------------------------------------------- CHAT Observing List Caroline Herschel Astronomy Tour ------------------------------------------- One inspiring way to make or deepen an acquaintance with the universe of deep sky observation is to explore the cosmic itinerary of the great astronomer Caroline Herschel (1750-1848). As an observer and a meticulous assistant to her brother William Herschel (1738-1822), she shared in the discovery and cataloguing of over 2500 nebulae, clusters, and galaxies. Drawing upon a catalogue in manuscript form which she had compiled as a main source for his own General Catalogue (GC) of 1864, her nephew John Herschel (1792-1871) remarked that in the course of this monumental project "I learned fully to appreciate the skill, diligence, and accuracy which that indefatigable lady brought to bear on a task which only the most boundless devotion could have induced her to undertake or enabled her to accomplish."[1] The Caroline Herschel Astronomy Tour (CHAT) centers on objects which she discovered or independently rediscovered, while also surveying others of more uncertain status which in any event she helped to document and catalogue, plus some of the already-known Messier objects which she observed and identified in the early months of her systematic observations for comets and nebulae. In addition to these 42 deep sky objects (DSO's), the lunar crater C. Herschel, named in her honor in 1935, is also included, making a CHAT observing list of 43 items in all. The CHAT list offers a sampling from two larger deep sky observing lists of special interest to the amateur astronomy: the Messier catalogue of about 110 objects[2]; and the approximately 2500 Herschel objects chronicled by Caroline and her brother William, with various subsets such as the "Herschel 400" recognized by amateur groups[3]. She also discovered or independently rediscovered eight comets, the initial focus of her systematic telescopic observations starting in 1782; rapidly these "sweeps for comets" led to the "sweeps for nebulae" by which she and her brother revolutionized astronomy. Just as the Earth's "diurnal motion" or rotation had been confirmed by telescopic observations of "other planets," so now our larger "situation within a system of stars," the Milky Way, would be illuminated by "the view of so many sidereal systems, some of which we may discern to be of a most surprising extent and grandeur," as William Herschel wrote in a preface to their first catalogue of 1000 nebulae published in 1786.[4] After about 140 years of further exploration, cataloguing, and some lively debates, astronomers reached the current consensus that many of these Herschel objects were indeed "sidereal systems" or island universes like our Milky Way galaxy, while many others were actually nebulae or clusters of assorted types within our own galaxy -- "sidereal subsystems," perhaps one might say. From a certain historical perspective, Caroline Herschel's observations of variable stars such as Algol (ß Per) using the inexact techniques then available[5] link her to the later work of Henrietta Swan Leavitt, whose discovery (1908-1912) of the period-luminosity relationship in Cepheid variables was to provide a reliable "yardstick" for measuring galactic and intergalactic distances, including those of the many deep sky objects discovered and catalogued by the Herschels.[6] * * * --------------------------- Sources: CHAT and CHAT-Plus --------------------------- With great humility, I emphasize that almost all the information used in compiling the CHAT observing list has been borrowed from invaluable articles and tables by others, with much of this material conveniently available on the World Wide Web. Two articles provided me with convenient lists and tables from which I drew up the first draft of the CHAT list. Harmut Frommert and Christine Kronberg, "Carole Herschel's Deep Sky Objects" Jane Houston Jones, 2002, "The Caroline Herschel Objects," _Sky and Telescope_, v. 104, no. 5 (November 2002), pp. 107-111 In turn, Frommert and Kronberg draw on recent articles by Michael Hoskin suggesting some revisions and additions to the list of objects discovered or independently rediscovered by Caroline Herschel, and also chronicling her early Messier observations. Michael Hoskin, with an Appendix by Brian Marsden, 2005, "Caroline Herschel as Observer," _Journal for the History of Astronomy_, v. 36, part 4, no. 125 (November 2005), pp. 373-406. Michael Hoskin, 2006, "Caroline Herschel's Catalogue of Nebulae," _Journal for the History of Astronomy_, v. 37, part 3, no. 128 (August 2006), pp. 251-255. As to these observations of known Messier objects, a word of caution and encouragement is in order. The CHAT list includes only her explorations of these objects through July, 1783. Hoskin reports that by the end of that year "she had viewed 41 Messier objects, two-fifths of the total"[7], with the 1781 version of Messier's catalogue then available listing 103 objects, as he explains.