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.

Lunar eclipse, where the shadow of the Earth can cast the Moon in darkness around the full moon phase. From left to right, the smaller Moon is hidden in the shadow of the larger Earth, which prevents the Sun's light from reaching it.
[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 a lunar eclipse (above), while only a portion of the Earth is shadowed in a solar eclipse?

Solar eclipse, where the shadow of the Moon can cast a portion of the Earth in darkness around the new moon phase. From left to right, the larger Earth is partially hidden in the shadow of the smaller Moon, which prevents the Sun's light from reaching it.
[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 lies 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?

Diagram of the inner solar system showing the central Sun and the Earth in orbit around it on the left side. Planet Mars (orbiting at a larger radius) is first shown further to the left on the same side of the Sun and so at opposition (as it lies on the opposite side of the Earth than the Sun) and second on the right side on the opposite side of the Sun and so at conjunction (because it lies on the same side of the Earth as the Sun).
[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).

Animation showing how the orbital motions of the Earth and Mars cause the position of Mars in the sky when viewed from Earth to slowly shift eastward, then westward (retrograde), and then eastward again. The retrograde motion occurs because Earth orbits the Sun faster than Mars does, and so over time catches up with and laps its neighbor which lies further from the Sun.
[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 impossible 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, ...)?

Diagram of the inner solar system showing the central Sun and the Earth in orbit around it on the left side. Planet Venus (orbiting at a smaller radius) is shown in three positions. In each case the half of Venus which faces the Sun is shown as bright and the half which faces away is shown as dark. A line bisects each Venus showing the portion which faces Earth and can be seen by us. As Venus changes position around the Sun relative to the Earth, the portion of the lit half that we can see grows larger and smaller.
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]

Diagram of the inner solar system showing the central Sun and the Earth in orbit around it on the left side. Planet Mars (orbiting at a larger radius) is shown in three positions. In each case the half of Mars which faces the Sun is shown as bright and the half which faces away is shown as dark. A line bisects each Mars showing the portion which faces Earth and can be seen by us. As Mars changes position around the Sun relative to the Earth, the portion of the lit half that we can see grows larger and smaller.
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]