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The Untimely Mitchel’s The Discovery End of Amateur Mountains of ’s CCDs? on Mars Moons Page 30 Page 52 Page 60 60 SOLAR SYSTEMHISTORY AY 2020 Finding JupitFinding I M our understandingofthesolarsystemand The discoveryoftheJoviansatellitesenhanced changed thecourseofhistory. cal history, challenging pointsturning in astronomi- momentous most one ofthe was worlds four ofthese discovery The fame. lasting to vaulted Galileo fi the Documenting 11th, four. hesaw onJanuary Then, planet. regal the ging atfi what observed atitand telescope built newly his aimed Galilei Galileo gods. ofthe king itthe named Romans the that wonder sky. It’s little night the in object brightest third the On January 7, January 1610,On when surged Jupiter’s eminence Moon, Venus, and occasionally Mars, Jupiter is usually usually is Jupiter Mars, occasionally Venus,Moon, and f a enjoyed always has sky, Jupiter ofthe hierarchy n the ront-row seat. With its prominence exceeded only by the the by only exceeded prominence its With ront-row seat. • SKY &TELESCOPE SKY an Seventy-nine rst appeared to be three nearby stars hug- stars nearby three be to appeared rst rst planetary moons other than our own, own, our than other moons planetary rst d Counting: Counting: d

by F. MichaelWitkoski

known for determining that the planets describe elliptical elliptical describe planets the that for determining known (best Kepler Johannes astronomer German fellow by gestion asug- following nomenclature this proposed Marius Simon Jupiter. with associated myths from characters all , that they were offi mid-1800s the until Itwasn’t Stars.” “Medicean the satellites four the ofTuscany, naming Duke by Grand the II deMedici, Cosimo patron, honor his to was intent immediate His scope. tele- his in visible stars myriad the and Moon, ofthe craters and mountains the ofJupiter’s satellites, observations his phlet, the exclusive focus of the fi ofthe focus exclusive the occupy didn’t planet our which —onein system another was a of Creation center the was Earth that belief established the round which all heavenly bodies revolved. Suddenly there there Suddenly revolved. bodies heavenly all which round orbits around the Sun — an accomplishment that ranks as as ranks that accomplishment —an Sun the around orbits In March 1610, fi his March In published Galileo one of the greatest intellectual achievements in history). history). in achievements intellectual greatest one ofthe Sidereus deserves credit for more than just the naming of naming the just than for more credit deserves However, digging deeper, it’s apparent that Marius Marius that it’s apparent deeper, digging However, cially named , , , and and Ganymede, Io, Europa, named cially ( The Starry Messenger Starry The rmament. ndings in apam- in ndings ), an account of ), account an

CALLISTO, EUROPA: NASA / JPL / DLR; GANYMEDE: NASA / JPL er’s Moons

IO: NASA / JPL / UNIVERSITY OF ARIZONA; JUPITER: NASA / JPL-CALTECH / SWRI / MSSS / KEVIN M. GILL Marius’s death in 1625. in Marius’s death confl abitter triggered and fi satellites the observed have Iovialis leo’s fi 1610 8, Gali- January after to one day —just translates 1582, in implemented reform for calendar adjusting after 29, December 1609, dated are which, observations mented fi tq k regularly Jupiter observes who anyone As Glass theLooking Through but r satellites, the observed heindependently fact, In moons. the journey far from the planet. Ganymede Ganymede planet. the from far journey never which Europa, for Ioand often most happens shadow. planet’s This the within eclipse through or traverse ter’s disk Jupi- behind hide can They visible. all always Juno (Jupiter).Imagesnottoscale. Europa, andIo),Voyager 2(Callisto),and by thesespacecraft:Galileo(Ganymede, ter. Theindividualimageswere recorded Callisto, Ganymede,Europa, Io,andJupi- family portraitshows(fr cult to verify than those of Galileo. Marius’s fi ofGalileo. those than verify to cult nows, the four Galilean satellites aren’t aren’t satellites Galilean four the nows, GALILEO’S MOONSThiscomposite ecords of his work are less complete and more dif- more and complete less workare ofhis ecords rst observations! In 1614 In observations! published rst Marius ( The World ofJupiter The om lefttoright) ict that continued until until continued that ict ) in which he claimed to to heclaimed which ) in rst. This angered Galileo Galileo angered This rst. rst docu- Mundus Mundus skyandtelescope.org AY 2020 • M

