This chain reaction converts hydrogen nuclei into helium nuclei and releases energy (particles) and gamma-rays. This thermonuclear reaction is thought to take on the order of a billion years to complete, and occurs under tremendously high pressure and temperature.

PROTOSTAR A protostar is a star that is still forming and nuclear fusion has not yet begun.

PROTOSUN A protosun is a star that is still forming, during the early stages of a solar system. The protosun is the central material, which will eventually become a sun (a star), when nuclear fusion begins.

PROVISIONAL NAME A provisional name is a temporary name gicen to an object, like a newly-discovered asteroid or a moon.

PROXIGEAN SPRING TIDE The Proxigean Spring Tide (also called the proxigean tide) is a rare, unusually high tide. This very high tide occurs when the moon is both unusually close to the Earth (at its closest perigee, called the proxigee) and in the New Moon phase (when the Moon is between the Sun and the Earth). The proxigean spring tide occurs at most once every 1.5 years.

PROXIGEE For an object orbiting the Earth, the proxigee is the closest perigee. The perigee is the point in each elliptical orbit which is closest to the Earth, but the perigee varies a small amount from orbit to orbit.

PROXIMA CENTAURI The closest star to us is the Sun. Other than that, the closest star is Proxima Centauri, aka Alpha Centauri C (the dimmest star in the Alpha centauri system). Proxima Centauri is about 4.2 light-years from the Sun. It has an absolute magnitude of 15.5 and an apparent magnitude of +11.05 (variable). Spectral type M5.5Vc.

PTOLEMY Claudius Ptolemaeus or Ptolemy (about 87-150) was a Greek astronomer and mathematician who wrote about his belief that all celestial bodies revolved around the Earth. His writings influenced people's ideas about the universe for over a thousand years, until the Copernican System (with a heliocentric solar system) was accepted.

PTOLEMAIC SYSTEM The Ptolemaic System is an outdated view of the solar system written about by Ptolemy (about 87-150) in his major work, Almagest (Mathematical Syntaxis). Ptolemy believed that all the planets and the sun orbited around the Earth in the order: Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. He also devised a system of circular orbits within orbits (called epicycles) to explain the way the planets orbited around the Earth. It was not until the 1500's when the Copernican System (with a heliocentric solar system) was accepted.

PUCK Puck is one of the 18 moons of Uranus. Its diameter is about 96 miles (154 km) and its mass is unknown. It orbits at a distance of about 53,500 miles (86,010 km) from Uranus. Puck was discovered by NASA's Voyager 2 in 1985.

PULSAR A pulsar is a rapidly spinning neutron star that emits energy in pulses. Pulsars were discovered in 1967 by S. Jocelyn Bell Burnell (1943- ), who was a Cambridge University astronomy graduate student at the time. Her graduate advisor (Anthony Hewish) was given a share of the 1974 Nobel Prize, but Bell was ignored. No one had any idea what these unusual objects were at the time, so the name little green men (LGM) was used. Soon, Thomas Gold suggested that pulsars were rapidly-spinning neutron stars, the remnants of a supernova.

PYTHAGORAS Pythagoras of Samos (569-475 BC) was a Greek philosopher, mathematician, and astronomer who founded a philosophical and religious school, the Pythagorean school in Croton, Italy. Pythagoras believed that the Earth was a sphere at the center of the Universe. He correctly realized that the morning star and the evening star were the same object, the planet Venus. Pythagoras (or the Pythagoreans) made a number of fundamental mathematical discoveries: that for a right triangle, the sum of the squares of the two shorter sides is equal to the square of the hypotenuse (known as the Pythagorean theorem); that the sum of the angles of a triangle is equal to two right angles; and that irrational numbers exist. A 142 km wide lunar crater was named for Pythagoras (Latitude 63.5°, Longitude 63.0°).