Earth and Space News: Christiaan Huygens Titan discovery March 1655

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Wednesday, February 27, 2013

Cassini-Huygens Mission Map Identifies 12 Titanean Mountains

Summary: A Cassini-Huygens mission map identifies 12 Titanean mountains via data collected by the spacecraft’s Visual Mapping Spectrometer and Radar Mapper.

A Cassini-Huygens mission map identifies 12 Titanean mountains from data collected by the spacecraft’s Visual Mapping Spectrometer (VIMS) and its Radar Mapper.

The NASA (National Aeronautics and Space Administration) Image and Video Library and the NASA JPL (Jet Propulsion Laboratory) Photojournal websites added the map to their image galleries on Dec. 12, 2012. The map of Saturnian moon Titan marks the locations of 12 mountains officially named by the International Astronomical Union (IAU) between Dec. 5, 2011, and Nov. 13, 2012.

The IAU’s Gazetteer of Planetary Nomenclature website, maintained by the U.S. Geological Survey’s Astrogeology Science Center, details the convention for naming Titanean mountains (Latin: mons, “mountain;” montes, “mountains”). Titan’s mountains are named after mountains and peaks located in Middle-earth, the fictional setting in fantasies by English writer John Ronald Reuel “J.R.R.” Tolkien (Jan. 3, 1892-Sept. 2, 1973).

Angmar Montes are centered at minus 10 degrees south latitude, 221.9 degrees west longitude, according to the International Astronomical Union’s (IAU) Gazetteer of Planetary Nomenclature. Angmar’s northernmost and southernmost latitudes extend to minus 9.5 degrees south and minus 10.5 degrees south, respectively. The Titanean geological formation’s easternmost and westernmost longitudes reach 219.3 degrees west and 224.6 degrees west, respectively. Angmar Montes span 230 kilometers.

Dolmed Montes are centered at minus 11.6 degrees south latitude, 216.8 degrees west longitude. Dolmed’s northernmost and southernmost latitudes occur at minus 8.05 degrees south and minus 14.6 degrees south, respectively. The Titanean geological structure registers easternmost and westernmost longitudes at 213.7 degrees west and 218 degrees west, respectively. Dolmed’s length covers 400 kilometers.

Doom Mons is centered at minus 14.65 degrees south latitude, 40.42 degrees west longitude. The Aztlan darklands region mountain marks northernmost and southernmost latitudes at minus 14 degrees south and minus 15.3 degrees south, respectively. The cryovolcanic construct, with its eponymous peak, records easternmost and westernmost longitudes at 40 degrees west and 41.13 degrees west, respectively. Doom’s length measures 63 kilometers.

Echoriath Montes are centered at minus 7.4 degrees south latitude, 213.8 degrees west longitude. The Titanean mountain range obtains northernmost and southernmost latitudes at minus 6.2 degrees south and minus 8.8 degrees south, respectively. The Titanean geological formation’s easternmost and westernmost longitudes are found at 204.8 degrees west and 224.5 degrees west, respectively. Echoriath Montes stretch for 930 kilometers.

Erebor Mons is centered at minus 4.97 degrees south latitude, 36.23 degrees west longitude. The Quivira region mountain’s northernmost and southernmost latitudes reach minus 4.3 south and minus 5.49 south, respectively. Erebor’s easternmost and westernmost longitudes occur at 35.68 degrees west and 36.67 degrees west, respectively. Erebor Mons has a length of 50 kilometers.

Gram Montes are centered at minus 9.9 degrees south latitude, 207.9 degrees west longitude. The northernmost and southernmost latitudes are recorded at minus 9.3 degrees south and minus 11 degrees south, respectively. Easternmost and westernmost longitudes are reached at 205.5 degrees west and 211.2 degrees west, respectively. Gram Montes span 260 kilometers.

Irensaga Montes are centered at minus 5.68 degrees south latitude, 212.71 degrees west longitude. The Adiri region mountain range registers northernmost and southernmost latitudes at minus 5 degrees south and minus 6.65 degrees south, respectively. The mountain range’s easternmost and westernmost longitudes extend to 210.54 degrees west and 214.73 degrees west, respectively. Irensaga Montes span 194 kilometers.

