Our Solar System – The inner planets

Everybody (well, almost) knows that our planet is located in a solar system where we revolve around our Sun together with seven other planets (once there were nine; a few years ago, Pluto was downgraded from planet to minor planet or dwarf planet). All planets move on almost circular orbits, and their orbits are located in almost the same plane. Moreover, all planets move in the same direction, let us say prograde (i.e. inverse to the sense of the clock hands spinning). This is true for an observer that looks at this plane from (let’s say) above it. An observer “below” this plane (i.e. on the other side) would see a retrograde motion of the planets. Basically, the perspective on everything in the Universe strongly depends on the location from which things are observed.

The inner planets are those who have the orbit with a radius smaller than Earth’s orbit radius. Starting from the closest to the Sun, they are Mercury and Venus. Mercury was known to the Sumerians around 3,000 B.C., while the oldest surviving astronomical manuscript mentioning Venus dates back to 1,600 B.C. and was written in Babylon.

Their motion is depicted in the animation below. This is what somebody which has the position fixed with respect to the Sun would see it.



Figure 1. Sun-bound view on the planetary motion.

Our ancestors did not even imagine this. Their perspective was earthbound, so they saw how this motion takes place with respect to the Earth. For them, the motion of the inner planets was intricate and counter-intuitive (see the animation below), nurturing  superstitions and generating serious scientific questions which remained unanswered for many centuries.


Figure 2. Earth-bound view of the planetary motion

At the beginning, the Earth was considered to be the centre of the Universe (and unfortunately, there are many scientific illiterates today still to believe it). The first heliocentric model was proposed by Aristarchus of Samos, that came to contradict the geocentric model of Heraclides Ponticus (although some claim the latter is the first to propose a heliocentric model).

The first breakthrough (and also a huge drawback) came with Claudius Ptolemy, who offered a quite accurate planetary model that could predict eclipses and explain the retrograde motion (we’ll come back to this one shortly). His model stated that planets move around Earth as a result of the composition of two motions: (1) the rotation on a small circle, called epicycle and (2) the motion of the centre of the epicycle on anotherr circle, called deferent.

If we take another look at Figure 2, we realise that Ptolemy was right: the man spoke what he saw… from Earth. The epicycle is the orbit of the planet as seen from the Sun, while all planets have the same deferent, the apparent orbit of the Sun around Earth. Too bad that Ptolemy put the later in the centre of the solar system. His error propagated for almost one and a half millennia, until Nicolaus Copernicus formulated (again) the first heliocentric model, sometime around 1510.


Figure 3. Copernican system (photo credit: Wikipedia)

Planetary retrograde motion

This phrasing obviously refers to the apparent motion of a planet with respect to Earth or another celestial body which is not the Sun. As depicted in the animation in Figure 2, at some point (let us take Mercury as example) the planet seems to move backwards with respect to Earth, namely around the point where its distance to the blue planet is minimal. Astronomically, it does not mean anything more than the two planets (Mercury and Earth) come at their closest relative distance (and no, folks, there is no energy emerging when this retrograde motion happens at the same time when our natural satellite, the Moon, is both at perigee and full).

Please stay tuned for the next articles. Meanwhile, I leave you contemplate both Sun-bound and Earth-bound apparent motions in one single synchronised animation.



The animations in this article were made by using Geogebra.

Vladimir Martinusi, Phd

Research Scientist


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