Philips Atlas of the Universe

Hour angle: The time which has elapsed since a

celestial body crossed the meridian of the observer.

Hour circle: A great circle on the celestial sphere

which passes through both poles of the sky. The zero

hour circle corresponds to the observer’s meridian.

Hubble Constant: The relationship between the

distance of a galaxy and its recessional velocity.

Its value is of the order of 70 kilometres per second

per megaparsec.

I

Inferior planets: Mercury and Venus, whose orbits

lie closer to the Sun than does that of the Earth. When

their right ascensionsare the same as that of the

Sun, so that they are approximately between the Sun

and the Earth, they reach inferior conjunction. If the

declinationis also the same as that of the Sun, the

result will be a transit of the planet.

Infra-red radiation: Radiation with wavelengths

longer than that of red light, but shorter than

microwaves. Infra-red sources in the sky are studied

either from high-altitude observatories (as at Mauna

Kea) or with space techniques. In 1983 the Infra-Red

Astronomical Satellite (IRAS) carried out a full survey

of the sky in infra-red.

Ion:An atom which has lost or gained one or more

electrons; it has a corresponding positive or negative

electrical charge, since in a complete atom the positive

charge of the nucleus is balanced out by the combined

negative charge of the electrons. The process of

producing an ion is termed ionization.

Ionosphere: The region above the stratosphere, from

about 65 up to about 800 kilometres. Ionization of the

atoms in this region (see ion) produces layers which

reflect radio waves, making long-range communication

over the Earth possible. Solar events have effects

upon the ionosphere, and produce ionospheric storms;

on occasion, radio communication is interrupted.

Irradiation: The effect which makes brightly lit or

self-luminous bodies appear larger than they really are.

For example, the Moon’s bright crescent appears larger

in diameter than the Earth-lit part of the disk.

J

Julian day: A count of the days, starting from 12 noon

on 1 January 4713 BC. The system was introduced by

Scaliger in 1582. The ‘Julian’ is in honour of Scaliger’s

father, and has nothing to do with Julius Caesar or the

Julian Calendar. Julian days are used by variable star

observers, and for reckonings of phenomena which

extend over very long periods of time.

K

Kepler’s Laws of Planetary Motion: The three

important laws announced by J. Kepler between 1609

and 1618. They are:

(1) The planets move in elliptical orbits, the Sun being

located at one focus of the ellipse, while the other focus

is empty.

(2) The radius vector, or imaginary line joining the

centre of the planet to the centre of the Sun, sweeps

out equal areas in equal times.

(3) The squares of the sidereal periods of the planets

are proportional to the cubes of their mean distances

from the Sun (Harmonic Law).

Kiloparsec: 1000 parsecs, or 3260 light-years.

Kirkwood gaps: Regions in the belt of asteroids

between Mars and Jupiter in which almost no asteroids

move. The gravitational influence of Jupiter keeps

these zones ‘swept clear’; an asteroid which enters a

Kirkwood region will be regularly perturbed by Jupiter

until its orbit has been changed. They were first noted

by the American mathematician Daniel Kirkwood.

L

Laser (Light Amplification by the Simulated

Emission of Radiation): A device which emits a

beam of light made up of rays of the same wavelength

(coherent light) and in phase with one another. It can

be extremely intense. Laser beams have already been

reflected off the Moon.

Latitude, celestial: The angular distance of a celestial

body from the nearest point on the ecliptic.

Librations, lunar: Although the Moon’s rotation is

captured with respect to the Earth, there are various

effects, known as librations, which enable us to

examine 59 per cent of the total surface instead of

only 50 per cent, although no more than 50 per cent

can be seen at any one time. There are three librations:

in longitude (because the Moon’s orbital velocity is not

constant), in latitude (because the Moon’s equator is

inclined by 6 degrees to its orbital plane), and diurnal

(due to the rotation of the Earth).

Light-year: The distance travelled by light in one year.

It is equal to 9.46 million million million kilometres.

Limb: The edge of the visible disk of the Sun. Moon,

a planet, or the Earth (as seen from space).

Local Group of galaxies: The group of which

our Galaxy is a member. There are more than two

dozen systems, of which the most important are the

Andromeda Spiral, our Galaxy, the Triangulum Spiral

and the two Clouds of Magellan.

Longitude, celestial: The angular distance from

the vernal equinox to the foot of a perpendicular

drawn from a celestial body to meet the ecliptic.

It is measured eastwards along the ecliptic from zero

degrees to 360 degrees.

Lunation (synodical month): The interval between

successive new moons: 29 days 12 hours 44 minutes.

See also synodic period.

Lyot filter (monochromatic filter):A device used for

observing the Sun’s prominences and other features of

the solar atmosphere, without the necessity of waiting

for a total eclipse.It was invented by the French

astronomer B. Lyot.

M

Mach number:The velocity of a vehicle moving in

an atmosphere divided by the velocity of sound in the

same region. Near the surface of the Earth, sound

travels at about 1200 kilometres per hour; so Mach 2

would be 2 
 1200 2400 kilometres per hour.

Magnetic storm: A sudden disturbance of the

Earth’s magnetic field, shown by interference with

radio communication as well as by variations in the

compass needle. It is due to charged particles sent

out from the Sun, often associated with solar flares.

A magnetic crochet is a sudden change in the Earth’s

magnetic field due to changing conditions in the lower

ionosphere. The crochet is associated with the flash

phase of the flare, and commences with it; the storm

is associated with the particles, which reach the Earth

about 24 hours later.

Magnetohydrodynamics: The study of the

interactions between a magnetic field and an

electrically conducting fluid. The Swedish scientist

H. Alfven is regarded as the founder of

magnetohydrodynamics.

Magnetosphere: The region round a body in which

that body’s magnetic field is dominant. In the Solar

System, Jupiter has the largest magnetosphere;

the other giants, as well as the Earth and Mercury,

have pronounced magnetic fields, but the Moon,

Venus and Mars do not.

Magnitude: This is really a term for ‘brightness’, but

there are several different types. (1) Apparent orvisual

magnitude: the apparent brightness of a celestial body

as seen with the eye. The brighter the object, the lower

the magnitude. The planet Venus is of about magnitude

 41 / 2 ; Sirius, the brightest star, 1.4; the Pole Star, 2;

stars just visible with the naked eye, 6; the faintest

stars that can be recorded with the world’s largest

telescopes, below 30. A star’s apparent magnitude

is no reliable key to its luminosity. (2) Absolute

magnitude: the apparent magnitude that a star would

have if seen from a standard distance of 10 parsecs

(32.6 light-years). (3) Photographic magnitude: the

magnitude derived from the size of a star’s image on

a photographic plate. (4) Bolometric magnitude: this

refers to the total radiation sent out by a star, not

merely to visible light.

Main Sequence:The well-defined band from the

upper left to lower right of a Hertzsprung-Russell

Diagram. The Sun is typical Main Sequence star.

Maser (Microwave Amplification by Simulated

Emission of Radiation): The same basic principle

as that of the laser, but applied to radio wavelengths

rather than to visible light.

Mass: The quantity of matter that a body contains.

It is not the same as weight, which depends upon

local gravity; thus on the Moon an Earthman has only

one-sixth of his normal weight, but his mass remains

unaltered.

Meridian, celestial: The great circle on the celestial

sphere which passes through the zenith and both

celestial poles. The meridian cuts the observer’s

horizon at the exact north and south points.

Messier numbers: Numbers given by the 18th-century

French astronomer Charles Messier to various nebulous

ATLAS OF THE UNIVERSE

Glossary

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