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

Megaparsec
One million parsecs, a distance equal to 3 260 000 light years.

Melpomene
One of the `minor planets` of our solar system. Discovered by Hind on 14 Jun 1852. Diameter in km: 162. More data in TABLE 13. THE MINOR PLANETS

Mensa
English name: The Table (Mountain). Genitive name: Mensae. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Mensae
The constellation Mensa. English name: The Table (Mountain). Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Mercury
SOLAR SYSTEM - PLANETARY ORBITAL DATA
Average dist.from Sun (AU) - 0.387
Min. distance from Sun (AU) - 0.307
Max. distance from Sun (AU) - 0.467
Eccentricity of orbit - 0.206
Inclination to ecliptic(°) - 7.00
SOLAR SYSTEM - PLANETARY PERIODS AND MOTIONS
Sidereal period (days or years) - 87.969 d
Mean orbital velocity(km/sec.) - 47.87
Sidereal period of axial rotation - 58.646 d
Inclination of equator to ecliptic(°) - 0.01
SOLAR SYSTEM - PLANETARY PHYSICAL DATA
Equatorial dia.(km) - 4 878
Polar dia.(km) - 4 878
Mass (Earth=1) - 0.06
Volume (Earth=1) - 0.06
Oblateness (Earth=1) - 0
Surface gravity (Water=1) - 0.378
Density - 5.43

Meridian
The imaginary line that passes from north to south horizons via the zenith.

Meteor
A streak of light, lasting a few seconds at most, produced when a meteoroid enters the Earth's atmosphere. Most meteoroids are barely the size of a grain of sand and are thought to be cometary debris. In most cases the object is destroyed by friction in Earth's ionosphere at a height of about 100 to 160 km (60 to 100 miles).

Meteorite.Any meteoroid
which strikes the surface of the Earth or another body. They are generally larger objects closely associated with asteroid type debris.

Meteoroid
Any small solid object in space. Most meteoroids are barely bigger than a grain of sand.

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

Metis
One of the `minor planets` of our solar system. Discovered by Graham on 16 Apr 1848. Diameter in km: 158. More data in TABLE 13. THE MINOR PLANETS

Metonic cycle
The period of 6 939.6 days, or 19 calendar years, after which the Moon's phases recur on the same day of the year. This period is also equal to 253 lunations.

Micrometeorite
A very small particle of interplanetary debris, too small to cause the luminous flash associated with meteors.

Micron
One thousandth of a millimetre. The symbol used is µ (mu).

Microscopii
The constellation Microscopium. English name: The Microscope. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Microscopium
English name: The Microscope. Genitive name: Microscopii. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Milky Way, The
(1) Another name for The Galaxy. (2) The broad band of faint light that stretches across the sky (when the time is favourable) as seen from a dark site. It is the result of countless millions of stars of The Galaxy seen edge on from the viewpoint of Earth. The Milky Way is brightest in the Northern hemisphere in the constellation of Cygnus, and in the Southern hemisphere in the constellation of Crux.

Million
1 000 000 - one thousand, thousand.

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

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

Mock Sun
An effect caused by ice crystals in Earth's atmosphere which refract sunlight and cause the appearance of two diffuse patches of light 22° either side of the Sun. These patches are termed Parhelia or Sundogs and often appear on the rim of a halo surrounding the Sun. The patches often have coloured fringes, red on one side, blue on the other.

Monoceros
English name: The Unicorn. Genitive name: Monocerotis. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Monocerotis
The constellation Monoceros. English name: The Unicorn. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Month, Anomalistic
The interval between two successive perigee passages of the Moon, equal to 27.55 days.

Month, Nodical or Draconic
The interval between successive passages of the Moon through one of its nodes, equal to 27.21 days.

Month, Sidereal
The revolution period of the Moon relative to the stars, equal to 27.32 days.

Month, Synodic
The interval between two successive New Moon's (a lunation), equal to 29.53 days.

Month, Tropical
The time taken for the Moon to return to the same celestial longitude (7 seconds shorter than the sidereal month).

Moon, Blue
The second Full Moon that occurs during a calendar month. Since the Moon takes 29.53 days to complete a cycle of phases a Blue Moon cannot occur in February. The last Blue Moon was on 30th November 2001, there are none during 2002 and 2003. The next Blue Moon occurs on 31st July 2004. Blue Moon's occur roughly every two and a half years so are not very common. Hence the term 'Once in a Blue Moon'.

