Today – May 10, 2018 – the planet Jupiter is closest to Earth for this year.
Yet the night of May 8-9 was Jupiter’s opposition, when Earth flew between Jupiter and the sun, placing Jupiter opposite the sun in our sky. You’d think Jupiter should be closest to Earth right at opposition … and yet it wasn’t. It’s closest to Earth for 2018 today, May 10 at 12 UTC, coming to within 409 million miles (658 million km). Jupiter was at opposition on May 9 at 1 UTC.
Jupiter’s closest approach to Earth for the year always falls on or near its opposition date. Why near? Why not always on the date of opposition?
The reason is that the orbits of Earth and Jupiter aren’t perfect circles, and, moreover, their orbits aren’t exactly on the same exact plane. They’re both very nearly circular, and go around the sun on almost the same plane, but not quite. Jupiter’s distance from the sun varies by only about 10.1% between perihelion (closest point to sun) and aphelion (most distant point from sun). If our planets’ orbits were perfect circles, then yes, we’d be closest together on the date Earth passes between Jupiter and the sun. But Earth’s orbit, and Jupiter’s orbit, are elliptical, like circles someone sat down on.
Jupiter’s elliptical orbit means its distance from the sun varies (as does Earth’s distance from the sun). Jupiter passed aphelion – its farthest point from the sun in its orbit – on February 16, 2017, an event that occurs once in each of the giant planet’s 11.9-year orbits around the sun. At aphelion, it lay about 507 million miles (816 million km) from the sun.
Since February 16, 2017, Jupiter has been heading toward perihelion – its closest point to the sun. It will reach its perihelion point on January 20, 2023, when it’ll lie 460 million miles (741 million km) from the sun.
Earth’s perihelion comes every year in early January. So we’re getting a bit farther from the sun each day now.
So Jupiter is now getting closer to the sun – bit by bit, closer and closer – every earthly day. And Earth is getting farther from the sun – bit by bit, farther and farther – every day. And that’s how Jupiter and Earth can be closest for 2018 about one and one-half days after our planet’s May 9, 2018 pass between Jupiter and the sun.
Jupiter can be closest to us today (May 10) because it’s closer to the sun today than it was at its May 9 opposition, while the Earth is farther from the sun today than it was when Jupiter was at opposition.
Bottom line: You’d think Jupiter would be closest to Earth on the day we pass between it and the sun. We did that on May 9, 2018, though, and our 2 worlds are closest on May 10. Why?