61 Solar System History

and Callisto, on the other hand, are t BY ANY OTHER NAME In a bid to curry almost always visible. The Galilean favor, Galileo initially named the four new moons have orbits that are virtually Jovian moons Cosmica Sidera (“Cosimo’s stars”), but Cosimo de Medici graciously perfectly circular and, consequently, suggested he refer to the newly discovered veer as far to the east of Jupiter as objects simply as Medicea Sidera (“the they do to the west. Although the Medician stars”). In this illustration, Galileo is four moons orbit in nearly the same depicted showing Cosimo the Jovian moons. plane aligned with Jupiter’s equa- tor, from our Earthly perspective we received a few reports over the years, usually view the Jovian system from but the very nature of such observa- slightly above or below the planet’s tions makes them diffi cult to verify. equator, so the satellites rarely appear strung out in a straight line. Moving Heaven and Earth Galileo’s telescopes were opti- As is well known, Galileo’s discover- cally poor by today’s standards — his ies and the inevitable implication observations we can accomplish with that Earth no longer occupied a modern binoculars. Indeed, under special place in the cosmos prompted favorable conditions, Galileo’s satel- consternation within the formi- lite worlds are bright enough to be dable Catholic establishment. In the discerned with the unaided eye were early decades of the 16th century, it not for their proximity to Jupiter Nicolaus Copernicus labored over a and its overpowering luminosity. Ganymede, at magnitude lengthy exposé on heliocentricity, a Sun-centered scenario 4.6 and a diameter of 5,262 kilometers, is the brightest and that opposed the prevailing Earth-centered model that the largest, while 4,821-km-diameter Callisto is the dimmest at ancient Greeks, particularly Aristotle and later Ptolemy magnitude 5.7. Europa has a diameter of only 3,122 km, mak- had promulgated. also contradicted Catholic ing it the smallest of the four, but its relatively high albedo dogma, and as such was considered heresy. Copernicus’s allows it to shine at magnitude 5.3. Closest to Jupiter in ideas are spelled out in detail in a six-part book, De revolu- distance is magnitude-5.0 Io, which spans 3,643 km. tionibus orbium coelestium (On the Revolutions of the Heavenly Are there any confi rmed naked-eye sightings of the Jovian Spheres), issued at the time of his death in 1543. moons? There is one recorded pre-telescopic observation from While Copernicus was hesitant about publishing his ideas, the 4th century BC. Chinese astronomer Gan De recorded “a Galileo was confi dent his own observations had conclusively small reddish star” next to Jupiter, which the 20th-century verifi ed that the was not Earth-centered. He traveled Chinese astronomer and historian Xi Zezong has interpreted to Rome in 1611 and invited ecclesiastical authorities to view as a possible sighting of Ganymede. This magazine has Jupiter through his telescope. Some readily accepted the valid-

tt A NEW VIEW In 1610, Galileo published Sidereus nuncius (The Starry Messen- ger), the title page from which is reproduced here. This slender volume contained his pioneering telescopic observations, including draw- ings of Jupiter’s satellites. Although a scientifi c triumph, Sidereus nuncius also set Galileo on a collision course with the Church.

t NEW MOONS Galileo’s drawings depicting Jupiter’s four largest moons fi rst ap- peared in Sidereus nuncius. His efforts provided important observational support for the Copernican model of a Sun-

centered solar system. GALILEO AND COSIMO DE MEDICI: CHRONICLE / ALAMY STOCK PHOTO; SIDEREUS NUNCIUS: PUBLIC DOMAIN / WIKIMEDIA SOURCE SCIENCE / SPL DRAWINGS: MOON JOVIAN COMMONS;

62 MAY 2020 • SKY & TELESCOPE ity of what appeared to be a minia- t GALILEO’S NEMESIS Simon Marius not ture analog to the solar system. Oth- only gave the the names ers struggled to fi nd reasons to deny they now bear, but he also proved to be a thorn in the great scientist’s side by claiming what was clearly obvious, appealing to have discovered the satellites fi rst and to the word of Scripture, which publishing a book to bolster his assertion. espoused a centralized, motionless Earth. Still others conjectured that chronometers that a truly reliable the instrument was faulty and didn’t method for determining longitude at portray reality. sea existed. However, Jesuit priest and astron- After relocating to Paris in 1672 omer Christopher Clavius was an to become Cassini’s assistant, Rømer early endorser of Galileo’s asser- continued to observe Jupiter’s satel- tions. Clavius was also responsible lites, which led him to conjecture for much of the analysis behind the that the speed of light is fi nite. This 1582 reform of the calendar, which position was counter to the prevailing more accurately synchronized it with view that light propagated instanta- the length of the year, thus keeping neously from a source. Both he and holy days (particularly Easter) in step Cassini recognized that as Earth and with the seasons. Ironically, Clavius Jupiter move apart in their respective adhered to the geocentric view, yet he orbits, the interval between eclipses didn’t turn a blind eye to the reality of the satellites increases, and when presented by Galileo’s telescope. they approach each other, it decreases. Rømer primarily moni- Galileo’s observations and scientifi c musings triggered tored Io, which, being closest to Jupiter, orbits more frequently years of dispute with the Church, resulting in his eventual and therefore provides more data points. His conclusion was trial and subsequent lifetime house arrest. Multiple factors that a fi nite speed of light accounted for the observed differ- played a part in this outcome, including the overt bluntness ences, although at the time methods suffi ciently accurate for of the books Galileo authored, coupled with his sometimes determining the speed of light were unavailable. abrasive personality. Teaching and writing about Copernican- ism was forbidden, and Revolutions was placed on the “Index” More and More Moons — a list of books Catholics were prohibited from reading. In 1892, American astronomer E. E. Barnard discovered Jupi- ter’s fi rst post-Galilean satellite. Barnard detected the dim Down-to-Earth Matters little moon (listed at magnitude 14.1) with the newly com- While Galileo’s observations ultimately validated a Sun- missioned 36-inch refractor at Lick Observatory on Mount centered view, observations of Jupiter’s moons contributed to Hamilton in California. This was to be the last visual discov- other advancements of the era, particularly for determining ery of a planetary satellite. At 167 km in diameter, Amalthea longitude, essential for navigation and cartography. (as it was formally named in 1976) is only 3% percent the size Although latitude is easy to reckon by measuring the altitude of stars above the horizon, no equivalent technique existed for determining longitude. In the 1670s Danish astronomer Ole Rømer devised a method of calculating longi- tude by timing the eclipses of Jupiter’s satellites. He made his observations from Uraniborg, ’s observatory near Copenhagen, in concert with Giovanni Domenico Cassini, who documented the same events from Paris. By comparing their results the two could calculate the difference in longi- tude between the two locations. Although a valid procedure, it proved unworkable from the deck of a ship navigating turbulent oceans. Many other scientists and inventors toiled over the puzzle using various means, but it wasn’t u ntil the late 1700s when John Harrison and others built accurate

