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parallax
the apparent shift of a relatively nearby object against a fixed background due to the motion of the observer; astronomers observe the parallax of stars to measure their distances
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parsec
the distance an object would have to be from Earth so that its parallax when viewed from two points separated by 1 AU would be one arcsecond; equal to 3.26 light-years.
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patera
an irregular, saucer-shaped volcanic structure
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penumbra
1. the region of a shadow from which part of the light source remains visible
2. the lighter region of a sunspot surrounding its dark center (umbra)
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penumbral eclipse
an event that occurs when the moon passes into the outer ring of Earth's shadow (penumbra), causing a slight shading in the moon's appearance
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periastron
the point in an object's orbit at which it is closest to the star it orbits
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perigee
the point in a satellite’s orbit when it is closest to Earth
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perihelion
the point in an object’s orbit when it’s closest to the sun
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period
the time interval for a regular event to take place
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periodic comet
a comet that has been seen to orbit the sun more than once (Comet Halley was the first recognized as periodic)
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phase
the regular cycle of changes in the appearance of a moon or planet
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photometer
a detector that measures the amount of light coming from an object
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photometry
the measurement of light intensities
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photons
individual “particles/waves” of light
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photosphere
the visible surface of the sun
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photovoltaic
conversion of light energy into electricity
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pixel
short for “picture element,” the individual light detectors on a CCD chip
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planet
a large rocky or gaseous body that orbits a star
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planetary nebula
a glowing shell of gas ejected by a dying, low-mass star
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planetary nebulae
glowing shells of gas ejected by dying, low-mass stars
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planetesimals
asteroid-size bodies in a young planetary system that collide to form larger bodies
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planisphere
a two-dimensional map of the sky with an adjustable overlay to show the part of the sky visible at any time of the night or year
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plasma
a gas that has been heated to a state where it contains ions and free-floating electrons; also known as ionized gas
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plasmasphere
a region of cold, high-density plasma above the ionosphere
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plate tectonics
a theory that describes how Earth’s crust is broken into plates and how those plates move across Earth’s surface
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polar cap
an icy region at the north or south pole of a planet
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polarization
a state in which the directions of the electric or magnetic field in an electromagnetic wave changes in a regular pattern; light from celestial objects is often polarized
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polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs) are a class of very stable organic molecules. They are flat molecules made only of carbon and hydrogen atoms. PAH molecules are quite common and highly carcinogenic. They are one of the by-products of combustion from automobiles and airplanes, and some are present in charcoal broiled hamburgers.
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position angle
the direction in the sky of one celestial object from another, measured eastward from due north
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power
the ability of a telescope or binoculars to increase the apparent size of a distant object
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precession
the slow, periodic change in the direction an object’s rotational axis caused by the gravitational influence of another body
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primary lens
a telescope's main lens which gathers light and brings it to a focus
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primary mirror
a telescope's main mirror which gathers light and brings it to a focus
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prime meridian
the line of longitude that runs through Greenwich, England
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prism
a wedge-shaped piece of glass that breaks white light into its constituent colors
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prograde
objects that move or appear to move in the same direction of most solar system bodies, or for moons, the same direction as the planet rotates
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prominence
a large eruption of gas streaming off the surface of the sun into the corona
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proper motion
the apparent yearly motion of a star across the sky
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proton
a subatomic particle that resides in an atom’s nucleus and possesses a positive electric charge
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protoplanet
a body that is accreting gas, dust, and rocks en route to becoming a full-fledged planet
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protoplanetary disk
a disk of gas and dust that surrounds a newborn star; planets form from collisions of particles inside the disk
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protostar
a cloud of hot, dense gas and dust that is gravitationally collapsing to form a star
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pulsar
a rapidly rotating neutron star that bathes Earth in regular pulses of electromagnetic radiation
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