How Far Is Jupiter From Earth

Happy 1st Birthday, Neptune — 165 Years in the Making

It was exactly one year ago this week that astronomers found a brand new planet in our solar system — a giant ball of ice and gas some 17 times the mass of the Earth, orbiting the Sun beyond Uranus. Ordinarily, a discovery this astonishing would have been the lead story in every media outlet in the world — but it's pretty much guaranteed that no living human heard even a rumor about the announcement.

That's because we automatically think of the word "year" in very local terms. Technically, a year is the time it takes any planet to complete one orbit of its star. Where we live, that adds up to 365 days and change. But for Neptune, discovered in 1846, a single orbit takes a bit less than 165 Earth years — and the planet has now completed exactly one of those Neptunian years since it was first spotted by the German astronomer Johann Gottfried Galle, at the Berlin Observatory. It has thus at last returned to the same point in the sky as it was when it was first identified.

In honor of the occasion, the people who run the Hubble Space Telescope have taken a set of four anniversary photos of the big blue planet, one every four hours or so to capture a full rotation (Neptune's year may be much longer than ours, but its day lasts only 16 hours). It's now early winter in Neptune's northern hemisphere — a season that will last 40 Earth years or so. The planet's solid surface is invisible, shrouded by a thick atmosphere made mostly of hydrogen and helium. White clouds of methane ice swirl through the Neptunian air — a feature nobody expected when Voyager 2 took the first and last closeups of the planet in 1989. Neptune also has a faint ring, but it's a poor cousin to Saturn's magnificent multicolored tutu.

The fact that Neptune was found at all was a big enough deal in its own right. It was only the second planet in history that was ever discovered. The first was Uranus, spotted by the musician-turned-astronomer William Herschel in 1781. (Mercury, Venus, Mars, Jupiter and Saturn were never really discovered, since they'd always been clearly visible to anyone who looked up.)

But the way Neptune was found was perhaps even more important. Herschel had stumbled on Uranus while surveying the skies from his back garden with his home-built telescope; his genius was in building such an exquisite instrument in the first place and then in recognizing that the object he spotted was highly unusual and worthy of following up.

Blogstronomy: How far away from Earth is Europa?

Europa orbits the planet Jupiter at an average distance of just over 670,000 kilometres. Compared to the 750,000,000 kilometre orbit of Jupiter around the Sun, this is pretty much nothing at all, so it makes sense to treat Europa as if it is the same distance away from us as Jupiter is. Another reason to do this is that due to Earth's rotation around the Sun, Jupiter's rotation around the Sun, and Europa's rotation around Jupiter, it's almost meaningless to put an actual value on how far away we are from Europa because it changes all the time. However, in the post How far away from the Sun is Jupiter? I also talked a little bit about how far Jupiter is from the Earth and, although the distance changes (for the same reasons outlined above), put some numbers on the greatest and smallest possible distances between the two. The answer, to any sensible degree of accuracy, would be pretty much exactly the same for Europa. If you're desperate for some greater accuracy, then all you need to do is read the article linked above, then subtract Europa's apoapsis* from Jupiter's closest approach to the Earth, and add it on to the furthest apart that Earth and Jupiter find themselves, but it won't make a lot of difference.

The View from the Center of the Universe, Discovering Our Extraordinary Place in the Cosmos

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