Philips Atlas of the Universe

Movements of the Planets

T

he word ‘planet’ really means ‘wanderer’, and the

planets were first identified in ancient times by their

movements against the starry background. Because their

orbits are not greatly inclined to that of the Earth – less

than 4° for all the planets apart from Mercury and the

exceptional Pluto – they seem to keep to a well-defined

band around the sky, termed the Zodiac. There are twelve

official Zodiacal constellations, though a thirteenth,

Ophiuchus (the Serpent-bearer) does cross the zone for

some distance.

The ‘inferior’ planets, Mercury and Venus, are closer

to the Sun than we are, and have their own way of behav-

ing. They seem to stay in the same general area of the sky

as the Sun, which makes them awkward to observe – par-

ticularly in the case of Mercury, where the greatest elonga-

tion from the Sun can never be as much as 30 degrees.

They show phases similar to those of the Moon, from new

to full, but there are marked differences. At new phase, the

dark side of the planet is turned towards us, and we cannot

see it at all unless the alignment is perfect, when the planet

will appear in transit as a dark disk crossing the face of the

Sun. This does not happen very often; Venus was last in

transit in 1882, and will not be so again until 8 June 2004.

Transits of Mercury are less uncommon; the last was on

6 November 1993; followed by that of 15 November 1999.

(En passant, there can surely be nobody now living who

can remember seeing a transit of Venus!)

When an inferior planet is full, it is on the far side of

the Sun, and is to all intents and purposes out of view. At

other times the phase may be crescent, half (dichotomy),

or gibbous (between half and full). At new, the planet is at

inferior conjunction; when full, it is at superior conjunc-

tion. These movements mean that the inferior planets are

best seen either in the west after sunset, or in the east

before sunrise. They never remain above the horizon

throughout a night.

The superior planets, the orbits of which lie beyond

that of the Earth in the Solar System, can reach superior

conjunction – though for obvious reasons they can never

pass through inferior conjunction. When seen at right

angles to the Sun, they are said to be at quadrature. When

near quadrature Mars can show an appreciable phase –

down to 85 per cent – so that when viewed through a

telescope its shape resembles that of the Moon a day or

two from full. The giant planets are so far away that their

phases are inappreciable.

When the Sun, the Earth and a planet are lined up,

with the Earth in the mid position, the planet is at opposi-

ATLAS OF THE UNIVERSE

O

rb

ito

fM

erc

Orb ury

ito

fV

en

us

Sun

3 1

4

2

Earth

Sun

E2

E1 M1

M2

M3

 Phases of Mercury.

(1) New. (2) Dichotomy

(half-phase). (3) Full. (4)

Dichotomy. For the sake of

clarity, I have not taken the

Earth’s movement round

the Sun into account in this

diagram.

 Retrograde motion of

Mars. As the Earth catches

Mars up and passes it, the

movement will seem to be

retrograde, so that between

3 and 5 Mars will appear to

go backwards in the sky –

east to west, against the

stars, instead of west to east.

OrbitofEarth

OrbitofMars

Sun 1

7

6

5 4

1

2

3

4

5

6 7 3 2 1 2

5 4 3

6

7

▲ Movements of Mars.

With the Earth at E1 and

Mars at M1, Mars is in

opposition. A year later the

Earth has come back to E1,

but Mars has only reached

M2. The Earth has to catch

Mars up before there is

another opposition, with the

Earth at E2 and Mars at M3.

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