Planetary Phases and Planetary Motion
By Ian Ridpath
The phases of the Moon are very familiar to us, but the inner planets, Mercury and Venus, can also show phases as they move around their orbits. In fact, it was Galileo's discovery of the phases of Venus in the early seventeenth century that confirmed the planets orbit the Sun and not the Earth.
With one exception, the outer planets do not show appreciable phases because, as seen from Earth, they always appear fully illuminated by the Sun. The exception is Mars, which can appear slightly gibbous (i.e. not quite circular) at certain positions in its orbit, when it lies at an angle of about 90 degrees from the Sun, relative to the Earth.
As seen through a telescope, both Mercury and Venus display a full range of phases, but Venus is much the easier of the two planets to see since it is brighter and farther away from the Sun.
When Venus is at crescent phase, in the morning or evening sky, it is close to Earth and appears large enough for the phase to be visible through binoculars.
As Venus moves away from the Sun in the morning sky, the phase increases from a crescent, reaching half phase at the time of greatest elongation. The distance between Venus and Earth is increasing, so its disc is becoming smaller. Continuing on around its orbit, Venus increases in phase until, at superior conjunction, it is fully illuminated - but then it is on the far side of the Sun and cannot be seen.
After superior conjunction, Venus emerges into the evening sky and begins to decrease in phase as it approaches Earth, becoming a slim crescent again as it moves towards inferior conjunction. Venus is at its brightest about five weeks before and after inferior conjunction, when its phase is about 27%, i.e. a thick crescent.
The Motions of the Planets
Planets orbit the Sun like cars lapping a gigantic racetrack, but they all must obey the strict lane discipline dictated by gravity: traffic in the inner lanes moves faster than the outer ones, in accordance with Kepler's Laws. Mercury, the innermost planet, hurries around the Sun every 88 days, while remote Pluto is a dawdler, taking 248 of our years to complete one lap.
From Earth's position in lane three of the planetary race track we see the planets in lanes one and two (Mercury and Venus) swing from one side of the Sun to the other and back again, appearing alternately in the evening and morning sky. When moving from the evening to morning sky these planets overtake us on the inside lane, lining up with the Sun at inferior conjunction. When moving from the morning back to the evening sky they lie on the far side of the Sun, passing through an alignment known as superior conjunction.
The widest separation of the inner planets from the Sun on each is termed greatest elongation - for Venus this is around 46 degrees but for Mercury, whose orbit is markedly elliptical, it can range from 18 degrees to 28 degrees.
Planets in the outer lanes can appear in any direction relative to the Sun, as seen from Earth. At times they can lie directly opposite the Sun in the sky, an alignment known as opposition. When they are behind the Sun they are said to be at conjunction (outer planets cannot, of course, come to inferior conjunction).
When we overtake an outer-lane planet something strange happens: that planet seems to slow down and go into reverse (retrograde motion) before resuming its normal forward motion. This is purely an illusion caused by the relative speeds of the Earth and planet - the same effect is seen when a car in an adjacent lane on a motorway appears to be moving backwards as you overtake it.
The average interval between two conjunctions or oppositions is known as a planet's synodic period; this differs from the orbital period because it is measured relative to the moving Earth.