Philips Atlas of the Universe

F

Faculae: Bright temporary patches on the surface

of the Sun, usually (although not always) associated

with sunspots. Faculae frequently appear in a position

near which a spot group is about to appear, and may

persist for some time in the region of a group which

has disappeared.

First Point of Aries: The vernal equinox. The right

ascensionof the vernal equinox is taken as zero, and

the right ascensions of all celestial bodies are referred

to it. See equinox.

First Point of Libra: The autumnal equinox. See

equinox.

Flares, solar: Brilliant outbreaks in the outer part of

the Sun’s atmosphere, usually associated with active

sunspot groups. They send out electrified particles

which may later reach the Earth, causing magnetic

storms and aurorae (seeaurora); they are also

associated with strong outbursts of solar radio

emission. It has been suggested that the particles

emitted by flares may present a hazard to astronauts

who are in space or on the unprotected surface of

the Moon.

Flare stars: Faint red dwarf stars which may brighten

up by several magnitudes over a period of a few

minutes, fading back to their usual brightness within

an hour or so. It is thought that this must be due to

intense flare activity in the star’s atmosphere. Although

the energies involved are much higher than for solar

flares, it is not yet known whether the entire stellar

atmosphere is involved, or only a small area, as in

the case of flares on the Sun. Typical flare stars are

UV Ceti and AD Leonis.

Flash spectrum: Just before the Moon completely

covers the Sun at a total solar eclipse, the Sun’s

atmosphere is seen shining by itself, without the usual

brilliant background of the photosphere. The dark

lines in the spectrum then become bright, producing

what is termed the flash spectrum. The same effect

is seen just after the end of totality.

Flocculi: Patches on the Sun’s surface, observed

by instruments based on the principle of the

spectroscope.Bright flocculi are composed of

calcium; dark flocculi are made up of hydrogen.

Focal length: The distance between a lens (or mirror)

and the point at which the image of an object at infinity

is brought to focus. The focal length divided by the

aperture of the mirror or lens is termed the focal ratio.

Fraunhofer lines: The dark absorption lines in the

Sun’s spectrum. named in honour of the German

optician J. von Fraunhofer, who first studied and

mapped them in 1814.

Free fall: The normal state of motion of an object in

space under the influence of the gravitational pull of

a central body; thus the Earth is in free fall around the

Sun, while an artificial satellite moving beyond the

atmosphere is in free fall around the Earth. While no

thrust is being applied, a lunar probe travelling between

the Earth and the Moon is in free fall; the same applies

for a probe in a transfer orbit between the Earth

and another planet. While a vehicle is in free fall,

an astronaut will have no apparent ‘weight’, and will

be experiencing zero gravity or weightlessness.

Fringe region: The upper part of the exosphere.

Atomic particles in the fringe region have little chance

of collision with one another, and to all intents and

purposes they travel in free orbits, subject to the

Earth’s gravitation.

G

g: Symbol for the force of gravity at the Earth’s surface.

The acceleration due to gravity is 9.75 metres per

second per second at sea level.

Galaxies: Systems of stars; our Galaxy contains about

100,000 million stars, but is not exceptional in size.

Galaxies are of various shapes; some are spiral, some

elliptical, some irregular. The most remote galaxies

known are at least 18,000 million light-years away;

all, apart from those of our Local Group, are receding

from us, so that the entire universe is expanding.

Galaxy, the: The Galaxy of which our Sun is a

member.

Gamma-rays: Extremely short-wavelength

electromagnetic radiations. Cosmic gamma-ray

sources have to be studied by space research methods.

Gegenschein (or counterglow):A very faint glow

in the sky, exactly opposite to the Sun; it is very

difficult to observe, and has never been satisfactorily

photographed. It is due to tenuous matter spread

along the main plane of the Solar System, so that

it is associated with the Zodiacal Light.

Geocentric:Relative to the Earth as a centre – or as

measured with respect to the centre of the Earth.

Geocorona:A layer of very tenuous hydrogen

surrounding the Earth near the uppermost limit of

the atmosphere.

Geodesy:The science which deals with the Earth’s

form, dimensions, elasticity, mass, gravitation and

allied topics.

Geophysics: The science dealing with the physics

of the Earth and its environment. Its range extends

from the interior of the Earth out to the limits of the

magnetosphere. In 1957–8 an ambitious international

programme, the International Geophysical Year

(IGY), was organized to undertake intensive studies

of geophysical phenomena at the time of a sunspot

maximum. It was extended to 18 months, and was so

successful that at the next sunspot minimum a more

limited but still extensive programme was organized,

the International Year of the Quiet Sun (IQSY).

Gibbous: A phase of the Moon or a planet which is

more than half, but less than full.

Gravitation: The force of attraction which exists

between all particles of matter in the universe.

Particles attract one another with a force which is

directly proportional to the product of their masses

and inversely proportional to the square of the

distance between them.

Great circle: A circle on the surface of a sphere

(such as the Earth, or the celestial sphere) whose plane

passes through the centre of the sphere. Thus a great

circle will divide the sphere into two equal parts.

Green flash (or green ray): When the Sun is setting,

the last visible portion of the disk may flash brilliant

green for a very brief period. This is due to effects

of the Earth’s atmosphere, and is best observed over

a sea horizon. Venus has also been known to show a

green flash when setting.

Greenwich Mean Time (GMT):The time reckoned

from the Greenwich Observatory in London, England.

It is used as the standard throughout the world. Also

known as Universal Time (UT).

Greenwich Meridian:The line of longitude which

passes through the Airy Transit Circle at Greenwich

Observatory. It is taken as longitude zero degrees,

and is used as the standard throughout the world.

Gregorian reflector: A type of reflecting telescope

(see reflector)in which the incoming light is reflected

from the main mirror on to a small concave mirror

placed outside the focus of the main mirror; the light

then comes back through a hole in the main mirror

and is brought to focus. Gregorian reflectors are not

now common.

H

HI and H II regions: Clouds of hydrogen in the

Galaxy. In HI regions the hydrogen is neutral, and

the clouds cannot be seen, but they may be studied

by radio telescopes by virtue of their characteristic

emission at a wavelength of 21 centimetres. In HII

regions the hydrogen is ionized (see ion), generally

in the presence of hot stars. The recombination of

the ions and free electrons to form neutral atoms gives

rise to the emission of light, by which the H II regions

can be seen.

Halation ring: A ring sometimes seen around a star

image on a photograph. It is purely a photographic

effect.

Halo:(1) A luminous ring around the Sun or Moon,

due to ice crystals in the Earth’s upper atmosphere.

(2) The galactic halo: The spherical-shaped star cloud

around the main part of the Galaxy.

Hertzsprung-Russell Diagram (H/R Diagram):

A diagram in which stars are plotted according to

spectral type and luminosity. It is found that there is

a well-defined band known as the Main Sequence

which runs from the upper left of the Diagram (very

luminous bluish stars) down to the lower right (faint

red stars); there is also a giant branch to the upper

right, while the dim, hot white dwarfs lie to the lower

left. H/R Diagrams have been of the utmost importance

in studies of stellar evolution. If colour index is used

instead of spectrum, the diagram is known as a

colour-magnitude diagram.

Hohmann orbit: See transfer orbit.

GLOSSARY

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