On May 10, 2018, the giant planet Jupiter will be closest to Earth for all of 2018.
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 would be closest to Earth on the day of opposition. But it isn’t. Why not?
In 2018, Jupiter’s opposition comes about a day-and-a-half before its closest point to Earth:
Jupiter’s opposition May 9 at 1 UTC (May 8 at 8 p.m. CDT).
Jupiter closest May 10 at 12 UTC (7 a.m CDT).
At its closest, Jupiter comes to within 409 million miles (658 million km).
Why isn’t Jupiter closest on the day of opposition? They would be, if the orbits of Earth and Jupiter were perfect circles and if our two worlds orbited on exactly on the same exact plane. They don’t. Both Earth and Jupiter have orbits that are very nearly circular. They go around the sun on almost the same plane. But not quite.
Consider that, because Jupiter’s orbit is elliptical, not circular, its distance from the sun varies. Likewise, Earth’s orbit is elliptical, not circular. Our distance from the sun varies, too.
Jupiter’s orbit takes 11.9 Earth-years. Earth’s orbit takes one year.
Right now, we’re headed toward a perihelion of Jupiter. In other words, every single day, Jupiter is closer to the sun than it was the day before. Are you beginning to see how it can be closer to Earth after we go between it and the sun?
Not yet? Keep reading …
Jupiter passed aphelion – its farthest point from the sun in its orbit – on February 16, 2017. Jupiter will reach perihelion – its closest point – on January 20, 2023. So Jupiter is getting closer to the sun each day. And what is Earth doing?
Earth’s perihelion happened in early January. So Earth is getting a bit farther from the sun each day now.
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-a-half days after our planet’s pass between Jupiter and the sun.
Understand? If not, check out these two links … or let’s talk in the comments below …
Bottom line: You’d think Jupiter would be closest to Earth on the day we pass between it and the sun. We did that during the night of May 8, 2018, for clocks in North America. Yet our 2 worlds are closest on May 10. Why?
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.