P

Parallax

The difference in apparent direction of an object as seen from two different locations; conversely, the angle at the object that is subtended by the line joining two designated points. (See parallax, horizontal.)

 

Parallax, annual

See parallax, heliocentric.

 

Parallax, diurnal

See parallax, geocentric.

 

Parallax, geocentric

The angular difference between the topocentric and geocentric directions toward an object.

 

Parallax, heliocentric

The angular difference between the geocentric and heliocentric directions toward an object; it is the angle subtended at the observed object.

 

Parallax, horizontal

The angular difference between the topocentric and a geocentric direction toward an object when the object is on the astronomical horizon.

 

Parallax in altitude

The angular difference between the topocentric and geocentric direction toward an object when the object is at a given altitude.

 

Parsec

The distance at which one astronomical unit (AU) subtends an angle of one second of arc; equivalently the distance to an object having an annual parallax of one second of arc. One parsec is 1/sin (1″) = 206264.806 AU, or about 3.26 light-years.

 

Penumbra

1. The portion of a shadow in which light from an extended source is partially but not completely cut off by an intervening body. 2. The area of partial shadow surrounding the umbra.

 

Pericenter

The point in an orbit that is nearest to the origin of the reference system. (See perigee; perihelion.)

pericenter, argument of:

one of the Keplerian elements. It is the angle measured in the orbit plane from the ascending node of a reference plane (usually the ecliptic) to the pericenter.

 

Perigee

The point in an orbit that is nearest to the Earth. Perigee is sometimes used with reference to the apparent orbit of the Sun around the Earth.

 

Perihelion

The point in an orbit that is nearest to the Sun.

 

Period

The interval of time required to complete one revolution in an orbit or one cycle of a periodic phenomenon, such as a cycle of phases. (See phase.)

 

Perturbations

1. Deviations between the actual orbit of a celestial body and an assumed reference orbit. 2. The forces that cause deviations between the actual and reference orbits. Perturbations, according to the first meaning, are usually calculated as quantities to be added to the coordinates of the reference orbit to obtain the precise coordinates.

 

Phase

1. The name applied to the apparent degree of illumination of the disk of the Moon or a planet as seen from Earth (cresent, gibbous, full, etc.). 2. The ratio of the illuminated area of the apparent disk of a celestial body to the entire area of the apparent disk; i.e., the fraction illuminated. 3. Used loosely to refer to one aspect of an eclipse (partial phase, annular phase, etc.). (See lunar phases.)

 

Phase angle

The angle measured at the center of an illuminated body between the light source and the observer.

 

Photometry

A measurement of the intensity of light, usually specified for a specific wavelength range.

 

Planetocentric coordinates

Coordinates for general use, where the z-axis is the mean axis of rotation, the x-axis is the intersection of the planetary equator (normal to the z-axis through the center of mass) and an arbitrary prime meridian, and the y-axis completes a right-hand coordinate system. Longitude of a point is measured positive to the prime meridian as defined by rotational elements. Latitude of a point is the angle between the planetary equator and a line to the center of mass. The radius is measured from the center of mass to the surface point.

 

Planetographic coordinates

Coordinates for cartographic purposes dependent on an equipotential surface as a reference surface. Longitude of a point is measured in the direction opposite to the rotation (positive to the west for direct rotation) from the cartographic position of the prime meridian defined by a clearly observable surface feature. Latitude of a point is the angle between the planetary equator (normal to the z-axis and through the center of mass) and normal to the reference surface at the point. The height of a point is specified as the distance above a point with the same longitude and latitude on the reference surface.

 

Polar motion

The quasi-periodic motion of the Earth’s pole of rotation with respect to the Earth’s solid body. More precisely, the angular excursion of the CIP from the ITRS z-axis. (See Celestial Intermediate Pole (CIP); International Terrestrial Reference System (ITRS).)

 

Polar wobble

See wobble, polar.

 

Pole, celestial

Either of the two points projected onto the celestial sphere by the Earth’s axis. Usually, this is the axis of the Celestial Intermediate Pole (CIP), but it may also refer to the instantaneous axis of rotation, or the angular momentum vector. All of these axes are within 0″.1 of each other. If greater accuracy is desired, the specific axis should be designated.

 

Pole, Tisserand mean

The angular momentum pole for the Earth about which the total internal angular momentum of the Earth is zero. The motions of the Celestial Intermediate Pole (CIP) (described by the conventional theories of precession and nutation) are those of the Tisserand mean pole with periods greater than two days in a celestial reference system (specifically, the Geocentric Celestial Reference System (GCRS)).

 

Precession

The smoothly changing orientation (secular motion) of an orbital plane or the equator of a rotating body. Applied to rotational dynamics, precession may be excited by a singular event, such as a collision, a progenitor’s disruption, or a tidal interaction at a close approach (free precession); or caused by continuous torques from other solar system bodies, or jetting, in the case of comets (forced precession). For the Earth’s rotation, the main sources of forced precession are the torques caused by the attraction of the Sun and Moon on the Earth’s equatorial bulge, called precession of the equator (formerly known as lunisolar precession). The slow change in the orientation of the Earth’s orbital plane is called precession of the ecliptic (formerly known as planetary precession). The combination of both motions—that is, the motion of the equator with respect to the ecliptic—is called general precession.

 

Proper motion

The projection onto the celestial sphere of the space motion of a star relative to the solar system; thus the transverse component of the space motion of a star with respect to the solar system. Proper motion is usually tabulated in star catalogs as changes in right ascension and declination per year or century.