Philips Atlas of the Universe

Reflector: A telescope in which the light is collected

by means of a mirror.

Refraction: The change in direction of a ray of light

when passing from one transparent substance into

another.

Refractor: A telescope which collects its light by

means of a lens. The light passes through this lens

(object-glass)and is brought to focus; the image

is then magnified by an eyepiece.

Resolving power: The ability of a telescope to

separate objects which are close together; the larger

the telescope the greater its resolving power. Radio

telescopes (see radio astronomy)have poor

resolving power compared with optical telescopes.

Retardation: The difference in the time of moonrise

between one night and the next. It may exceed one

hour, or it may be as little as a quarter of an hour.

Retrograde motion: In the Solar System, movement

in a sense opposite to that of the Earth in its orbit;

some comets, notably Halley’s, have retrograde

motion. The term is also used with regard to the

apparent movements of planets in the sky; when

the apparent motion is from east to west, relative

to the fixed stars, the direction is retrograde. The

term may be applied to the rotations of planets. Since

Uranus has an axial inclination of more than a right

angle, its rotation is technically retrograde; Venus

also has retrograde axial rotation.

Reversing layer: The gaseous layer above the bright

surface or photosphere of the Sun. Shining on its

own, the gases would yield bright spectral lines; but

as the photosphere makes up the background, the

lines are reversed, and appear as dark absorption or

Fraunhofer lines. Strictly speaking, the whole of the

Sun’s chromosphere is a reversing layer.

Right ascension: The right ascension of a celestial

body is the time which elapses between the

culmination of the First Point of Ariesand the

culmination of the body concerned. For example,

Aldebaran in Taurus culminates 4h 33m after the

First Point of Aries has done so; therefore the right

ascension of Aldebaran is 4h 33m. The right ascensions

of bodies in theSolar System change quickly.

However, the right ascensions of stars do not change,

apart from the slow cumulative effect of precession.

Roche limit: The distance from the centre of a planet,

or other body, within which a second body would be

broken up by gravitational distortion. This applies only

to an orbiting body which has no appreciable structural

cohesion, so that strong, solid objects, such as artificial

satellites, may move safely well within the Roche limit

for the Earth. The Roche limit lies at 2.44 times the

radius of the planet from the centre of the globe, so

that for the Earth it is about 9170 kilometres above

ground-level. Saturn’s ring system lies within the

Roche limit for Saturn.

RR Lyrae variables: Regular variable stars whose

periods are very short (between about 1^1 / 4 hours and

about 30 hours). They seem to be fairly uniform in

luminosity; each is around 100 times as luminous

as the Sun. They can therefore be used for distance

measures, in the same way as Cepheids. Many of

them are found in star clusters, and they were formerly

known as cluster-Cepheids. No RR Lyrae variable

appears bright enough to be seen with the naked eye.

S

Saros:A period of 18 years 11.3 days, after which

the Earth, Moon and Sun return to almost the same

relative positions. Therefore, an eclipse of the Sun

or Moon is liable to be followed by a similar eclipse

18 years 11.3 days later. The period is not exact, but

is good enough for predictions to be made – as was

done in ancient times by Greek philosophers.

Satellite:A secondary body orbiting a primary. The

Earth has one satellite (the Moon); Jupiter has 53,

Saturn 30, Uranus 23, Neptune 11 and Pluto one,

while Mercury and Venus are unattended.

Schmidt telescope (or Schmidt camera): A type

of telescope which uses a spherical mirror and a

special glass correcting plate. With it, relatively wide

areas of the sky may be photographed with a single

exposure; definition is good all over the plate. In its

original form, the Schmidt telescope can be used only

photographically. The largest Schmidt in use is the

122-centimetre instrument at Palomar.

Scintillation: Twinkling of stars. It is due entirely

to the effects of the Earth’s atmosphere; a star will

scintillate most violently when it is low over the

horizon, so that its light is passing through a thick layer

of atmosphere. A planet, which shows up as a small

disk rather than a point, will generally twinkle much

less than a star.

Seasons: Effects on the climate due to the inclination

of the Earth’s axis. The fact that the Earth’s distance

from the Sun is not constant has only a minor effect

upon our seasons.

Second of arc:One 360th of a degree. See arc

minute, arc second.

Secular acceleration: Because of friction produced

by the tides, the Earth’s rotation is gradually slowing

down; the ‘day’ is becoming longer. The average daily

lengthening is only 0.00000002 seconds, but over a

sufficiently long period the effect becomes detectable.

The lengthening of terrestrial time periods gives

rise to an apparent speeding-up of the periods of the

Sun, Moon and planets. Another result of these tidal

phenomena is that the Moon is receding from the

Earth slowly.

Seeing: The quality of the steadiness and clarity

of a star’s image. It depends upon conditions in the

Earth’s atmosphere. From the Moon, or from space,

the ‘seeing’ is always perfect.

Seismometer:An earthquake recorder. Very sensitive

seismometers were taken to the Moon by the Apollo

astronauts, and provided interesting information about

seismic conditions there.

Selenography: The study of the Moon’s surface.

Sextant:An instrument used for measuring the

altitude of a celestial body above the horizon.

Seyfert galaxies: Galaxies with small, bright nuclei.

Many of them are radio sources, and show evidence

of violent disturbances in their nuclei.

Shooting-star:The luminous appearance caused by

a meteor falling through the Earth’s atmosphere.

Sidereal period:The time taken for a planet or other

body to make one journey around the Sun (365.2 days

in the case of the Earth). The term is also used for

a satellite in orbit around a planet. Also known as

periodic time.

Sidereal time:The local time reckoned according

to the apparent rotation of the celestial sphere. It is

zero hours when the First Point of Aries crosses the

observer’s meridian. The sidereal time for any observer

is equal to the right ascension of an object which lies

on the meridian at that time. Greenwich sidereal time

is used as the world standard (this is, of course, merely

the local sidereal time at Greenwich Observatory).

Solar apex:The point on the celestial sphere towards

which the Sun is apparently travelling. It lies in the

constellation Hercules; the Sun’s velocity towards the

apex is 19 kilometres per second. The point directly

opposite in the sky to the solar apex is termed the

solar antapex. This motion is distinct from the Sun’s

rotation around the centre of the Galaxy, which

amounts to about 320 kilometres per second.

Solar constant:The unit for measuring the amount

of energy received on the Earth’s surface by solar

radiation. It is equal to 1.94 calories per minute per

square centimetre. (A calorie is the amount of heat

needed to raise the temperature of 1 gram of water

by 1 degree C.)

Solar flares:See flares, solar.

Solar parallax: The trigonometrical parallax of the

Sun. It is equal to 8.79 seconds of arc.

Solar System: The system made up of the Sun, the

planets, satellites, comets, asteroids, meteoroids and

interplanetary dust and gas.

Solar time, apparent: The local time reckoned

according to the Sun. Noon occurs when the Sun

crosses the observer’s meridian, and is therefore

at its highest in the sky.

Solar wind: A steady flow of atomic particles

streaming out from the Sun in all directions. It was

detected by means of space probes, many of which

carry instruments to study it. Its velocity in the

neighbourhood of the Earth exceeds 965 kilometres

per second. The intensity of solar wind is enhanced

during solar storms.

Solstices:Times when the Sun is at its northernmost

point in the sky (declination 23^1 / 2 °N, around 22 June),

or at its southernmost point (23^1 / 2 °S, around 22

December). The dates of the solstices vary somewhat,

because of the calendar irregularities due to leap years.

GLOSSARY

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