Moon, Harvest
The Full Moon that occurs nearest the time of the autumnal equinox. So-called because this will occur around the time of harvest.

Moon, Hunter's
The Full Moon that occurs after Harvest Moon. So-called bacause this will occur around the start of the hunting season.

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

Musca
English name: The Fly. Genitive name: Muscae. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Muscae
The constellation Musca. English name: The Fly. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Nadir
The point on the celestial sphere directly below the observer. The nadir is directly opposite the zenith.

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

Nebula, Dark
A dense cloud of gas and dust in space that appears as a dark silhouette against a brighter background. In many cases dark nebulae are part of a larger nebulae complex which provides the light background against which the dark nebulae appears. At infra-red wavelengths young stars not visible at visual wavelengths are often found to be forming. Dark nebulae sometimes appears as a dark feature against a starry background but without any other associated nebulae.

Nebula, Emission
A gas cloud which is close to a star or stars. The radiation from the stars excites the gas so that it emits light of its own. Most of this gas is hydrogen which emits the red/orange light which is typical of this type of nebulae.

Nebula, Nebulae
Any cloud of gas and dust in space. The three nebulae types listed below may appear on their own but are frequently seen in combination.

Nebula, Reflection
A cloud of gas and dust which lies close to a star or stars which reflects light from those stars. This type of nebulae is often part of a larger nebulae complex (although it is sometimes seen without the other nebulae types above) and the reflection is typically blue in colour due to the dust within the nebulae.

Neptune
SOLAR SYSTEM - PLANETARY ORBITAL DATA
Average dist.from Sun (AU) - 30.069
Min. distance from Sun (AU) - 29.811
Max. distance from Sun (AU) - 30.327
Eccentricity of orbit - 0.009
Inclination to ecliptic(°) - 1.77
SOLAR SYSTEM - PLANETARY PERIODS AND MOTIONS
Sidereal period (days or years) - 164.793 y
Mean orbital velocity(km/sec.) - 5.48
Sidereal period of axial rotation - 16.110 h
Inclination of equator to ecliptic(°) - 28.31
SOLAR SYSTEM - PLANETARY PHYSICAL DATA
Equatorial dia.(km) - 49 528
Polar dia.(km) - 48 682
Mass (Earth=1) - 17.20
Volume (Earth=1) - 58.0
Oblateness (Earth=1) - 0.017
Surface gravity (Water=1) - 1.125
Density - 1.76

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

Neujmin 1
A comet in our solar system with an orbital period of 18.2years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Neujmin 2
A comet in our solar system with an orbital period of 5.43years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Neujmin 3
A comet in our solar system with an orbital period of 10.6years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Neutrino
A particle that has no charge, and little or no mass.

Neutron
An atomic particle with no charge and a mass almost equal to a proton.

Neutron star
The remnant of a very massive star that has undergone a supernova explosion. Neutron stars send out rapidly changing radio emissions and are frequently termed as pulsars.

Newtonian reflector
A reflecting telescope in which the light is collected by a concave primary mirror at the rear of the telescope and then reflected back to a flat secondary mirror angled at 45 degrees mounted near the front of the telescope. The light is then reflected through the side of the telescope tube and on into an eyepiece. It is the simplest type of reflecting telescope.

NGC
New General Catalogue. A list of some 7000 plus deep-sky objects.

Nodes
The points at which the orbit of the Moon, a planet or a comet intersect with the plane of the ecliptic; south to north (Ascending Node), north to south (Descending Node).

Norma
English name: The Level. Genitive name: Normae. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Normae
The constellation Norma. English name: The Level. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Nova
A star that suddenly flares up to several times its normal brightness. Typically they remain bright for a short period before fading back into obscurity.

Nucleus
The central part of an atom containing two particles; a proton and (with the exception of hydrogen) and a neutron.

Nutation
A slow 'nodding' of the Earth's axis due to the gravitational tug of the Moon on the Earth's protuberant equator.

Nysa
One of the `minor planets` of our solar system. Discovered by Goldschmidt on 27 May 1857. Diameter in km: 68. More data in TABLE 13. THE MINOR PLANETS

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

Oblateness
The measure of how much a rotating object deviates from being a perfect sphere. A perfect sphere would have an oblateness of 0.0 (0%). The Earth for example is oblate to a factor of 0.0034 (0.34%), while Saturn, the most oblate of the planets has a figure of 0.108 (10.8%). Generally, the planets have an equatorial bulge and in the case of Saturn, its equatorial diameter is some 10.8% greater than its polar diameter.

