Today – April 8, 2017 – the planet Jupiter is closest to Earth for this year.
Yet yesterday was Jupiter’s opposition, when Earth flew between Jupiter and the sun, placing Jupiter opposite the sun in our sky. You’d think Jupiter was closest to Earth for 2017 yesterday as well … and yet it wasn’t. It’s closest to Earth for 2017 today, April 8, coming to within 414 million miles (666 million km).
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. They’re both very nearly circles, 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 just passed an aphelion 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, Jupiter has been heading toward another perihelion. It will reach its perihelion on January 20, 2023, when it’ll lie 460 million miles (741 million km) from the sun.
Jupiter is only 0.00002 AU closer to the sun on April 8 than on April 7, but the Earth is 0.000282 AU farther from the sun on April 8 than on April 7. So Earth and Jupiter are closer on April 8 because the Earth is moving outward (away from the sun) while Jupiter is moving inward (toward the sun). The Earth’s outward change in distance appears to be greater than Jupiter’s inward change of distance.
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 2017 one day after our planet’s April 7, 2017 pass between Jupiter and the sun.
It can be closest to us today (April 8) because it’s closer to the sun today than it was yesterday, while the Earth is farther from the sun today than it was yesterday.
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 April 7, 2017, though, and our 2 worlds are closest on April 8. Why?