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

Differential rotation
The rotation of a body such as a gaseous planet or the Sun so that different parts are rotating at different speeds. For example, a star or planet which rotates faster at its equator than it does at its poles.

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

Direct motion
Movement of revolution or rotation of a celestial object in the same sense as that of the Earth. For example, the Sun moves across the sky each day from east to west, an effect of the Earth's rotation on its axis. But, against the background of stars the Sun actually moves from west to east over the course of a year, the effect of Earth's orbit around the Sun. This west to east movement is direct motion. The Moon behave in exactly the same way. The planets also exhibit this west to east motion most of the time but owing to our perspective of the planets they occasionally appear to move in the reverse sense, known as retrograde motion.

Disk
The appearance or face of a planet, star or moon as seen from Earth.

Diurnal
Another name for daily.

Doppler effect
The change in frequency (or wavelength) of light (or other radiation) caused by the motion of an object or the observer. An object receding from an observer would exhibit a frequency shift toward a lower frequency (red shift) and vice-versa.

Dorado
English name: The Goldfish. Genitive name: Doradus. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Doradus
The constellation Dorado. English name: The Goldfish. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Doris
One of the `minor planets` of our solar system. Discovered by Goldschmidt on 19 Sep 1857. Diameter in km: 246. More data in TABLE 13. THE MINOR PLANETS

Double star
A star made up of two components. They are either genuinely associated (binary stars) or they appear close by chance (optical pair or binary).

Draco
English name: The Dragon. Genitive name: Draconis. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Draconids
Meteor shower. Most active day is Oct.10. Very feeble shower.

Draconis
The constellation Draco. English name: The Dragon. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

du Toit
A comet in our solar system with an orbital period of 15.0years. More data in TABLE 14. NOTABLE PERIODIC COMETS

du Toit-Martley
A comet in our solar system with an orbital period of 5.21years. More data in TABLE 14. NOTABLE PERIODIC COMETS

du Toit-Neujmin-Delporte
A comet in our solar system with an orbital period of 6.37years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Early-type star
Any hot star of spectral class O, B or A. Sometimes referred to as 'early spectral-type.

Earth
SOLAR SYSTEM - PLANETARY ORBITAL DATA
Average dist.from Sun (AU) - 1.000
Min. distance from Sun (AU) - 0.983
Max. distance from Sun (AU) - 1.017
Eccentricity of orbit - 0.017
Inclination to ecliptic(°) - 0.00
SOLAR SYSTEM - PLANETARY PERIODS AND MOTIONS
Sidereal period (days or years) - 365.256 d
Mean orbital velocity(km/sec.) - 29.79
Sidereal period of axial rotation - 23.934 h
Inclination of equator to ecliptic(°) - 23.44
SOLAR SYSTEM - PLANETARY PHYSICAL DATA
Equatorial dia.(km) - 12 756
Polar dia.(km) - 12 714
Mass (Earth=1) - 1.00
Volume (Earth=1) - 1.00
Oblateness (Earth=1) - 0.0034
Surface gravity (Water=1) - 1.000
Density - 5.52

Earthshine
The faint luminosity seen of the night side of the Moon, especially when the Moon appears at a crescent phase. It is caused by light reflected from the Earth onto the Moon.

Eccentricity
The measure of how non-circular an object's orbit is. 0 = a perfect circle; any figure between 0 and 1 = an ellipse; 1 = a parabola; any figure greater than 1 = a hyperbola. Eccentricity may sometimes be expressed as a percentage. Eccentricity may be calculated by dividing the distance between the two foci of the ellipse and the length of the major axis of the ellipse.

Eclipse, lunar
The passage of the Moon through Earth's shadow. Lunar eclipses may be either total or partial. Totality may last up to one and three quarter hours although the period of totality is on average shorter.

Eclipse, solar
The passage of the Moon in front of the Sun so that the Moon is directly in front of the Sun. Totality may last for a little over 7 minutes under favourable conditions. Partial eclipses occur when the Sun is incompletely covered. Annular eclipses occur when the Moon it near the farthest part of its orbit and hence appears smaller. In this case a bright ring of sunlight is seen around the dark body of the Moon. Technically speaking, a solar eclipse could be termed as an occultation of the Sun by the Moon.

Ecliptic
The apparent yearly path of the Sun among the stars.