Merlock Montes are centered at minus 8.9 degrees south latitude, 211.8 degrees west longitude. The Titanean geological formation marks northernmost and southernmost latitudes at minus 8.4 degrees south and minus 9.6 degrees south, respectively. The mountains record easternmost and westernmost longitudes at 209.8 degrees west and 214.2 degrees west, respectively. Merlock Montes cover 200 kilometers.

Mindolluin Montes are centered at minus 3.3 degrees south latitude, 208.96 degrees west longitude. The Adiri region mountain range’s northernmost and southernmost latitudes reach to minus 1.78 degrees south and minus 4.27 degrees south, respectively. Mindolluin’s easternmost and westernmost longitudes extend to 205.7 degrees west and 213.23 degrees west, respectively. Mindolluin Montes traverse 340 kilometers.

Mithrim Montes are centered at minus 2.16 degrees south latitude, 127.42 degrees west longitude. The Xanadu region mountain range obtains northernmost and southernmost latitudes at minus 1.2 degrees south and minus 3.2 degrees south, respectively. The Titanean range registers easternmost and westernmost longitudes at 126 degrees west and 128.9 degrees west, respectively. Mithrim Montes cover 147 kilometers.

Rerir Montes are centered at minus 4.8 degrees south latitude, 212.1 degrees west longitude. The Titanean geological formation’s northernmost and southernmost latitudes occur at minus 4.4 degrees south and minus 5.2 degrees south, respectively. Rerir’s easternmost and westernmost longitudes are found at 208.3 degrees west and 216.5 degrees west, respectively. Rerir Montes span 370 kilometers.

Taniquetil Montes are centered at minus 3.67 degrees south latitude, 213.26 degrees west longitude. The Adiri region mountain range records northernmost and southernmost latitudes at minus 2.9 degrees south and minus 4.16 degrees south, respectively. The range posts easternmost and westernmost longitudes of 211.84 degrees west and 214.77 degrees west, respectively. Taniquetil Montes traverse 130 kilometers.

On Dec. 5, 2011, the International Astronomical Union (IAU) approved names for six of the 12 mountainous Tolkien namesakes identified on the Cassini-Huygens mission map: Angmar, Dolmed, Echoriath, Gram, Merlock and Rerir. Doom, Erebor, Irensaga, Mindolluin, Mithrim and Taniquetil received official name approval on Nov. 13, 2012.

The takeaways for the Cassini-Huygens mission’s map identifying 12 Titanean mountains are that the 2012 map is based on data collected by the Cassini-Huygens spacecraft’s Visual Mapping Spectrometer (VIMS) and its Radar Mapper; that, according to International Astronomical Union (IAU) convention, mountains on Saturnian moon Titan are named after mountains and peaks in the works of English writer J.R.R. Tolkien; that six of the Titanean mountainous features received official name approval on Dec. 5, 2011; and that six names were approved on Nov. 13, 2012.

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Cassini-Huygens mission map of Saturnian moon Titan identifies locations of 12 mountains named by the International Astronomical Union (IAU) between Dec. 5, 2011, and Nov. 13, 2012; NASA ID PIA16598; 2012-12-14; image credit NASA/JPL-Caltech/University of Arizona/USGS: May be used for any purpose without prior permission, via NASA JPL Photojournal @ https://photojournal.jpl.nasa.gov/catalog/PIA16598;
Generally not subject to copyright in the United States; may use this material for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations and Internet Web pages; general permission extends to personal Web pages, via NASA Image and Video Library @ https://images.nasa.gov/details-PIA16598; via NASA JPL @ https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA16598

Saturn’s rings obscure part of largest Saturnian moon Titan (center-lower left) in image obtained April 28, 2006, by the Cassini-Huygens spacecraft’s narrow-angle camera (NAC), at an approximate distance of 1.8 million kilometers (1.1 million miles) from Titan; NASA ID PIA08188; image addition date 2006-05-30; image credit NASA/JPL/Space Science Institute: May be used for any purpose without prior permission, via NASA JPL Photojournal @ https://photojournal.jpl.nasa.gov/catalog/PIA08188;
Generally not subject to copyright in the United States; may use this material for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations and Internet Web pages; general permission extends to personal Web pages, via NASA Image and Video Library @ https://images.nasa.gov/details-PIA08188