[8] Indeed a longer version of the CHAT list might well include _all_ the Messiers, since there are two lines of evidence pointing to her knowledge of them all, as Hoskin discusses. First, there is a letter in which Nevil Maskelyne, the Astronomer Royal, says "a decade" after Caroline Herschel's early observations of 1782-1783 that she "knows all the nebulae [in Messier's list] at sight, which he [William] esteems necessary to distinguish new Comets that may appear from them."[9] Comet hunting was the original purpose of Messier's list, but of course familiarity with it was also essential for the newer pursuit of "sweeps for nebulae" not previously known. The second line of evidence offered by Hoskin[10] is a statement by Caroline herself in her later years as to the "Requisites" for those who might in future use a special heirloom, her evidently favorite reflecting telescope or "sweeper," in hunting comets[11]: 1. The name of every star as far as the 1st, 2nd, 3rd and 4th magnitude must be known at sight. 2. Every Nebula [of Messier] must be known so well as to be found in the sweeper in one minute. Thus it would seem that an astronomer meeting Caroline's standard of competence might do well in a modern Messier Marathon, where the aim at a propitious time of year and latitude is to view all of the 110 or so currently catalogued Messier objects in a single night. (Observers adept in this event will explain that in fact a technique of quickly "hopping" from one object to the next is required mainly in the critical periods near dusk and dawn, with other portions of the marathon often quite leisurely, affording ample opportunity to view and appreciate many Messier objects in a more contemplative fashion.) While not all amateur astronomers may relish such an "athletic" approach -- at least at certain phases of a marathon -- to these beautiful objects, the above quoted remarks both of Caroline Herschel herself and her professional peer Nevil Maskelyne suggest that the CHAT list and project be structured so as to encourage familiarity with the full complement of Messiers. To the basic CHAT project based on the list of 43 objects given below, we may accordingly add a "CHAT-Plus" category whose list comprises these objects plus all of the remaining 83 Messiers, or 126 objects in all (125 DSO's plus the lunar crater C. Herschel). An advantage of the CHAT-Plus category is that it offers an incentive to travel further in deep sky explorations that may take many different roads. For an observer beginning with the basic CHAT list, the CHAT-Plus concept may lend extra impetus and relish to a broader quest to become familiar with all of the Messiers as well as more objects in the Herschel catalogues and NGC/IC. In a different scenario, someone who has already completed the full Messier list could then observe the additional 16 objects on the CHAT list, and arrive at a CHAT-Plus level of accomplishment. Given the great popularity of the Messier catalogue, this scenario might prove quite common in actuality. An important point for beginners in deep sky observing, including myself, is that the basic CHAT list consists mostly of objects which Caroline Herschel viewed and recorded in her first two years or so of systematic observations (1782-1784). Thus it is not so surprising that they offer a fine point of embarkation for an amateur astronomer's itinerary of explorations in galactic and intergalactic space. ----------------------------------- A note on telescopes and magnitudes ----------------------------------- As Jane Houston Jones (2002) remarks in her engaging article, the objects discovered or independently rediscovered by Caroline Herschel "are generally bright and easy"[12] to find -- as are many of the already-known Messier objects which she identified and often delighted to observe. Jones also notes the faintness of NGC 381 and NGC 891. Interestingly, Hoskin (2005) regards the attribution of these objects to Caroline in the early Herschel catalogues of 1786 and 1789 as clerical errors where other objects are meant[13]. In any event, Caroline did the observing which provides a basis for the CHAT list with notably modest equipment, especially by current standards. She began her systematic observations in August of 1782 with a very small telescope "consisting of a Tube with two glasses such as are commonly used in a finder," with a magnification of 14.5X and a field of about 3 degrees, 10 minutes, as Hoskin documents.