Egeria
One of the `minor planets` of our solar system. Discovered by De Gasparis on 2 Nov 1850. Diameter in km: 244. More data in TABLE 13. THE MINOR PLANETS

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

Electron
An atomic particle that carries a negative charge. It orbits the nucleus of an atom.

Element, optical
Any single part of an optical train such as a mirror or lens. In the case of lenses or eyepieces they may comprise several pieces of glass, each one known as an element. The combination of elements is used to correct the faults that would often be present in a single element lens or eyepiece.

Elongation
The angular distance between the Sun and a planet, or between a planet and a satellite, as seen from Earth.

Emersion
A term used to describe when an object re-emerges after an occultation or eclipse.

Emission lines
Specific wavelengths of light that are brighter than adjoining wavelengths seen in spectra.

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

Ephemeris
A table or list of the predicted position of an object such as a planet.

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

Epoch
An instant in time for which the positions of celestial objects are given.

Equator, celestial
The projection of the Earth's equator onto the celestial sphere.

Equatorial mounting
A type of mounting for a telescope (or other instrument) which is set up so that one axis of motion is parallel to the Earth's axis. This arrangement means that only one axis is required to be driven to keep an object in the field of view.

Equinox
The equinoxes are the two points at which the ecliptic intersects with the celestial equator. The Vernal equinox (or First Point of Aries) is where the ecliptic crosses the celestial equator from south to north, the Sun reaches this point around the 21st March. The opposite equinox, the autumnal equinox, is where the ecliptic crosses the celestial equator from north to south. The Sun reaches this point around the 22nd September. The term equinox stems from Latin, equi = equal, nox = night. At the equinoxes the day and night are of equal duration.

Equulei
The constellation Equuleus. English name: The Little Horse. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Equuleus
English name: The Little Horse. Genitive name: Equulei. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Eridani
The constellation Eridanus. English name: The River Eridanus. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Eridanus
English name: The River Eridanus. Genitive name: Eridani. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

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

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

Escape velocity
The minimum speed that an object must attain to escape from the surface of the planet or other body without being given any extra impetus. The escape velocity of Earth is 11.2 kilometres per second.

eta Aquarids
Meteor shower. Most active day is May 4. Very fast meteors. Poorly seen from the U.K.

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

Eugenia
One of the `minor planets` of our solar system. Discovered by Goldschmidt on 27 Jun 1857. Diameter in km: 244. More data in TABLE 13. THE MINOR PLANETS

Eunomia
One of the `minor planets` of our solar system. Discovered by De Gasparis on 29 Jul 1851. Diameter in km: 260. More data in TABLE 13. THE MINOR PLANETS

Euphrosyne
One of the `minor planets` of our solar system. Discovered by Ferguson on 1 Sep 1854. Diameter in km: 270. More data in TABLE 13. THE MINOR PLANETS

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

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

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

Euterpe
One of the `minor planets` of our solar system. Discovered by Hind on 8 Nov 1853. Diameter in km: 116. More data in TABLE 13. THE MINOR PLANETS

Exosphere
The outermost part of the Earth's atmosphere. It is an ill-defined, highly rareified zone which starts at a height of about 700km (435 miles) and extends into the vacuum of space.

Extinction
The apparent reduction in brightness of a celestial object when it is low in the sky and much of its light is absorbed by Earth's atmosphere.

Eyepiece (or occular)
The lens, or lens combination, at the eye end of a telescope. It is responsible for the magnification of the object under scrutiny. There are a variety of eyepiece types with different characteristics. Eyepieces will be stated as having a certain focal length and it is this figure, usually in millimetres, which when divided into the focal length of the telescope, dictates the magnifying factor that a given eyepiece will yield.

Faculae
Bright patches seen of the photosphere during solar observation.

Faye
A comet in our solar system with an orbital period of 7.34years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Fides
One of the `minor planets` of our solar system. Discovered by Luther on 5 Oct 1855. Diameter in km: 96. More data in TABLE 13. THE MINOR PLANETS

Finlay
A comet in our solar system with an orbital period of 6.76years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Fireball
A very brilliant meteor.

Flares, solar
Brilliant eruptions in the outer part of the Sun's atmosphere. They can usually only be seen with the aid of special filters and instruments. They are often associated with areas of activity on the Sun such as sunspots. Occasionally, these eruptions may cause charged particles to reach the Earth which may in turn cause auroral displays.

