In everyday use, the word “opposition” has a somewhat negative connotation. But in observational astronomy, opposition is a good thing.
Each planet beyond Earth in the solar system—Mars, Jupiter, Saturn, Uranus, and Neptune—is said to be at opposition when it is opposite the Sun as seen from Earth. In other words, Sun, Earth, and outer planet are in a line, with Earth in the middle.
This happens because, moving outward from the Sun, each planet’s orbit is larger than the one before, and each planet travels slower in its orbit than the one before. Therefore, each planet takes longer to complete its orbit than any of the planets closer to the Sun.
For example, Jupiter was at opposition this past September. Since Jupiter takes longer to chug around the Sun (4,332 days) than Earth does (365 days), Earth catches up to and passes Jupiter about every 13 months, slipping between Jupiter and the Sun.
The significance of opposition to hobby astronomers is three-fold:
1) When a planet is at or near opposition, it is closer to Earth than at other points in its orbit;
2) A planet at opposition is fully illuminated by the Sun, from our vantage point on Earth; and
3) A planet at opposition is visible all night long, rising at sunset and reaching its highest point in the sky around midnight.
In short, a planet at or near opposition is close, bright, and up all night. This combination presents planetary observers with optimal viewing conditions and the opportunity to see detail on the planet’s surface they might not otherwise spot. Not to mention an excellent excuse to stay up all night, no matter what the neighbors think.
Here’s a related question recently dropped into my Ask An Astronomer box at a local science center’s evening event; it was submitted by Tara:
“Jupiter just went into its closest opposition since 1963. A site I found on the internet said it wouldn’t be this close again until 2022. Why the uneven time difference if this is true?”
Wonderful question and observation, Tara. And you need know but one thing to understand why: the orbits of the planets are not perfect circles. Instead, planetary orbits are elliptical, or oval. The eccentricity of each planet’s elliptical orbit differs, that is, the degree to which the oval is stretched out from the circular. In addition, the planets’ orbits are inclined, or tilted, with respect to the plane of Earth’s orbit around the Sun. These variables inserted into the distance-at-opposition equation mean the possible distances separating Earth and another planet when they rendezvous at opposition are, well, astronomical.