u COPERNICAN REVOLUTION Polish astronomer Nicolaus Co- pernicus is credited with having fi rst described a heliocentric solar system in detail in his book De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), published in 1543, the year

PUBLIC DOMAIN / WIKIMEDIA COMMONS (2) COMMONS WIKIMEDIA / DOMAIN PUBLIC of his death.

skyandtelescope.org • MAY 2020 63 Solar System History

of Ganymede, and orbits much nearer t BIG GLASS It’s easy to imagine how stunned to Jupiter than its four larger siblings Galileo would have been had he been able to do. Consequently, Amalthea is a virtu- utilize the Lick Observatory’s 36-inch behemoth telescope. This refractor was the instrument E. E. ally impossible target through amateur Barnard used to make the last visual discovery telescopes, regardless of size or quality — of a planetary satellite. Jupiter is roughly 6 million times brighter than Amalthea, but only about 1,500 But of all the satellites orbiting Jupiter times brighter than the Galilean moons. the only ones that we can accurately Jupiter’s sixth satellite, Himalia, was describe as “worlds” are the four Gali- discovered in 1904 by Charles Dillon Per- leo discovered. In the 1970s, Io, Europa, rine on photographic plates taken with Ganymede, and Callisto returned to the 36-inch Crossley refl ector telescope the forefront of scientifi c interest as the at Lick Observatory. Although slightly Pioneer, Voyager, and Galileo spacecraft fainter than Amalthea, Himalia orbits far returned scores of detailed close-up from Jupiter’s overpowering brightness photographs of each one. These images and is more easily visible. Nevertheless, established the distinct personalities of at least a 10-inch telescope and optimal the Galilean moons. Io is a multicolored viewing conditions are required for this volcanic wonderland; Ganymede fea- 14.2-magnitude object. Additionally, one tures grooved water-ice terrain; Callisto’s would need to separate it from the multitudes of nearby fi eld ancient surface is pockmarked with impact craters; and stars of similar appearance. Perrine also discovered Jupiter’s Europa is a world covered with a water-ice crust encasing a seventh moon, Elara, a year later. silicate rock surface, sporting the smoothest surface of any In subsequent decades, discoveries trickled in, but the dam solar system body. Could there be an ocean residing beneath fi nally broke in 2000 when highly sensitive ground-based Europa’s surface — one that might harbor primitive micro- detection methods came online. The vast majority of Jupiter’s organisms? Enthusiastic scientists are discussing plans for currently known, 79 satellites have been discovered in the satellite missions to learn more about this exciting world. past two decades, mostly by a team lead by Scott S. Sheppard What began with a small spyglass more than 400 years ago at the Carnegie Institution for Science. These are tiny objects, continues unabated to this day. generally only a few kilometers in diameter. However, in 2019 Sheppard’s team announced the discovery of 20 new satellites ¢ MIKE WITKOSKI volunteers at Muddy Run Observatory (lo- orbiting Saturn, bringing the ringed planet’s total to 82, thus cated approximately 100 miles west of Philadelphia), where the

crowning it the reigning moon king — at least for now. Galilean satellites are a big hit on public viewing nights. EUROPA COMMONS; WIKIMEDIA / DOMAIN PUBLIC REFRACTOR: LICK CLIPPER BASE IMAGE: NASA / SETI / JPL-CALTECH INSTITUTE; EUROPA CLOSE-UP: NASA / UNIVERSITY / JPL-CALTECH OF ARIZONA

MYSTERY MOON What lurks below the intricate icy surface of Europa? The proposed Europa Clipper orbiter is designed to help scientists fi nd out. The launch date of the mission, however, is still “TBD.”

64 MAY 2020 • SKY & TELESCOPE