Obliquity of the ecliptic
The angle between the ecliptic and the celestial equator: 23 degrees, 26 minutes and 45 seconds. This angle is also an expression of the tilt of Earth's axis.

Occultation
The covering-up of one celestial object by another. For example, a total solar eclipse is an occultation of the Sun by the Moon.

Octanis
The constellation Octans. English name: The Octant. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Octans
English name: The Octant. Genitive name: Octanis. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Olbers
A comet in our solar system with an orbital period of 69.6years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Open Cluster
A loose association of stars with no clearly defined shape. Often referred to as 'Galactic Clusters' since they are all found close to the galactic plane (of our galaxy). There are a little over 1000 of these clusters and they may contain just a dozen stars or as many as 500. The clusters occupy a region of space of only a few tens of light years across and are formed in the same region of space from a nebula, only in time do the stars disperse. In some instances traces of the original nebula can still be seen.

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

Ophiuchi
The constellation Ophiuchus. English name: The Serpent-Holder. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Ophiuchids
Meteor shower. Most active day is Jun.9. Low activity from several radiants. Poorly seen from U.K.

Ophiuchus
English name: The Serpent-Holder. Genitive name: Ophiuchi. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Opposition
The position of a planet when it is opposite the Sun in the sky. At this point the Sun, Earth and the planet are roughly in line and the planet will rise at about the same time that the Sun sets: the planet is thus visible virtually all night. A good example is when the Moon is Full, it could be said to be in opposition.

Orbit
The path of a celestial body around its parent body.

Orion
English name: Orion (the Hunter). Genitive name: Orionis. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Orion (the Hunter)
The constellation Orion. Genitive name: Orionis. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Orionids
Meteor shower. Most active day is Oct.20. Fast meteors with fine, often persistent trains.

Orionis
The constellation Orion. English name: Orion (the Hunter). Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Orrery
A model showing the Sun and planets. The model is usually capable of being moved mechanically so that the planets move at the correct speed around the Sun relative to each other. Some versions of an Orrery have just the Sun, Earth and Moon.

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

Oterma
A comet in our solar system with an orbital period of 7.88years. More data in TABLE 14. NOTABLE PERIODIC COMETS

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

Pales
One of the `minor planets` of our solar system. Discovered by Goldschmidt on 19 Sep 1857. Diameter in km: 176. More data in TABLE 13. THE MINOR PLANETS

Pallas
One of the `minor planets` of our solar system. Discovered by Olbers on 28 Mar 1802. Diameter in km: 580x470. More data in TABLE 13. THE MINOR PLANETS

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

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

Parallax, trigonometric
The apparent shift of an object when viewed from two different directions. This shift may be used to gauge the distance of an object out to distances of about 1 000 light years or more. The object is observed six months apart and the amount it shifts relative to the background stars is measured in arcseconds. No star subtends an angle of one arcsecond, so the angles measured this way are extremely small. However, if the angle is measurable it is possible, by using trigonometry, to measure the distance to a star.

Parsec (pc)
Parallax second. The distance at which one astronomical unit subtends an angle of one arcsecond. One parsec is equal to 3.26 light years, or 206 265 astronomical units, or 30.86 million million kilometres.

Parthenope
One of the `minor planets` of our solar system. Discovered by De Gasparis on 11 May 1850. Diameter in km: 156. More data in TABLE 13. THE MINOR PLANETS

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

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

Pavo
English name: The Peacock. Genitive name: Pavonis. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Pavonis
The constellation Pavo. English name: The Peacock. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Pegasi
The constellation Pegasus. English name: Pegasus. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Pegasus
English name: Pegasus. Genitive name: Pegasi. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

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

Penumbra
(1). The lighter part of a sunspot. (2). The area of partial shadow either side of the main umbra cone of shadow cast by the Earth.

Perigee
The position of the Moon in its orbit when it is closest to the Earth.

Perihelion
The position of the Earth (or other planet) in its orbit when it is closest to the Sun.

Period
The interval between successive occurrences of a cyclical event.

Perrine-Mrkos
A comet in our solar system with an orbital period of 6.77years. More data in TABLE 14. NOTABLE PERIODIC COMETS

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

Perseids
Meteor shower. Most active day is Aug.12. Rich, consistent shower. Swift, bright trains.