Flocculi
Patches on the Sun's surface visible with spectroscopic equipment.

Flora
One of the `minor planets` of our solar system. Discovered by Hind on 18 Oct 1847. Diameter in km: 162. More data in TABLE 13. THE MINOR PLANETS

Focal length
The distance between a lens or mirror and the point where it brings light to a focus.

Focal ratio
The focal length of a telescope divided by its aperture (opening) or primary mirror diameter.

Forbes
A comet in our solar system with an orbital period of 6.27years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Fornacis
The constellation Fornax. English name: The Furnace. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Fornax
English name: The Furnace. Genitive name: Fornacis. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Fortuna
One of the `minor planets` of our solar system. Discovered by Hind on 22 Aug 1852. Diameter in km: 198. More data in TABLE 13. THE MINOR PLANETS

Fraunhofer lines
Dark absorption lines seen in the spectrum of the Sun.

Frequency
The number of waves passing a point in a given time, usually 1 second. Frequency is measured in hertz and is equal to the speed of the wave divided by their wavelength. Longer wavelengths have lower frequency and vice-versa.

Galactic Cluster
Another name for an open star cluster. They are often termed Galactic Clusters because they are found mainly in the plane of our galaxy (The Milky Way). If you were to view our galaxy from afar, you would find that all the open/galactic clusters lie within the spiral arms of the galaxy. See also Open Cluster.

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

Galaxy
A system made up of stars, nebulae and interstellar matter. Many galaxies, but not all, are spiral in form.

Galaxy, The
The star system of which the Sun is a member. The Galaxy is also known as the Milky Way galaxy. It is spiral in shape, contains about 100 000 million stars and is approximately 100 000 light years across. The nucleus of the Galaxy is roughly 30 000 light years thick tapering away to about 10 000 light years thickness in the spiral arms. The whole system is slowly spinning taking about 200 million years to rotate once. The Galaxy is the second largest member of the Local Group of galaxies, a loose cluster of galaxies roughly dumbbell in shape and about 4 million light years long.

Gale
A comet in our solar system with an orbital period of 11.3years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Gamma-rays
Electromagnetic radiation of extremely short wavelength, shorter than X-rays.

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

Gauss
A unit of measurement of a magnetic field. Earth has a surface magnetic field strength of between 0.3 and 0.6 Gauss.

Gegenschein
A very faint skyglow opposite the Sun, very difficult to observe, caused by very thinly spread interplanetary material. Often known by its English name, Counterglow.

Gehrels 1
A comet in our solar system with an orbital period of 14.5years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Gehrels 2
A comet in our solar system with an orbital period of 7.98years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Gehrels 3
A comet in our solar system with an orbital period of 8.14years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Gemini
English name: The Twins. Genitive name: Geminorium. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Geminids
Meteor shower. Most active day is Dec.13. Rich shower of medium speed meteors. Many bright events.

Geminorium
The constellation Gemini. English name: The Twins. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Geocentric
Meaning: As seen from the centre of Earth. A system of co-ordinates.

Geodesy
The study of the shape, mass, size and other features of the Earth.

Giant stars
Stars that are swelling in size as they approach the end of their lives. Giant stars are often no more massive than the Sun but they have expanded to great size and are therefore less dense but highly luminous.

Gibbous
The phase of the Moon or a planet when it is between half and fully illuminated.

Giclas
A comet in our solar system with an orbital period of 6.96years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Gravity
The force of attraction between two or more masses. This force is dependant on both the masses themselves as well as the distance between them. Being a force, gravity is strictly speaking measured in Newton's but is commonly measured in Kilogrammes.

Great circle
A circle on the surface of a sphere whose plane passes through the centre of that sphere.

Gruis
The constellation Grus. English name: The Crane. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Grus
English name: The Crane. Genitive name: Gruis. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Gunn
A comet in our solar system with an orbital period of 6.82years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Halo, galactic
A roughly spherical shaped region around the main part of the Galaxy.

Harmonia
One of the `minor planets` of our solar system. Discovered by Goldschmidt on 31 Mar 1856. Diameter in km: 116. More data in TABLE 13. THE MINOR PLANETS

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