{"id":13239,"date":"2018-11-22T14:59:43","date_gmt":"2018-11-22T22:59:43","guid":{"rendered":"http:\/\/www.preposterousuniverse.com\/blog\/?p=13239"},"modified":"2018-11-22T14:59:43","modified_gmt":"2018-11-22T22:59:43","slug":"thanksgiving-13","status":"publish","type":"post","link":"https:\/\/preposterousuniverse.com\/blog\/2018\/11\/22\/thanksgiving-13\/","title":{"rendered":"Thanksgiving"},"content":{"rendered":"

This year we give thanks for an historically influential set of celestial bodies, the moons of Jupiter<\/a>. (We’ve previously given thanks for the Standard Model Lagrangian<\/a>, Hubble’s Law<\/a>, the Spin-Statistics Theorem<\/a>, conservation of momentum<\/a>, effective field theory<\/a>, the error bar<\/a>, gauge symmetry<\/a>, Landauer’s Principle<\/a>, the Fourier Transform<\/a>, Riemannian Geometry<\/a>, the speed of light<\/a>, and the Jarzynski equality<\/a>.)<\/p>\n

\"\"<\/a>For a change of pace this year, I went to Twitter and asked for suggestions for what to give thanks for in this annual post. There were a number of good suggestions, but two stood out above the rest: @etandel<\/a> suggested Noether’s Theorem<\/a>, and @OscarDelDiablo<\/a> suggested the moons of Jupiter. Noether’s Theorem, according to which symmetries imply conserved quantities, would be a great choice, but in order to actually explain it I should probably first explain the principle of least action<\/a>. Maybe some other year.<\/p>\n

And to be precise, I’m not going to bother to give thanks for all<\/em> of Jupiter’s moons. 78 Jovian satellites have been discovered thus far, and most of them are just lucky pieces of space debris that wandered into Jupiter’s gravity well and never escaped. It’s the heavy hitters — the four Galilean satellites<\/a> — that we’ll be concerned with here. They deserve our thanks, for at least three different reasons!<\/p>\n

Reason One: Displacing Earth from the center of the Solar System<\/strong><\/p>\n

Galileo discovered the four largest moons of Jupiter — Io<\/a>, Europa<\/a>, Ganymede<\/a>, and Callisto<\/a> — back in 1610, and wrote about his findings in Sidereus Nuncius<\/a> (The Starry Messenger<\/em>). They were the first celestial bodies to be discovered using that new technological advance, the telescope. But more importantly for our present purposes, it was immediately obvious that these new objects were orbiting around Jupiter, not around the Earth.<\/p>\n

\"\"<\/p>\n

All this was happening not long after Copernicus had published his heliocentric model of the Solar System in 1543, offering an alternative to the prevailing Ptolemaic geocentric model. Both models were pretty good at fitting the known observations of planetary motions, and both required an elaborate system of circular orbits and epicycles — the realization that planetary orbits should be thought of as ellipses didn’t come along until Kepler published Astronomia Nova<\/em><\/a> in 1609. As everyone knows, the debate over whether the Earth or the Sun should be thought of as the center of the universe was a heated one, with the Roman Catholic Church prohibiting<\/a> Copernicus’s book in 1616, and the Inquisition putting Galileo on trial<\/a> in 1633. <\/p>\n

Strictly speaking, the existence of moons orbiting Jupiter is equally compatible with a heliocentric or geocentric model. After all, there’s nothing wrong with thinking that the Earth is the center of the Solar System, but that other objects can have satellites. However, the discovery brought about an important psychological shift. Sure, you can<\/em> put the Earth at the center and still allow for satellites around other planets. But a big part of the motivation for putting Earth at the center was that the Earth wasn’t “just another planet.” It was supposed to be the thing around which everything else moved. (Remember that we didn’t have Newtonian mechanics at the time; physics was still largely an Aristotelian story of natures and purposes, not a bunch of objects obeying mindless differential equations.)<\/p>\n

The Galilean moons changed that. If other objects have satellites, then Earth isn’t that<\/em> special. And if it’s not that special, why have it at the center of the universe? Galileo offered up other arguments against the prevailing picture, from the phases of Venus to mountains on the Moon, and of course once Kepler’s ellipses came along the whole thing made much more mathematical sense than Ptolemy’s epicycles. Thus began one of the great revolutions in our understanding of our place in the cosmos.<\/p>\n