For further information:

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Available via The University of Arizona PIRL (Planetary Image Research Laboratory) @ https://pirlwww.lpl.arizona.edu/~jani/barnes-mtnschan-jgr07.pdf
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Brown, R.H.; K.H. Baines; G. Bellucci; J.-P. Bibring; B.J. Buratti; F. Capaccioni; P. Cerroni; R.N. Clark; A. Coradini; D.P. Cruikshank; P. Drossart; V. Formisano; R. Jaumann; Y. Langevin; D.L. Matson; T.B. McCord; V. Mennella; E. Miller; R.M. Nelson; P.D. Nicholson; B. Sicardy; and C. Sotin. “The Cassini Visual and Infrared Mapping Spectrometer (VIMS) Investigation.” Space Science Reviews, vol. 115, issue 1-4 (November 2004): 111-168.
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Available @ https://sci.esa.int/web/conferences/-/35018-titan-from-discovery-to-encounter-conference-presentations

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Angmar Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn. Last updated April 28, 2014.
Available @ https://planetarynames.wr.usgs.gov/Feature/14892

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Categories (Themes) for Naming Features on Planets and Satellites.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Documentation > Surface Feature Categories.
Available @ https://planetarynames.wr.usgs.gov/Page/Categories

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Dolmed Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/14893

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Doom Mons.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/15044

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Echoriath Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/14894

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Erebor Mons.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/15045

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Gram Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/14895

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Irensaga Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/15046

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Merlock Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/14896

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Mindolluin Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/15047

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Mithrim Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/15049

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Rerir Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/14897

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Taniquetil Montes.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Feature/15050

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Target: The Moon.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
Available @ https://planetarynames.wr.usgs.gov/Page/MOON/target

International Astronomical Union (IAU) / U.S. Geological Survey (USGS) Gazetteer of Planetary Nomenclature. “Target: Titan.” USGS Astrogeology Science Center > Gazetteer of Planetary Nomenclature > Nomenclature > Saturn.
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Available @ https://photojournal.jpl.nasa.gov/catalog/PIA09856

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Available @ https://photojournal.jpl.nasa.gov/catalog/PIA09858

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Available @ https://photojournal.jpl.nasa.gov/catalog/PIA14909

Lavoie, Sue, site mgr. “PIA14922: Colorful Colossuses and Changing Hues.” NASA JPL Photojournal > Saturn. Image addition date 2012-08-29.
Available @ https://photojournal.jpl.nasa.gov/catalog/PIA14922

Lavoie, Sue, site mgr. “PIA16598.” NASA JPL Photojournal > Saturn. Image addition date 2012-12-14.
Available @ https://photojournal.jpl.nasa.gov/catalog/PIA16598

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Lopes, R.M.C.; R.L. Kirk; K.L. Mitchell; A. LeGall; J.W. Barnes; A. Hayes; J. Kargel; L. Wye; J. Radebaugh; E.R. Stofan; M.A. Janssen; C.D. Neish; S.D. Wall; C.A. Wood; J.I. Lunine; and M.J. Malaska. “Cryovolcanism on Titan: New Results From Cassini RADAR and VIMS.” JGR Reports, vol. 118, issue 3 (March 2013): 416-435.
Available via Wiley Online @ https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgre.20062

Marriner, Derdriu. “Christiaan Huygens Discovered Saturnian Satellite Titan March 25, 1655.” Earth and Space News. Wednesday, March 21, 2012.
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Moore, Patrick, Sir. Philip’s Atlas of the Universe. Revised edition. London UK: Philip’s, 2005.

Wednesday, March 21, 2012

Christiaan Huygens Discovered Saturnian Satellite Titan March 25, 1655

Summary: Dutch astronomer Christiaan Huygens discovered Saturnian satellite Titan March 25, 1655, with a telescope he designed with his brother Constantijn.

