The path of a planet in the sky as seen from the Earth results from a combination of the effects of the. orbital motion of both the Earth and the planet. The form of the observed motion depends primarily on whether the planet is nearer the Sun than the Earth (an INFERIOR PLANET), or further away (a SUPERIOR PLANET). Mars is a typical superior planet . When the Earth and Mars are in line with the Sun and on the same side, Mars is said to be at OPPOSITION. At this time Mare is nearest to the Earth and best placed for observation. Figure 9.6 illustrates the terms opposition, CONJUNCTION, QUADRATURE and PHASE ANGLE for Mars. These terms are also used for the other superior planets: Jupiter, Saturn, Uranus, Neptune and Pluto. Technically, opposition is defined to occur when the celestial longitude of the planet differs from that of the Sun by 180o”. Conjunction occurs when the difference is zero, and quadrature when it is 90°.

For Mars the greatest phase angle is 470, for Jupiter it is 12o and for the other superior planets it is smaller still. As Mars and the Earth move round the Sun the varying distance between them causes the angular diameter of Mars to vary from 25 arc set at opposition to 3.5 arc see at conjunction.

One of the two inferior planets; the other is Mercury. As Venus moves along its orbit it appears successively at INFERIOR CONJUNCTION, GREATEST WESTERN ELONGATION, SUPERIOR CONJUNCTION, GREATEST EASTERN ELONGATION before returning to inferior conjunction. At its positions of greatest elongation Venus appears 48° away from the Sun in the sky. The planet is best placed for visual observations from the Earth when it is near these positions. During its orbital motion Venus turns varying parts of its illuminated hemisphere towards the Earth so that it shows phases similar to the Moon. At inferior conjunction the dark side is towards Earth, but, as Venus moves, the illuminated hemisphere is gradually turned towards Earth: at greatest western elongation half the visible hemisphere is illuminated and at superior conjunction all of it is. In the other half of the orbit the phases occur in the reverse order. Meanwhile the apparent diameter of Venus is varying between 64.5 arc sec at inferior conjunction, when it is nearest to Earth, and 9.9 arc sec at superior conjunction when it is furthest away. Because of its varying phase and distance Venus does not attain its brightest apparent magnitude at either conjunction, but near to its positions of great elongation. The changing phases make it an interesting subject for binoculars or a small telescope.

Mercury shows the same phases as Venus but because of its smaller orbit its greatest elongations are 28° from the Sun. At inferior conjunction, Venus and Mercury may pass in front of the Sun’s disc. Such a TRANSIT does not occur every time because of the inclination of the planets’ orbits to that of the Earth. it can be seen that transits of Mercury are much more common than those of Venus. Transits can only occur when the planet is near one of the nodes of its orbit, since otherwise it passes above or below the Sun. Mercury passes through its nodes on 9 May and 10 November, Venus on 7 June and 9 December and so transits can only occur close to these dates.

Because the orbits of all the planets are inclined to that of the Earth, each planet is seen to move along a different path against the background of stars. None of the inclinations is very great and as a result the apparent paths of all the planets are confined to a narrow band in the sky called the ZODIAC.

The name SIDEREAL PERIOD is given to the true time of revolution of a planet round the Sun relative to the distant stars. The SYNODIC PERIOD is the average time of revolution relative to the line joining the Earth to the Sun, that is the average time between two successive oppositions or corresponding conjunctions.

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