[14] One might perhaps compare this small refractor with the telescopes used in Galileo's era, or with larger pairs of modern astronomical binoculars. It was with this scope that she discovered NGC 2360 on 26 February 1783, and independently rediscovered the "missing" M48, for which Messier had given a mistaken declination[15]. More generally, the wide field of view would be apt in sweeps for comets and nebulae alike. In July of 1783, evidently much impressed both with Caroline's skill and diligence in her sweeps and with the treasures of deep sky astronomy she was discovering, her brother William presented her with a Newtonian reflector offering, as Hoskin emphasizes, not only more powerful optics but most admirable ergonomics for surveying the heavens while "comfortably seated," with a minimum of motion on the part of the observer.[16] This reflector, or "small sweeper," evidently became Caroline's favorite telescope, with its focal length of 27 inches and an aperture of 4.2". This scope had two eyepieces: one at 24X with a field of about 2 degrees, 15 minutes; and the other at the slightly higher power of 30X with a narrower field of about 1 degree, 30 minutes. Hoskin documents these specifications for the instrument together with the estimate of David W. Hughes that the limiting magnitude would be around 12.0.[17] Caroline's happy and fruitful sessions with this scope, as reflected in part by the CHAT list, illustrate at once the virtues of wider fields of view and lower magnifications for much deep sky observation, and the wise maxim that the best telescope is the one used most easily, eagerly, and often. By March of 1790, she was also using a larger reflector William had fashioned for her with a focal length of 63"; and an aperture of 9.2", or "more than twice that of its predecessor," as Hoskin notes, with a limiting magnitude estimated at around 13.7.[18] In contrast to the smaller reflector, this "five-foot sweeper" -- telescopes then often being described by focal length -- can offer an object lesson as to the less happy side of user ergonomics. Because the focal length of the scope considerably exceeded Caroline's height and no device like a modern observing chair was offered to accommodate her modest stature, she needed to stand on some kind of more or less awkward support in order to avail herself of the instrument's superior "light-gathering power."[19] While sometimes using the newer sweeper to good effect, with much value placed as Hoskin explains on "the exact measurements of position that it made possible," Caroline expressed her own leaning toward ease of use in a written remark of 6 August 1793 which he quotes: "Finding myself unable to bear the fatigue of standing, I took the small sweeper which was in good order and shewed objects very well."[20] Her larger telescope would do further service when her nephew John took it to Capetown (1834-1838) as part of a new generation of deep sky surveying and cataloguing, now including the southern skies.[21] It was the beloved "small sweeper," however, which Caroline herself kept as a heirloom during her later years after she returned to her native Hanover, leaving instructions for its inheritors as to the "Requisites" of effective comet-hunting, including ready knowledge of all the Messiers.[22] To sum up, many of the discoveries on the CHAT list were made with a 4.2-inch reflector; and some with a yet more basic refractor at a magnification of 14.5X, as were a number of the observations of known Messier objects. Thus users of smaller instruments should feel especially welcome to explore the deep sky realms of Caroline Herschel. ---------------------------------- Tony Flanders: Some open questions ---------------------------------- After the CHAT list was compiled, two companion articles appeared in _Sky and Telescope_ for August 2007, one by Michael Hoskin nicely summarizing his articles cited above[23]. The other, a short commentary by Tony Flanders[24] supplemented by a more detailed presentation on the World Wide Web[25], engagingly invites us to regard the identifications of certain deep sky objects discovered by Caroline Herschel as still quite open, and to try making new observations with telescopes comparable to hers as a strategy for better clarifying the issues. Taking her beloved small Newtonian sweeper with an aperture of 4.2" and eyepieces at 24X and 30X as a standard, Flanders proposes an 80mm f/6 refractor with 20mm and 16mm eyepieces as one reasonable equivalent.