Dutch Golden Age astronomer Christiaan Huygens discovered Saturnian satellite Titan March 25, 1655, with a telescope that he designed with his older brother, Constantijn.

In 1655, Christiaan Huygens (April 14, 1629-July 8, 1695) collaborated with his older brother, Dutch statesman and microscope and telescope maker Constantijn Huygens Jr., Lord of Zuilichem (March 10, 1628-Nov. 2, 1697), in designing and constructing a telescope. The telescope’s objective lens had a focal length of 10 Rhineland feet (Rheinländischer Fuß, equivalent to 12.36 inches), or approximately 337 centimeters, according to an April 2004 presentation at the European Space Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands, by Peter Louwman, Dutch amateur astronomer and owner of the Louwman Collection of Historic Telescopes in Wassenaar, western coastal Netherlands. The brothers equipped their creation with a single-lens eyepiece with a focal length of 3 Rhineland inches (79 millimeters), which yielded a magnification of about 43x (page 105).

Huygens described the new telescope as having a length of 12 Rhineland feet. Louwens explains that the practice at that time was to consider the telescope tube’s total length by including the eyepiece.

Christiaan is credited with inscribing, via a diamond, the focal length and the date of the lens’ final polishing, Feb. 3, 1655, along the rim of the objective lens as “X. 3 FEBR CIƆ IƆ CLV.” (Sometimes CIƆ was used to represent M, the Roman numeral for 1,000; IƆ, half of CIƆ, substituted sometimes for D, the Roman numeral for 500.)

Christiaan also inscribed “Admovere oculis distantia sidera mentis” (They brought the distant stars closer to our eyes) along the opposite rim. He extracted the phrase as a partial verse from lines 305 and 306 (Admovere oculis distantia sidera mentis aetheraque ingenio subposuere suo) of Fastorum Libri Sex (Six Books of the Calendar) by Roman poet Publius Ovidius Naso (March 20, 43 BCE-ca. 17 or 18 CE), known as Ovid.

The Huygens brothers’ 1655 telescope no longer exists in its entirety, according to Peter Louwman. The only part that has survived is the objective lens, which Louwman describes as “. . . by far the most important part . . .” The lens is often referenced as the “Admovere” lens.

The central Netherlands’ Universiteits Museum (University Museum), which maintains the historical collections of Universiteit Utrecht (Utrecht University), usually safeguards the objective lens in a vault. The lens is presently on temporary display at Rijksmuseum Boerhaave in Leiden, western Netherlands, according to Louwman.

Only the Admovere lens surfaced in 1867 in a collection of old instruments at Universiteit Utrecht (Utrecht University), according to March 21, 2005, article by Universiteit Utrecht’s Rob H. van Gent and Universiteitsmuseum U. The central Netherlands’ Universiteits Museum (University Museum), which maintains the university’s historical collections, usually safeguards the objective lens in a vault. The lens is presently on temporary display at Rijksmuseum Boerhaave in Leiden, western Netherlands, according to Louwman.

On March 25, 1655, at about 8 p.m., Christiaan Huygens aimed the newly built telescope at the solar system’s sixth planet, Saturn. He discerned a Saturnian companion that his extensive subsequent observations revealed to be the first satellite discovered in the Saturnian system.

In a letter dated June 13, 1655, to English clergyman and mathematician John Wallis (Dec. 3, 1616-Nov. 8, 1703), Huygens cryptically announced that recently he had observed something striking with his new telescope that no one else had seen (Perſpicillum mihi nuper paravi ipſe 12 pedum longitudine, quo vix aliud praeſtantius reperiri exiſtimo quam, quum antehac nemo viderit, quod ego recens obſervavi.) Huygens described his discovery in a 53-letter anagram, by transposing letters into a phrase from Ovid and adding a string of 17 single letters (Scribitur autem tranſpoſitis literis in hunc modum. Admovere oculis distantia sidera nostris, VVVVVVVCCCRRHNBQX.).