Reason Two: Measuring the speed of light<\/strong><\/p>\n

Time is what clocks measure. And a clock, when you come right down to it, is something that does the same thing over and over again in a predictable fashion with respect to other clocks. That sounds circular, but it’s a nontrivial fact about our universe that it is filled with clocks. And some of the best natural clocks are the motions of heavenly bodies. As soon as we knew about the moons of Jupiter, scientists realized that they had a new clock to play with: by accurately observing the positions of all four moons, you could work out what time it must be. Galileo himself proposed that such observations could be used by sailors to determine their longitude<\/a>, a notoriously difficult problem.<\/p>\n

Danish astronomer Ole R\u00f8mer<\/a> noted a puzzle when trying to use eclipses of Io to measure time: despite the fact that the orbit should<\/em> be an accurate clock, the actual timings seemed to change with the time of year. Being a careful observational scientist, he deduced that the period between eclipses was longer when the Earth was moving away from Jupiter, and shorter when the two planets were drawing closer together. An obvious explanation presented itself: the light wasn’t traveling instantaneously from Jupiter and Io to us here on Earth, but rather took some time. By figuring out exactly how the period between eclipses varied, we could then deduce what the speed of light must be.<\/p>\n

R\u00f8mer’s answer<\/a> was that light traveled at about 220,000 kilometers per second. That’s pretty good! The right answer is 299,792 km\/sec, about 36% greater than R\u00f8mer’s value. For comparison purposes, when Edwin Hubble first calculated the Hubble constant, he derived a value of about 500 km\/sec\/Mpc, whereas now we know the right answer is about 70 km\/sec\/Mpc. Using astronomical observations to determine fundamental parameters of the universe isn’t easy, especially if you’re the first one to to it.<\/p>\n

Reason Three: Looking for life<\/strong><\/p>\n

Here in the present day, Jupiter’s moons have not lost their fascination or importance. As we’ve been able to study them in greater detail, we’ve learned a lot about the history and nature of the Solar System more generally. And one of the most exciting prospects is that one or more of these moons might harbor life.<\/p>\n

\"\"<\/p>\n

It used to be common to think about the possibilities for life outside Earth in terms of a “habitable zone<\/a>,” the region around a star where temperatures allowed planets to have liquid water. (Many scientists think that liquid water is a necessity for life to exist — but maybe we’re just being parochial about that.) In our Solar System, Earth is smack-dab in the middle of the habitable zone, and Mars just sneaks in. Both Venus and Jupiter are outside, on opposite ends.<\/p>\n

But there’s more than one way to have liquid water. It turns out that both Europa and Ganymede, as well as Saturn’s moons Titan and Enceladus, are plausible homes for large liquid oceans. Europa, in particular, is thought to possess a considerable volume of liquid water underneath an icy crust — approximately two or three times as much water as in all the oceans on Earth. The point is that solar radiation isn’t the only way to heat up water and keep it at liquid temperatures. On Europa, it’s likely that heat is generated by the tidal pull from Jupiter, which stretches and distorts the moon’s crust as it rotates.<\/p>\n

Does that mean there could be life there? Maybe<\/a>! Nobody really knows. Smart money says that we’re more likely to find life on a wet environment like Europa than a dry one like Mars. And we’re going to look — the Europa Clipper<\/em><\/a> mission is scheduled for launch by 2025.<\/p>\n

If you can’t wait for then, go back and watch the movie Europa Report<\/em><\/a>. And while you do, give thanks to Galileo and his discovery of these fascinating celestial bodies.<\/p>\n","protected":false},"excerpt":{"rendered":"

This year we give thanks for an historically influential set of celestial bodies, the moons of Jupiter. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, the error bar, gauge symmetry, Landauer’s Principle, the Fourier Transform, Riemannian Geometry, the speed of light, and the […]<\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[28,32],"tags":[],"class_list":["post-13239","post","type-post","status-publish","format-standard","hentry","category-science","category-space"],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts\/13239","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/comments?post=13239"}],"version-history":[{"count":4,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts\/13239\/revisions"}],"predecessor-version":[{"id":13252,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts\/13239\/revisions\/13252"}],"wp:attachment":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/media?parent=13239"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/categories?post=13239"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/tags?post=13239"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}