[26] I might add that with a modern 4.5" f/8 Newtonian reflector, eyepieces at 38mm and 30mm would yield comparable magnifications. Flanders suggests that one potential source of uncertainty is that Caroline would sometimes observe a new object with her small sweeper and then invite her brother to study the object further with a much larger instrument, raising the possibility that sister and brother might sometimes study different objects thinking them to be the same.[27] Accordingly, Flanders urges that we compare the level of detail revealed by a modern telescope (for example in resolving an open cluster) with Caroline's description: if the telescope "consistently" shows more detail than her notes report, then one should "consider masking your telescope's aperture" to make its light-gathering ability more comparable to that of her small sweeper.[28] From this perspective, Flanders raises some questions about the identification of an open cluster which Caroline found in Cassiopeia as NGC 189, suggesting that this magnitude 8.8 object "would be difficult to see through her modest telescope," and asking if some other cluster or asterism in the area might fit her observations.[29] Also intriguingly problematic for Flanders is the identification of another of Caroline's open clusters as NGC 659. He proposes that she may actually have discovered the neighboring and "much bigger and brighter" NGC 663, only half a degree away (RA 01:46.3, Dec +61:13), finding it difficult to envision her observing NGC 659 without also noticing NGC 663. Also, he finds the smaller NGC 659 "very hard to resolve at low power in a small telescope," making NGC 663 an appealing alternative to consider.[30] While the CHAT list follows the Herschel catalogues and Hoskin as to the identifications of these objects, we might well also explore the possibilities raised by Flanders -- observing, for example, both the "canonical" NGC 659 and the alternative NGC 663. Both of these objects are well-known, and the latter is easy to observe at the same time as the former, thus "covering all bases." As for NGC 189, in questioning that identification Flanders leaves open the question of what other cluster or asterism Caroline might have observed in its stead. Searching for possible candidates could be a very creative exercise, and a reminder that not all astronomical and/or historical mysteries have answers neatly to be listed. ------------------------------- The CHAT list and its structure ------------------------------- The CHAT list includes 43 objects divided into six categories. There are five categories of deep sky objects (DSO's) which Caroline Herschel discovered, independently discovered, observed as known Messier objects, or which were attributed to her in the first two catalogues of nebulae which she and her brother published in 1786 and 1789. The sixth category consists of the lunar crater C. Herschel. --------------- CHAT Categories --------------- Category A includes 12 objects which Caroline Herschel discovered or independenly rediscovered. Category B includes two objects which are very likely independent rediscoveries as judged by Hoskin (2005, 2006) and also discussed by Frommert and Kronberg. Category C includes three objects attributed to "C.H."in the first Herschel catalogues of 1786 and 1789 (each with 1000 nebulae): NGC 2204, NGC 891, and NGC 381. Hoskin (2005), however, concludes that catalogue misnumberings occurred in these attributions, with the intended objects included in CHAT Category A: NGC 2360, NGC 205, and NGC 189. Regardless, these three objects are retained with due cautions as to the attributions because they are part of the Caroline Herschel tradition, reflect her astronomical work as a cataloguer even if not as an observer, and are discussed in the outstanding article by Jane Houston Jones (2002). Category D includes three objects which Caroline Herschel initially recorded as her first, third, and sixth discoveries among the nebulae, but which were later identified as M29, M46, and M93 -- in the case of M46 by she herself later in the same year of 1783, see Hoskin (2005), who also tells how in that year her brother William in his own sweeps for nebulae "encountered what he later realized had been M 79."