Planetary Orbits

Here are the positions of the Moon and the Earth (the Sun lies far off to the right of the figure) during a lunar eclipse. The Earth is blocking the sunlight from reaching the Moon, and we see its shadow pass over the Moon. Because the Moon's orbit is inclined by 5 degrees to the orbit of the Earth around the Sun, we have lunar eclipses occasionally rather than once every month.

 [NMSU, N. Vogt]

Here are the positions of the Earth and the Moon during a solar eclipse. The Moon is blocking the sunlight from reaching the Earth, for a portion of the Earth's surface. Why is the entire Moon shadowed in the lunar eclipse (above), while only a portion of the Earth is shadowed in the solar eclipse?

 [NMSU, N. Vogt]

We say that a planet is in opposition when it is on the opposite side of the Earth than the the Sun, and that a planet is in conjunction when it lie behind the Sun from the Earth. Can an inner planet, whose orbit lies within that of the Earth (Mercury or Venus) ever be observed at opposition?

 [NMSU, N. Vogt]

Because the Earth and the fellow planets orbit the Sun at different speeds, the paths of the other planets in the sky as observed from Earth can sometimes appear to move forwards (from west to east over a period of weeks), then backwards, and then forwards again as traced against the stars. We call this retrograde motion. Observe how from the surface of the Earth (the blue planet), the black path of the red planet appears to move forward, then backward, and then forward again. This happens because inner planets orbit faster than outer planets and so catch up with them (if they were racing around the Sun, we would say that they lapped them).

 [NMSU, N. Vogt]

Venus is an inner planet, by which we mean that it orbits that Sun within the orbit of the Earth. Because of this geometry, the Earth can never come between Venus and the Sun (Venus will never be observed at opposition). Could we ever observe Venus at midnight? At what time of the day would Venus be fully illuminated by the Sun (like the full Moon)? At what time of the day would Venus be completely hidden in shadow (like the new Moon)? Venus is not very bright in the sky, so it is nearly impossibly to pick out (by eye) when the Sun is shining. When would the best times be to observe Venus? And where would it appear at that time (directly overhead, at the horizon, ...)?

• What are the observed phases of an inner planet, like Venus?

 Venus and Earth are shadowed, to show the portion of each planet illuminated by the Sun at each orbital position. The green lines superimposed on Venus indicate the portion of the planet seen from Earth at each orbital position. [NMSU, N. Vogt]

• What are the observed phases of an outer planet, like Mars?

 Mars and Earth are shadowed, to show the portion of each planet illuminated by the Sun at each orbital position. The green lines superimposed on Mars indicate the portion of the planet seen from Earth at each orbital position. [NMSU, N. Vogt]