On March 5, 1656, approximately 11 and one-third months (11 months nine days) after his discovery, Huygens announced his discovery of the first Saturnian satellite in a three-page tract entitled De Saturni Luna Observatio Nova (A New Observation of Saturn’s Moon). He noted that he had made the discovery with his 12-foot telescope (Nostrum, quo Saturni aſſeclam reperimus, quinquagies diametrum rei viſae multiplicat, duodenos pedes aequans). He stated that he would be presenting the Saturnian moon’s orbital period, based on his past and present observations, in a work on the entire Saturnian system (Obſervationes praeterito praeſentique anno collectas, quibus periodus ipſius demonſtratur, tunc una edituri ſumus cum integrum Saturni ſyſtema perfecerimus.).

Approximately 11 and one-half months after his discovery, in a letter dated March 15, 1656, to John Wallis, Huygens deciphered his anagram. He gave the correct sequence, which used all 53 letters, as: Saturno luna sua circunducitur diebus sexdecim horis quatuor (Its moon circles Saturn in 16 days and four hours).

In 1659, Huygens published Systema Saturnium (The System of Saturn). In this 84-page compendium, he revealed his name for his discovery as Saturni Luna (Saturn’s Moon).

Sir John Herschel (March 7, 1792-May 11, 1871), whose father, Uranus discoverer William Herschel (Nov. 15, 1738-Aug. 25, 1822), discovered Saturn’s sixth and seventh moons March 13, 1789, gave names from Greek mythology to the seven then-known Saturnian satellites. In Results of Astronomical Observations Made During the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope, published in 1847, Sir John assigned Titan to “The bright satellite, discovered by Huyghens” (page 415).

The takeaways for Christiaan Huygens’ discovery of Saturnian satellite Titan on March 25, 1655, are that the Dutch Golden Age astronomer discovered the first satellite in the Saturnian system with a telescope that he and his older brother, Constantijn, had recently designed and built; that he named his discovery Saturni Luna (Saturn’s Moon); and that Sir John Herschel, whose father, Uranus discoverer William Herschel, discovered Saturn’s sixth and seventh moons in 1789, is credited with Saturni Luna’s official name, Titan.

Utrecht’s University Museum loaned the “Admovere” objective lens, the only remnant of the telescope with which Dutch Golden Age astronomer Christiaan Huygens discovered Titan March 25, 1655, as the first Saturnian satellite discovery, for display April 25 to Aug. 28, 2013, in an exhibition,“Constantijn and Christiaan Huygens -- A Golden Legacy,” in the Grote Kirk in The Hague: Universiteitsmuseum Utrecht @universiteitsmuseumutrecht, via Facebook April 24, 2013

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Dutch Golden Age astronomer Christiaan Huygens’ sketch of Saturn with his satellite discovery (A) and a fixed star (B), as viewed March 25, 1655, at 8 in the evening; C. Huygens, Systema Saturnium (1695), page 9: Not in copyright, via Internet Archive @ https://archive.org/details/CristianiHugeni00Huyg/page/9;
Not in copyright, via Biodiversity Heritage Library @ https://www.biodiversitylibrary.org/item/72050#page/253/mode/1up;
via Wikimedia Commons @ https://upload.wikimedia.org/wikipedia/commons/thumb/9/9d/The_System_of_Saturn_WDL4302.pdf/page27-2133px-The_System_of_Saturn_WDL4302.pdf.jpg

Ferrari, Cécile. “Saturn’s Satellites and Rings: Huygens’ Heritage.” Pages 281-290. In: Karen Fletcher, ed. Proceedings of the International Conference “Titan -- From Discovery to Encounter,” 13-17 April 2004, ESTEC, Noordwijk, Netherlands. ESA SP-1278. Noordwijk, Netherlands: ESA Publications Division, 2004.
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) @ http://adsabs.harvard.edu/full/2004ESASP1278..281F
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) PDF @ http://adsabs.harvard.edu/pdf/2004ESASP1278..281F

Geballe, Thomas R.; Sang J. Kim; Keith S. Noll; and Regis Courtin. “Three Micron Spectroscopy of Titan’s Hydrocarbons, HCN, and Haze.” Pages 355-364. In: Karen Fletcher, ed. Proceedings of the International Conference “Titan -- From Discovery to Encounter,” 13-17 April 2004, ESTEC, Noordwijk, Netherlands. ESA SP-1278. Noordwijk, Netherlands: ESA Publications Division, 2004.
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) @ http://adsabs.harvard.edu/full/2004ESASP1278..355G
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) PDF @ http://adsabs.harvard.edu/pdf/2004ESASP1278..355G

