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Derek Haselden - Astronomical Glossary Category: Meteorology and astronomy > Astronomy Date & country: 05/11/2007, UK Words: 796

Perseus
English name: Perseus. Genitive name: Persei. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Perseus
The constellation Perseus. Genitive name: Persei. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Perturbations
The disturbances in the orbit of a celestial object produced by the gravitational effects of other bodies.

Peters-Hartley
A comet in our solar system with an orbital period of 8.12years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Phases
The apparent change in shape of the Moon and inferior planets from Full to New. Mars sometimes displays a gibbous phase but no other planets show a significant phase as seen from Earth.

Pheonicis
The constellation Pheonix. English name: The Pheonix. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Pheonix
English name: The Pheonix. Genitive name: Pheonicis. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Phobos
Satellite of the planet Mars. See TABLE 6. MARS - SATELLITE DATA

Phocæa
One of the `minor planets` of our solar system. Discovered by Chacornac on 6 Apr 1853. Diameter in km: 72. More data in TABLE 13. THE MINOR PLANETS

Phoebe
Satellite of the planet Saturn. See TABLE 8. SATURN - SATELLITE DATA

Photometer
A device that measures the intensity of light from a particular source.

Photon
The smallest unit of light.

Photosphere
The bright visible surface of the Sun.

Pictor
English name: The Painter. Genitive name: Pictoris. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Pictoris
The constellation Pictor. English name: The Painter. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Pisces
English name: The Fish. Genitive name: Piscium. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Pisces Australis
English name: The Southern Fish. Genitive name: Piscis Australis. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Piscids
Meteor shower. Most active day is Sep.8. 3 radiants.

Piscis Australids
Meteor shower. Most active day is Jul.31

Piscis Australis
The constellation Pisces Australis. English name: The Southern Fish. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Piscium
The constellation Pisces. English name: The Fish. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Planet
Any large body orbiting a star. A somewhat arbitrary term since there appears to be no defining size that clearly differentiates between a planet or asteriod. For example, Pluto is widely regarded as the ninth planet of the Solar System yet its parameters do not clearly conform to the other eight planets of the Solar System. Although it seems unlikely to lose its planetary status, Pluto does appear to be an oddity when you consider such things as its orbit, size and other known data.

Planetary nebula
A shell of gas that surrounds a small, dense, hot star. The name is misleading because planetary nebulae have no connection with planets. They probably got their name because some of these nebulae appear disk-like and may have been initially mistaken for planets.

Planetoid
Another name for an asteroid or minor planet.

Planisphere
A circular map of the stars covered by a rotating mask that has a window or opening in it. The map and mask are the same size and have time and date markings on the outside. Lining up the time/date marks then expose a given area of the map through the mask so that the stars visible for a given time can then be seen. Planispheres are usually devised so that they can only be used within a certain latitude range.

Pluto
SOLAR SYSTEM - PLANETARY ORBITAL DATA
Average dist.from Sun (AU) - 39.482
Min. distance from Sun (AU) - 29.658
Max. distance from Sun (AU) - 49.305
Eccentricity of orbit - 0.249
Inclination to ecliptic(°) - 17.14
SOLAR SYSTEM - PLANETARY PERIODS AND MOTIONS
Sidereal period (days or years) - 247.920 y
Mean orbital velocity(km/sec.) - 4.75
Sidereal period of axial rotation - 6.387 d (R)
Inclination of equator to ecliptic(°) - 122.52
SOLAR SYSTEM - PLANETARY PHYSICAL DATA
Equatorial dia.(km) - 2 302
Polar dia.(km) - 2 250
Mass (Earth=1) - 0.0025
Volume (Earth=1) - 0.0057
Oblateness (Earth=1) - 0
Surface gravity (Water=1) - 0.067
Density - 1.10

Poles, celestial
The north and south points of the celestial sphere.

Polyhymnia
One of the `minor planets` of our solar system. Discovered by Chacornac on 28 Oct 1854. Diameter in km: 62. More data in TABLE 13. THE MINOR PLANETS

Pomona
One of the `minor planets` of our solar system. Discovered by Goldschmidt on 26 Oct 1854. Diameter in km: 92. More data in TABLE 13. THE MINOR PLANETS

Pons-Brooks
A comet in our solar system with an orbital period of 70.9years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Pons-Winnecke
A comet in our solar system with an orbital period of 6.36years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Populations, stellar
There are essentially two types of star regions. I - in which the brightest stars are hot and white; and II - in which the brightest stars are old Red Giants. This is an odd distinction because the first stars to have populated a given region would now be classified as Population II! In our own galaxy Population II stars are more numerous toward the centre of the galaxy while Population I stars are more numerous in the outer regions of the galaxy and spiral arms.

