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Parsec
P
Parsec
A unit of distance in space. It is equal to 3.26 light-years.
Penumbra
The outer region of the shadow cast during an eclipse, producing a partial eclipse.
Periapsis
The point in an orbit closest to the planet. (See also: Apse.)
Perigee
The point in an orbit closest to the Earth.
Perihelion
The point in an orbit at which a planet is closest to the Sun.
Phobos
One of the two moons of Mars.
Photosphere
The visible surface of the Sun or any other star. The convectional rising and falling of hot gases in the photosphere makes the surface look like grains of rice.
Pioneer
A series of NASA space probes that began in 1958 to study the Moon. The last ones were sent
to look at the distant planets and provide information about their environment before the Voyager probes were launched. Most of these probes are currently heading out into deep space.
Planet
A body in space that does not give out light, and that orbits a star. There is no hard-and-fast rule about how small a planet can be; Pluto is only a little bigger
than some asteroids, also known as minor planets. (See also: Earth; Mars; Jupiter; Mercury; Neptune; Saturn; Uranus; Venus.)
Planetesimal
A small rocky mass fewer than 100km across that was one of a vast number that formed in the early stages of the Solar System. As planetesimals came together (by accretion), they formed planets.
Plasma
A low-density gas that contains charged atoms.
Polaris, Pole star
The North Pole star, also called Alpha Ursa Minoris. It is a supergiant star 820 light-years from the Earth. It lies about 1° above the geographical North Pole. The South Pole star is Sigma Octanis.
Precession
The wobbling motion of a body
as it follows its orbit. The Earth wobbles as a result of the combined gravitational effects of the Sun, Moon, and other planets.
Pluto
Pluto (diameter 2,300km) is
the ninth planet from the Sun. Pluto orbits the Sun once every 248 years at a distance of 5.9 billion km, marking the very edge of the Solar System.
It is the smallest planet and quite unlike the neighboring gas giant planets. Some scientists think it is really no more than a large asteroid. Because of its distance from the Earth and its small size, it was not discovered until 1930.
Pluto is a frozen world with
an atmosphere of methane gas enveloping a cold rocky or possibly ice core. (See also: Voyager.)
Rock, nitrogen, methane, and ice
 Pluto is so small and distant that little is known about it for certain.
 Pluto on the left and its moon Charon
on the right, taken by NASA’s Hubble Space Telescope. Pluto was 2.6 billion miles away from Earth when this image was taken. Charon was not discovered until 1978.
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