|You are here: NMM Home / Learning / Online resources / GCSE Astronomy / The solar system / Moving worlds|
The word 'planet' derives from the Greek for 'wanderer' and the five bright planets were originally recognised as 'wandering stars' as they moved through the zodiac. The way they appear to move depends on both their distance from the Sun and whether they are inside or outside of the orbit of the Earth.
Consequently Mercury moves very quickly across the sky as it races around the Sun, whilst Saturn crawls round, taking almost 30 years to complete a single circuit.
Mercury and Venus are closer to the Sun than the Earth and are described as inferior planets.
The changing phases of an inferior planet as it moves around the Sun
Similarly, at greatest western elongation inferior planets are also at maximum distance from the Sun and at 50% phase, but are best seen just before sunrise.
The changing positions of Venus and Mercury. The lines show the apparent path of each planet in the evening sky as they move around the Sun
Transits of Mercury and Venus
If an inferior planet is directly between the Earth and the Sun and exactly in line with both bodies then for a few hours it will appear to cross or transit the solar surface, appearing as a silhouetted black disk.
Transits of Mercury occur 13 or 14 times each century. The last one took place on 7 May 2003 (see left) and the next will be in November 2006.
Transits of Venus are much rarer. Two successive transits separated by 8 years occur at intervals of 121.5 and 105.5 years. A pair were observed in the 19th century. The last pair, the first was seen on 8 June 2004 and the next can be seen in 2012.
The superior planets are those further from the Sun than the Earth and include Mars, the gas giants and Pluto.
Superior planets are best viewed at opposition, when the Sun and planet are on opposite sides of the Earth. The planet is then closest to the Earth, culminates at midnight and is visible for all or most of the night. At conjunction a superior planet is on the other side of the Sun from the Earth and so is invisible.
Just 85% of the visible hemisphere of Mars is illuminated at this time, giving it an obvious gibbous appearance but all the more distant planets show minimum phases of 99% or above.
With the exception of Pluto, both inferior and superior planets move within the zodiac constellations and remain close to the ecliptic.
Inferior planets move east to west and then west to east as they move around the Sun. The long-term movement of superior planets is from west to east. This is direct motion. But around opposition their motion appears to loop or zigzag and they temporarily move from east to west. This is retrograde motion. The stationary points are where the planet apparently changes direction.
Retrograde motion can be explained by considering the relative orbital speeds and inclinations of the superior planets and the Earth in the heliocentric model of the solar system. Planets further than the Earth from the Sun move more slowly in their orbits, so the Earth will catch up with them as the two planets move towards opposition.
Models of the solar system: from Ptolemy to Kepler
Explaining the movements of the planets and predicting their positions was complex. The models of the solar system evolved from geocentric (Earth-centred) to heliocentric (Sun-centred).
This basic model could not explain why planets underwent retrograde motion - they sometimes appeared to reverse direction and move in loops against the background stars. To account for this a more complicated version placed the Earth very slightly away from the centre of the planet's orbits - the deferent. Two further complications were added. Firstly planets were thought to travel in loops - epicycles - superimposed on their circular orbits. Secondly, the motion of the centre of a planetary epicycle was thought to be uniform (they kept the same speed) with respect to the equant - a point on the other side of the centre of the circle from the Earth.
Further variations in such a complicated system allowed astronomers to explain the observed motions of objects in the solar system and the geocentric model survived for the next 1500 years.
In his book De Revolutionibus he put forward the idea that the planets moved in circles around a stationary central Sun. This model explained the retrograde motion of the planets but could still not predict their exact locations.
The description also applies to a satellite as it moves around a planet.
These are explained below and relate to the diagram on the left.
2. Each planet moves so that an imaginary line joining the centre of the planet to the centre of the Sun sweeps out equal areas in equal times. In the diagram above the planet moves from A to B and from C to D in the same amount of time - the two shaded wedges have equal areas. This means that planets move fastest when they are closest to the Sun (perihelion) and slowest when they are furthest away (aphelion).
3. The square of the time taken for a planet to complete one orbit is proportional to the cube of its mean distance from the Sun.
This is expressed mathematically as:
where T is the orbital period of the planet, r is the mean distance of the planet from the Sun and k is a constant.
If two bodies are involved, this can be written as an equation:
where 1 and 2 are the two orbiting bodies.
Kepler's laws also apply to any system of satellites, for example the moons of Jupiter as well as to comets and asteroids in orbit around the Sun.
The new planets discovered in orbit around other stars also move in the same way.
For the special case of planets in orbit around our own Sun, a simpler version of Kepler's third law can be used. All the planetary orbits can be scaled with respect to the Earth, which takes 1 year to travel around the Sun in an orbit at a mean distance of 1 astronomical unit (AU).
Using Kepler's third law:
so in this case k = 1.
If all the other planetary orbits are set out in years and astronomical units, then their orbits can be scaled with respect to the Earth, just by measuring how long it takes them to travel around the Sun.
1. Saturn takes 29.5 years to orbit the Sun. Calculate Saturn's mean distance from the Sun.
In this case the simple version of Kepler's third law applies where k=1, T is in years and r is in astronomical units (AU).
29.52 = 1 x r3
So r = 9.5 AU
Here the full version of Kepler's third law is used where TD and TP are the orbital periods of Deimos and Phobos, rD and rP are the mean distances of Deimos and Phobos from Mars.
(TD/0.32)2 = (23500/9400)3
So TD = 1.3 days
Questions to think about
© NMM London