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Herschel, John. “Chapter VI. Observations of the Satellites of Saturn.” Pages 414-430. Results of Astronomical Observations Made During the Years 1864, 5, 6, 7, 8 at the Cape of Good Hope: Being the Completion of a Telescopic Survey of the Whole Surface of the Visible Heavens Commence in 1825. London UK: Smith, Elder and Co., 1847.
Available via Internet Archive @ https://archive.org/details/resultsofastrono00hers/page/414

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Available @ https://www.dhi.ac.uk/hartlib/view?docset=additional&docname=TCDLaa5T

Hom, Elaine J. “Roman Numerals: Conversion, Meaning & Origins.” Live Science > History. May 15, 2013.
Available @ https://www.livescience.com/32052-roman-numerals.html

Hugenii, Christiani. “De Saturni Luna Observatio Nova: Avertissement.” Oeuvres Complètes de Christiaan Huygens, tome quinzième Observations Astronomiques, Système de Saturne, Travaux Astronomiques 1658-1666: 165-170. La Haye [The Hague, the Netherlands]: Martinus Nijhoff, 1888.
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/22151160

Huygenii, Cristiani. Systema Saturnium: Sive, De Causis Mirandorum Saturni Phaenomenôn, et Comite Ejus Planeta Novo. Hagae Comitis [The Hague, Netherlands]: Adriani Vlacq, M.DC.LIX (1659).
Available via Biodiversity Heritage Library @ https://www.biodiversitylibrary.org/item/72050#page/223/mode/1up
Available via Internet Archive @ https://archive.org/details/CristianiHugeni00Huyg/
Available via Smithsonian Institution Libraries Digital Collections @ https://www.sil.si.edu/DigitalCollections/HST/Huygens/huygens.htm
Available via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:The_System_of_Saturn_WDL4302.pdf

Huygens, Christiaan. “No. 224. Christiaan Huygens à John Wallis [13 Juin 1655].” Oeuvres Complètes de Christiaan Huygens, tome premier Correspondance 1638-1656: 331-333. La Haye [The Hague, the Netherlands]: Martinus Nijhoff, 1888.
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/18426236

Huygens, Christiaan. “No. 272. Christiaan Huygens à John Wallis [15 Mars 1656].” Oeuvres Complètes de Christiaan Huygens, tome premier Correspondance 1638-1656: 392. La Haye [The Hague, the Netherlands]: Martinus Nijhoff, 1888.
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/18426297

Huygens, Christiaan. “Nouvelle Observation d’Une Lune de Saturne / Christiani Hugenii De Saturni Luna Observatio Nova.” Oeuvres Complètes de Christiaan Huygens, tome quinzième Observations Astronomiques, Système de Saturne, Travaux Astronomiques 1658-1666: 172-177. La Haye [The Hague, the Netherlands]: Martinus Nijhoff, 1888.
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/22151167

Koninklijke Nederlandse Akademie van Wetenschappen. “Huygens and the Improvement of the Telescope.” KNAW (Koninklijke Nederlandse Akademie van Wetenschappen) DWC Digitaal Wetenschopshistorisch Centrum > Biography > Christiaan Huygensweb > Scientific Work.
Available @ https://www.dwc.knaw.nl/biografie/christiaan-huygensweb/instrumenten-en-uitvindingen/huygens-and-the-improvement-of-the-telescope/?lang=en

Koninklijke Nederlandse Akademie van Wetenschappen. “Huygens and Titan, Saturn’s Moon.” KNAW (Koninklijke Nederlandse Akademie van Wetenschappen) DWC Digitaal Wetenschopshistorisch Centrum > Biography > Christiaan Huygensweb > Scientific Work.
Available @ https://www.dwc.knaw.nl/biografie/christiaan-huygensweb/wetenschappelijk-werk/huygens-and-titan-saturns-moon/?lang=en

Levy, David H. Skywatching. Revised and updated. San Francisco CA: Fog City Press, 1994.

