There are five (excluding the Earth) naked eye planets, Mercury, Venus, Earth, Mars, Jupiter, and Saturn ( sky view from Eugene ). Uranus is actually just bright enough to be seen with the unaided eye at a dark sight if you know where to look and someone points it out. The planets' motions share some common features:

• The planets move roughly in the ecliptic plane. This implies that the orbits of the planets are nearly co-planar with the orbit of the Earth about the Sun.

• Today, we know that the planets orbit the Sun in the same sense as does the Earth, the west-to-east direction counterclockwise as viewed from the NCP. We also observe a strange behavior in that the apparent motion of planets can appear more erratic. Planets usually move in the west-to-east direction through the stars on the Celestial Sphere, however, periodically, this motion reverses itself and the planets move east-to-west through the stars, undergoing what is called retrograde motion. This erratic motion was something all models for the Solar System had to explain.

• The planets' orbits are all roughly circular in shape.

The preceding green-highlighted facts are interesting and important because they suggest that our Solar System formed as a single unit rather than as individual planets which, later, were assembled into the Solar System.

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What are the interesting properties of the apparent (observed) motions of the planets?

Retrograde motion

The planets generally move west-to-east through the stars (prograde motion) but, peridiocally, the motion changes and they move east-to-west through the stars (retrograde motion). Below is the 2014 retrograde of Mars:


The retrograde motion continues for a few months and then the motion switches back to prograde. This seemingly inexplicable behavior is easily understood within the context of a Sun-centered (helio-centric) Solar System. This motion is harder to understand in the context of an Earth-centered (geo-centric) model. We return to this issue later.

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Morning and Evening Stars

Venus and Mercury are observed only in the early morning or early evening; Venus and Mercury can not be seen at midnight in Eugene. The explanation for this phenomenon is simple in the Sun-centered (helio-centric) model:


Because the orbits of Mercury and Venus lie inside the orbit of the Earth, we see that both planets have to stay rather close to the position of the Sun as they move through the sky. Venus must be within ~46 degrees of the Sun at all times while Mercury (because of its smaller orbit) must remain within 28 degrees of the Sun (Note -- due to the non-circular shape of Mercury's orbit, this limit can be as small as 18 degrees). This close proximity to the Sun means that we should be able to see Mercury and Venus only in the early evening (after the Sun has just set) or in the early morning (just before the Sun rises):


Morning and Evening stars can be explained in a tweaked geo-centric model. They, however, are naturally explained in a helio-centric model as shown above.

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Phases of Venus

Venus and Mercury both go through the full set of phases like the Moon; Mars does not. Why?

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Galileo discovered that Venus goes through phases in the 1600s. The observation of the phases of Venus requires a telescope. The geo-centric model when modified to explain morning and evening stars cannot simultaneously account for the phases of Venus. Consequently, we concentrate on looking at how the geo-centric model explains retrograde motion.

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