When we say an astronomical object is at opposition, we are saying it is opposite the sun’s position in the sky. When the sun is going down, the object at opposition is rising. At local midnight when the sun is beneath our feet on the opposite side of the earth, the object at opposition is highest in the sky. Likewise, the object is setting when the sun is rising.
At or close to opposition, an object is also closest to Earth. Combined with the fact that it’s also up all night, it’s a great time to look at that object in a telescope for the best view. But the object’s distance can vary from one opposition to the next. Mars was especially close to Earth during its 2003 opposition. At 0.373 astronomical units (34.6 million miles or 55.8 million kilometers), it was the red planet’s closest pass to Earth in about 60,000 years. The Mars opposition of 2012 was 0.674 astronomical units away. In 2018, Mars will pass 0.386 astronomical units away. These variations happen because Earth and Mars each orbit in ellipses rather than perfect circles and Mars’ orbital period isn’t a whole multiple of Earth’s. Earth and Mars are at different points on their ellipses from one opposition to the next.
Mars oppositions happen a little more than two years apart. The planet doesn’t go through it in 2017. For the planets farther out, oppositions happen a little more than one year apart. Because they move more slowly, they aren’t much past the places they were the next time Earth passes between them and the sun. But after enough time, their opposition dates shift by several weeks and then months.
Let’s note that an object at opposition isn’t usually exactly opposite the sun. If it is, its elongation is 180°. But because the earth will rarely pass perfectly in between the sun and that object, its elongation will almost always be a little less, say 178.4°. And planets closer to the sun than Earth never reach opposition. Instead, Mercury and Venus reach greatest elongation. The most Mercury gets from the sun is about 27°. For Venus, greatest elongation is about 46°.
Some planet oppositions in 2017 (all times and dates Universal Time)
Apr-07 21:38 Jupiter at opposition 4.455 AU (approx. 666.5 million km or 414.1 million miles) from Earth.
Jun-15 10:21 Saturn at opposition 9.043 AU (approx. 1353 million km or 840.6 million miles) from Earth.
Jul-10 04:38 Pluto at opposition 32.347 AU (approx. 4839 million km or 3006.8 million miles) from Earth.
Sep-05 05:27 Neptune at opposition 28.939 AU (approx. 4329.2 million km or 2690 million miles) from Earth.
Oct-19 17:34 Uranus at opposition 18.915 AU (approx. 2829.6 million km or 1758.3 million miles) from Earth.
Some asteroid oppositions in 2017 (all times and dates Universal Time)
Jan-17 22:36 Vesta at opposition 1.523 AU (approx. 227.8 million km or 141.6 million miles) from Earth.
Feb-20 03:03 Eunomia at opposition 1.851 AU (approx. 276.9 million km or 172.1 million miles) from Earth.
Jun-17 19:02 Hebe at opposition 1.548 AU (approx. 231.6 million km or 143.9 million miles) from Earth.
Jul-02 17:29 Juno at opposition 2.097 AU (approx. 313.7 million km or 194.9 million miles) from Earth.
Oct-30 08:47 Iris at opposition 0.849 AU (approx. 127 million km or 78.9 million miles) from Earth.