Portia
Satellite of the planet Uranus. See TABLE 9. URANUS - SATELLITE DATA

Praxidike
Satellite of the planet Jupiter. See TABLE 7. JUPITER - SATELLITE DATA

Precession
This is the slow movement of the celestial poles tracing out large circles on the celestial sphere. It is caused by a slow wobble in the Earth's axis due to the gravitational effects of the Sun and Moon on the Earth's equatorial bulge. A reasonable analogy is that of a spinning top - As the top slows down it will start to wobble. This 'Earth wobble' is very slow, one 'wobble' takes about 26 000 years to complete. The other effect is that the equinoxes are slowly moving or precessing, at the rate of about 50 arcseconds a year. The Vernal Equinox is also called the First Point of Aries but due to the effects of precession the Vernal Equinox is now in the constellation of Pisces and moving towards Aquarius.

Primary
A term used to describe the larger or brighter member of a pair of celestial objects.

Prime Meridian
The meridian on Earth's surface which passes through Greenwich Observatory. It is taken to be the dividing line between the east and west hemispheres, 0 degrees longitude.

Prometheus
Satellite of the planet Saturn. See TABLE 8. SATURN - SATELLITE DATA

Prominences
A mass of glowing gas, mainly hydrogen, that rises from the surface of the Sun.

Proper motion
The movement of an individual star on the celestial sphere.

Proserpina
One of the `minor planets` of our solar system. Discovered by Luther on 5 May 1853. Diameter in km: 88. More data in TABLE 13. THE MINOR PLANETS

Prospero
Satellite of the planet Uranus. See TABLE 9. URANUS - SATELLITE DATA

Proteus
Satellite of the planet Neptune. See TABLE 10. NEPTUNE - SATELLITE DATA

Proton
An atomic particle, part of the nucleus, that has a positive electrical charge.

Psyche
One of the `minor planets` of our solar system. Discovered by De Gasparis on 15 Mar 1852. Diameter in km: 248. More data in TABLE 13. THE MINOR PLANETS

Puck
Satellite of the planet Uranus. See TABLE 9. URANUS - SATELLITE DATA

Pulsar
A neutron star that is rapidly rotating and emitting radio waves. Pulsars are believed to vector the radio emissions so that if Earth lies in the line of sight they appear like a lighthouse (when seen by a radio telescope, that is).

Puppids-Velids
Meteor shower. Most active day is Dec.9. 2 of several radiants in Puppis, Vela and Carina.

Puppis
English name: The Stern. Genitive name: Puppis. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Puppis
The constellation Puppis. English name: The Stern. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Pyxidis
The constellation Pyxis. English name: The Mariner's Compass. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Pyxis
English name: The Mariner's Compass. Genitive name: Pyxidis. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Quadrantids
Meteor shower. Most active day is Jan.3. Blue & yellow meteors, medium speed. Radiant in Bootes.

Quadrature
The position of the Moon or another planet when it is at right angles to the Sun as seen from Earth. An example would be when the Moon is seen at First or Last Quarter phases.

Quasar
A very remote, highly luminous body. They are now known to be the cores of very energetic galaxies although precisely what makes them so luminous is uncertain.

Radial velocity
The movement of a celestial body either away from (a positive value) or toward (a negative value) the observer.

Radiant
The point in the sky where meteors of a given shower seem to originate or radiate from.

Radio waves
Electromagnetic radiation with a wavelength longer than infrared.

Red dwarf
A star that is smaller and cooler than the Sun. Most red dwarfs are about one-tenth the mass and diameter of the Sun.

Red giant
A star that is large and cool compared with the Sun but is many times the diameter of the Sun. These are stars that are nearing the end of their lives.

Regression of the nodes
This is a slow movement of the lunar nodes caused by the gravitational pull of the Sun. The lunar nodes move slowly westward, taking 18.6 years to complete one revolution.

Reinmuth 1
A comet in our solar system with an orbital period of 7.31years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Reinmuth 2
A comet in our solar system with an orbital period of 6.74years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Retardation
This is the time difference between Moon rise one day, and Moon rise the following day.

