Philips Atlas of the Universe

Spacesuit: Equipment designed to allow an astronaut

to operate outside the atmosphere.

Specific gravity: The density of any substance

compared with that of an equal volume of water.

Spectroheliograph: An instrument used for

photographing the Sun in the light of one particular

wavelength only. If adapted for visual use, it is known

as a spectrohelioscope.

Spectroscope: An instrument used to analyse the

light on a star or other luminous object. Astronomical

spectroscopes are used in conjunction with telescopes.

Without them our knowledge of the nature of the

universe would still be very rudimentary.

Spectroscopic binary: Abinary star whose

components are too close together to be seen

separately, but whose relative motions cause opposite

Doppler shifts which are detectable spectroscopically.

Speculum:The main mirror of reflecting telescope

(see reflector). Older mirrors were made of speculum

metal; modern ones are generally of glass.

Spherical aberration: The blurred appearance of

an image as seen in a telescope, due to the fact that

the lens or mirror does not bring the rays falling on

its edge and on its centre to exactly the same focus.

If the spherical aberration is noticeable, the lens or

mirror is of poor quality, and should be corrected.

Spicules: Jets up to 16,000 kilometres in diameter, in

the solar chromosphere. Each lasts for 4–5 minutes.

Spiral nebula:A now obsolete term for a spiral galaxy.

Star:A self-luminous gaseous body. The Sun is a

typical star.

Steady-state theory: A theory according to which

the universe has always existed, and will exist for ever.

The theory has now been abandoned by almost all

astronomers.

Stratosphere: The layer in the Earth’s atmosphere

lying above the troposphere. It extends from about

11 to about 64 kilometres above sea-level.

Sublimation: The change of a solid body to the

gaseous state without passing through a liquid

condition. (This may well apply to the polar caps

on Mars.)

Sundial: An instrument used to show the time, by

using an inclined style, or gnomon, to cast a shadow

on to a graduated dial. The gnomon points to the

celestial pole. A sundial gives apparent time; to

obtain mean time, the value shown on the dial must

be corrected by applying the equation of time.

Sun-grazers: Comets which atperihelion make

very close approaches to the Sun. All the sun-grazers

are brilliant comets with extremely long periods.

Sunspots: Darker patches on the solar photosphere;

their temperature is about 4000 degrees C (as against

about 6000 degrees C for the general photosphere),

so that they are dark only by contrast; if they could be

seen shining on their own, their surface brilliance

would be greater than that of an arc-light. A large

sunspot consists of a central darkish area or umbra,

surrounded by a lighter area or penumbra, which

may be very extensive and irregular. Sunspots tend

to appear in groups, and are associated with strong

magnetic fields; they are also associated with faculae

and with solar flares. They are most common at the

time of solar maximum (approximately every 11 years).

No sunspot lasts for more than a few months at most.

Supergiant stars: Stars of exceptionally low density

and great luminosity. Betelgeux in Orion is a typical

supergiant.

Superior conjunction: The position of a planet when

it is on the far side of the Sun as seen from Earth.

Superior planets:The planets beyond the orbit of

the Earth in the Solar System: that is to say, all the

principal planets apart from Mercury and Venus.

Supernova:A colossal stellar outburst. A Type I

supernova involves the total destruction of the white

dwarf component of a binarysystem; a Type II

supernova is produced by the collapse of a very

massive star. At its peak, a supernova may exceed

the combined luminosity of all the other stars of an

average galaxy.

Synchronous satellite:An artificial satellite moving

in a west-to-east equatorial orbit in a period equal

to that of the Earth’s axial rotation (approximately

24 hours): as seen from Earth the satellite appears

to remain stationary, and is of great value as a

communications relay. Many synchronous satellites

are now in orbit.

Synchrotron radiation: Radiation emitted by charged

particles moving at relativistic velocities in a strong

magnetic field. Much of the radio radiation coming

from the Crab Nebulais of this type.

Synodic period: The interval between successive

oppositions of a superior planet. For an inferior

planet, the term is taken to mean the interval between

successive conjunctions with the Sun.

Syzygy:The position of the Moon in its orbit when at

new or full phase.

T

Tektites: Small, glassy objects which are

aerodynamically shaped, and seem to have been

heated twice. It has been suggested that they are

meteorite, but it is now generally believed that they

are of terrestrial origin.

Telemetry:The technique of transmitting the results of

measurements and observations made on instruments

in inaccessible positions (such as unmanned probes in

orbit) to a point where they can be used and analysed.

Telescope:The main instrument used to collect the

light from celestial bodies, thereby producing an image

which can be magnified. There are two main types: the

reflector and the refractor. All the world’s largest

telescopes are reflectors, because a mirror can

be supported by its back, whereas a lens has to be

supported around its edge – and if it is extremely

large, it will inevitably sag and distort under its own

weight, thereby rendering itself useless.

Terminator: The boundary between the day and night

hemispheres of the Moon or a planet. Since the lunar

surface is mountainous, the terminator is rough and

jagged, and isolated peaks may even appear to be

detached from the main body of the Moon. Mercury

and Venus, which also show lunar-type phases, seem

to have almost smooth terminators, but this is probably

because we cannot see them in such detail (at least in

the case of Mercury, whose surface is likely to be as

mountainous as that of the Moon). Mars also shows

a smooth terminator, although it is now known that

the surface of the planet is far from being smooth

and level. Photographs of the Earth taken from space

or from the Moon show a smooth terminator which

appears much ‘softer’ than that of the Moon, because

of the presence of atmosphere.

Thermocouple: An instrument used for measuring

very small quantities of heat. When used in conjunction

with a large telescope, it is capable of detecting

remarkably feeble heat-sources.

Tides:The regular rise and fall of the ocean waters,

due to the gravitational pulls of the Moon and (to a

lesser extent) the Sun.

Time dilation effect:According to relatively theory,

the ‘time’ experienced by two observers in motion

compared with each other will not be the same. To an

observer moving at near the velocity of light, time will

slow down; also, the observer’s mass will increase until

at the actual velocity-of-light time will stand still and

mass will become infinite! The time and mass effects

are entirely negligible except for very high velocities,

and at the speeds of modern rockets they may be

ignored completely.

Transfer orbit (or Hohmann orbit):The most

economical orbit for a spacecraft which is sent to

another planet. To carry out the journey by the shortest

possible route would mean continuous expenditure of

fuel, which is a practical impossibility. What has to be

done is to put the probe into an orbit which will swing

it inwards or outwards to the orbit of the target planet.

To reach Mars, the probe is speeded up relative to the

Earth, so that it moves outwards in an elliptical orbit;

calculations are made so that the probe will reach the

orbit of Mars and rendezvous with the planet. To reach

Venus, the probe must initially be slowed down relative

to the Earth, so that it will swing inwards towards the

orbit of Venus. With a probe moving in a transfer orbit,

almost all the journey is carried out in free fall, so

that no propellant is being used. On the other hand,

it means that the distances covered are increased, so

that the time taken for the journey is also increased.

Transit:(1) The passage of a celestial body, or a point

on the celestial sphere, across the observer’s meridian;

thus the First Point of Aries must transit at 0 hours

sidereal time.(2) Mercury and Venus are said to be

in transit when they are seen against the disk of the

ATLAS OF THE UNIVERSE

Glossary

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