The short answer
Opposition is the moment a planet sits on the opposite side of Earth from the Sun. Seen from here, it rises around sunset, is highest around midnight and sets around sunrise, so it’s up all night. It is also near its closest to Earth, which makes it look bigger and brighter than at any other time in its cycle. For Mars, Jupiter, Saturn, Uranus and Neptune, the weeks around opposition are the best time of year to observe.
The geometry
Picture the Sun, Earth and an outer planet such as Jupiter. Earth moves faster around the Sun than any outer planet, so every so often it overtakes it on the inside track. At that moment the three line up, Sun, Earth, planet, and the planet is “opposite” the Sun in our sky.
The opposite arrangement, with the planet on the far side of the Sun, is called conjunction with the Sun. Then the planet is lost in the Sun’s glare for weeks. Between the two, a planet spends months in the morning sky, then all-night, then the evening sky. See what is a conjunction for the other meanings of that word.
Why it’s the best time to look
- It’s up all night. You can observe it in the evening, at a convenient hour, and it reaches its highest point around midnight (around 1 a.m. under daylight saving time).
- It’s near its closest. A closer planet shows a larger disk and more detail in a telescope. The effect is dramatic for Mars, modest for Jupiter and Saturn.
- It’s at its brightest. It is fully lit from our point of view. Saturn gets an extra boost: around opposition its rings brighten noticeably, because sunlight bounces straight back from the ring particles.
How often it happens
The gap between oppositions depends on how fast the planet moves compared with Earth. The slower the planet, the sooner Earth catches up with it again, so the far planets come to opposition roughly once a year, a little later each time.
| Planet | Roughly every | Notes |
|---|---|---|
| Mars | About 2 years and 2 months | Varies a lot in size and brightness from one opposition to the next |
| Jupiter | About 13 months | Moves about one constellation per year along the zodiac |
| Saturn | About 12½ months | Rings brighten noticeably near opposition |
| Uranus | A little over a year | Faint; needs a dark sky or binoculars and a chart |
| Neptune | A little over a year | Needs binoculars or a telescope |
Mars is the special case. Its orbit is noticeably elongated, so some oppositions bring it much closer than others. At a close opposition it can outshine Jupiter; at a distant one, its disk is small even in a good telescope. For the same reason, the date of Mars’s closest approach can differ by days from the date of opposition. For current dates, see sky events and planets visible tonight.
Mercury and Venus never reach opposition
The inner planets orbit inside Earth’s orbit, so they can never appear opposite the Sun. They always stay within a limited angle of it, which is why you see them only in the evening or morning twilight. For them, the best times to look are their greatest elongations, when they appear furthest from the Sun. How to find planets explains the difference.
The Moon has an opposition too, of a sort: when it is opposite the Sun, it’s full. That’s why the full Moon, like a planet at opposition, rises at sunset and is up all night.
Making the most of it
- Wait until it’s high. A planet low in the sky is viewed through more air and looks blurry. Around midnight is best.
- Use moderate magnification. More isn’t always better; see the best magnification for planets.
- Watch it move. Around opposition an outer planet appears to stop and drift backward against the stars for a few weeks, an effect called retrograde motion. It’s an illusion caused by Earth overtaking it.