Louwman, Peter. “Christiaan Huygens and His Telescopes.” Pages 103-114. In: Karen Fletcher, ed. Proceedings of the International Conference “Titan -- From Discovery to Encounter,” 13-17 April 2004, ESTEC, Noordwijk, Netherlands. ESA SP-1278. Noordwijk, Netherlands: ESA Publications Division, 2004.
Available @ https://www.esa.int/esapub/sp/sp1278/sp1278p1.pdf
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) @ http://adsabs.harvard.edu/full/2004ESASP1278..103L
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) PDF @ http://adsabs.harvard.edu/pdf/2004ESASP1278..103L

Moore, Patrick, Sir. Philip’s Atlas of the Universe. Revised edition. London UK: Philip’s, 2005.

Nemiroff, Robert; and Jerry Bonnell. “Huygens Discovers Luna Saturni.” NASA (National Aeronautics and Space Administration) APOD (Astronomy Picture of the Day). March 25, 2005.
Available @ https://apod.nasa.gov/apod/ap050325.html

Universiteitsmuseum Utrecht @universiteitsmuseumutrecht. “Dit is niet zomaar een stukje glas uit onze collectie. Voor zover bekend is dit de eerste lens die Christiaan Huygens sleep. In 1655 ontdekte hij hiermee de maan Titan van Saturnus. Voor deze ontdekking spraken sterrenkundigen van 'de geheimzinnige planeet met oren'. In de lens staat gegegraveerd: 'Admovere oculis distantia sidera nostris'. Weet jij wat dit betekent? Vanaf vandaag te bewonderen in de Huygenstentoonstelling in Den Haag. http://bit.ly/12b4YJZ.” Facebook. April 24, 2013. Available @ https://www.facebook.com/universiteitsmuseumutrecht/photos/a.240903229294605/541270965924495/

van Gent, Rob H.; and Tiemen Cocquyt. “350 Jaar Huygenslens.” NEMO Kennislink > Publicaties. March 21, 2005. Available @ https://www.nemokennislink.nl/publicaties/350-jaar-huygenslens/

van Helden, Albert. “Huygens, Titan, and Saturn’s Ring.” Pages 11-29. In: Karen Fletcher, ed. Proceedings of the International Conference “Titan -- From Discovery to Encouter,” 13-17 April 2004, ESTEC, Noordwijk, Netherlands. ESA SP-1278. Noordwijk, Netherlands: ESA Publications Division, 2004.
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) @ http://adsabs.harvard.edu/full/2004ESASP1278...11V
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) PDF @ http://articles.adsabs.harvard.edu/pdf/2004ESASP1278...11V

Verduin, C.J. (Kees). “Portraits of Christiaan Huygens (1629-1695).” Universiteit Leiden > Kees Verduin > A Short History of Probability and Statistics > Christiaan Huygens. Last modified March 2009.
Available @ https://www.leidenuniv.nl/fsw/verduin/stathist/huygens/acad1666/huygpor/

Walden, John H., Dr. “Huygens Saturn’s Ring.” Pages 63-69. In: Harlow Shapley and Helen E. Howarth, eds., A Source Book in Astronomy. Sources Books in the History of the Sciences. First edition. Second impression. New York NY; London UK: McGraw-Hill Book Company, Inc., 1929.
Available via HathiTrust @ https://catalog.hathitrust.org/Record/009506271
Available via Internet Archive @ https://archive.org/details/in.ernet.dli.2015.177246/page/n77

Yoder, Joella. “The Huygens Manuscript.” Pages 43-54. In: Karen Fletcher, ed. Proceedings of the International Conference “Titan -- From Discovery to Encounter,” 13-17 April 2004, ESTEC, Noordwijk, Netherlands. ESA SP-1278. Noordwijk, Netherlands: ESA Publications Division, 2004.
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) @ http://adsabs.harvard.edu/full/2004ESASP1278...43Y
Available via Harvard ADSABS (NASA Astrophysics Data System Abstracts) PDF @ http://adsabs.harvard.edu/pdf/2004ESASP1278...43Y