Reticuli
The constellation Reticulum. English name: The Net. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Reticulum
English name: The Net. Genitive name: Reticuli. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Retrograde motion
Orbital or rotational movement in the opposite sense to that of Earth's motion.

Reversing layer
The layer of the Sun's atmosphere above the photosphere.

Rhea
Satellite of the planet Saturn. See TABLE 8. SATURN - SATELLITE DATA

Right ascension (R.A.)
The angular distance of a celestial body from the Vernal Equinox, measured eastwards. This is one of the equatorial system of co-ordinates and it is measured in hours, minutes and seconds. Right ascension is also the difference in time between the culmination of the Vernal Equinox and the culmination of that object.

Roche limit
The distance from the centre of a planet that another body would have to be within to be broken up by the gravitational pull of the planet. This would only be true for an object that is not gravitationally cohesive.

Rosalind
Satellite of the planet Uranus. See TABLE 9. URANUS - SATELLITE DATA

Russell 1
A comet in our solar system with an orbital period of 6.10years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Russell 2
A comet in our solar system with an orbital period of 7.38years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Russell II
A comet in our solar system with an orbital period of 7.62years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Sagitta
English name: The Arrow. Genitive name: Sagittae. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Sagittae
The constellation Sagitta. English name: The Arrow. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Sagittarii
The constellation Sagittarius. English name: The Archer. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Sagittarius
English name: The Archer. Genitive name: Sagittarii. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Saros
The period after which the Sun, Earth and Moon return to approximately the same positions, this period being 18 years and 11.3 days. This period is very useful for eclipse prediction since one eclipse will be repeated by an almost identical event precisely one Saros later.

Saturn
SOLAR SYSTEM - PLANETARY ORBITAL DATA
Average dist.from Sun (AU) - 9.537
Min. distance from Sun (AU) - 9.021
Max. distance from Sun (AU) - 10.054
Eccentricity of orbit - 0.054
Inclination to ecliptic(°) - 2.48
SOLAR SYSTEM - PLANETARY PERIODS AND MOTIONS
Sidereal period (days or years) - 29.447 y
Mean orbital velocity(km/sec.) - 9.67
Sidereal period of axial rotation - 10.233 h
Inclination of equator to ecliptic(°) - 26.73
SOLAR SYSTEM - PLANETARY PHYSICAL DATA
Equatorial dia.(km) - 120 536
Polar dia.(km) - 108 728
Mass (Earth=1) - 95.16
Volume (Earth=1) - 764.0
Oblateness (Earth=1) - 0.098
Surface gravity (Water=1) - 0.916
Density - 0.70

Schaumasse
A comet in our solar system with an orbital period of 8.26years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Schwarzchild radius
The radius that a body must have if its escape velocity is to be equal to the velocity of light. This term is frequently encountered when discussing the properties of black holes.

Schwassmann-Wachmann 2
A comet in our solar system with an orbital period of 6.50years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Schwassmann-Wachmann 3
A comet in our solar system with an orbital period of 5.34years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Scintillation
Another term used to describe the twinkling of a star. It is caused by the Earth's atmosphere distorting the light from a star, mainly when the star is at a low altitude. Planets may also scintillate when close to the horizon but much less so than stars.

Scorpii
The constellation Scorpius. English name: The Scorpion. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Scorpius
English name: The Scorpion. Genitive name: Scorpii. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Sculptor
English name: The Sculptor. Genitive name: Sculptoris. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Sculptoris
The constellation Sculptor. English name: The Sculptor. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Scuti
The constellation Scutum. English name: The Shield. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Scutum
English name: The Shield. Genitive name: Scuti. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Secondary
The smaller or dimmer component of a pair of celestial objects that orbits the primary object.

Selenography
The study of the Moon's surface.

Serpens
English name: The Serpent. Genitive name: Serpentis. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Serpentis
The constellation Serpens. English name: The Serpent. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Setebos
Satellite of the planet Uranus. See TABLE 9. URANUS - SATELLITE DATA

Sextans
English name: The Sextant. Genitive name: Sextantis. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Sextantis
The constellation Sextans. English name: The Sextant. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Seyfert galaxies
A kind of so-called 'active' galaxy. Seyfert galaxies have fairly small but bright nuclei and weak spiral arms. These galaxies are also strong emitters of radio wave energy.