[31] Category E includes 22 known Messier objects which she had observed and recorded through July of 1783. As noted above, by the end of that year she had logged 41 Messiers, and is later reported to have known them all, as she herself wrote in her later years was a basic "Requisite" for anyone hunting comets. Category F, as noted above, includes the lunar crater C. Herschel located in Mare Imbrium. --------------------------- Explanation of list entries --------------------------- For the first five categories comprising deep sky objects, the list includes the following columns of information for each object: CH#: Where applicable, this is Caroline Herschel's own number assigned in a manuscript catalogue of 20 observations of nebulae she did not associate at the time with known objects, as documented by Hoskin (2006) and conveniently summarized by Frommert and Kronberg. The CHAT list includes 16 of these observations. The other four are CH15, an evident reobservation of her newly discovered NGC 225 (CH11); CH17 and CH18, not now identifiable; and CH4, the elusive "mystery object" sometimes identified with the enigmatic NGC 2349, which Jones (2002) humorously suggests might best be tackled "in the armchair some rainy night." Date: This is the date of Caroline's observation. Messier: Where applicable, this is the object's Messier number. NGC/IC: This is the object's number in the New General Catalogue (NGC) or Index Catalogue (IC), abbreviated in the list as "N" and "I" respectively, thus N2360 or I4665. Constel: This is the constellation in which the object appears, with a key to the abbreviations given after the list. RA: This is the object's right ascension. Dec: This is the object's declination. Type: This is the object's type, using these abbreviations: Galaxy (Gal); Open Cluster (OC); Open Cluster with Nebulosity (OC+N); Globular Cluster (GloC); and Planetary Nebula (PlaN). Status: This is the object's status, using these abbreviations: a discovery by Caroline Herschel (Discov); an original rediscovery (Redisc); an already known object (Known); a likely already known object (Known?); or an attribution in the 1786 or 1789 Herschel catalogue regarded by Hoskin (2005) as a misnumbering of another intended object(?). Taki8.5: This identifies the number of the chart where the object may be found in _Taki's 8.5 Magnitude Star Atlas_ by Taki Toshimi, freely available for downloading in PDF format at . Pages are formatted in the international A4 paper size. Mag: This is the magnitude of the object, or one value for it taken from a source like _Taki's 8.5 Magnitude Star Atlas_, the very helpful table of the Messiers compiled by Frommert and Kronberg, , or Jones (2002). A question mark after a Messier or NGC/IC identification for an object, as with "M752?" in Category B or "M29?" in Category D, means that the identication may involve some degree of uncertainty but is regarded as quite likely. Hoskin (2005) in fact regards the identication of NGC 752 as "very likely" and that of M29 as "almost certain."[32] Similarly, he confidently asserts in his 2006 article that Caroline "is ... an independent discoverer of IC 4665."[33] For the one object in Category F, the lunar crater C. Herschel, the general location in Mare Imbrium and the latitude, longitude, crater diameter, and crater depth are given. ________________________________________________________________________________ -------------------------------------------------------------------------------- CHAT Observing List: Caroline Herschel Astronomy Tour -------------------------------------------------------------------------------- A. Deep Sky Objects -- Discoveries or Independent Rediscoveries (12) -------------------------------------------------------------------------------- CH# Date Messier NGC/IC Constel RA Dec Type Status Taki8.5 Mag -------------------------------------------------------------------------------- 2 26/02/1783 ____ N2360 CMa 07:17.8 -15:37 OC Discov Chart 102 7.2 5 08/03/1783 M48 N2548 Hya 08:13.8 -05:45 OC Redisc Chart 77 5.8 7 23/07/1783 ____ N6866 Cyg 20:03.9 +44:10 OC Discov Chart 17 7.6 8 31/07/1783 ____ N6633 Oph 18:27.3 +06:31 OC Discov Chart 67 4.6 9 27/08/1783 M110 N205 And 00:40.4 +41:41 Gal Redisc Chart 37 8.1 10 23/09/1783 ____ N253 Scl 00:47.6 -25:18 Gal Discov Chart 109 7.2 11 27/09/1783 ____ N225 Cas 00:43.6 +61:46 OC Discov Chart 37 7.0 12 27/09/1783 ____ N189 Cas 00:39.6 +61:05 OC Discov Chart 37 8.8 14 30/10/1783 ____ N7789 Cas 23:57.4 +56:43 OC Discov Chart 14 6.7 16 12/05/1784 ____ N6819 Cyg 19:41.3 +40:11 OC Discov Chart 18 7.3 19 07/08/1787 ____ N7380 Cep 22:47.3 +58:08 OC+N Discov Chart 15 7.2 20 27/09/1783 ____ N659 Cas 01:44.4 +60:40 OC Discov Chart 36 7.9 ________________________________________________________________________________ -------------------------------------------------------------------------------- B. Deep Sky Objects -- Probable Independent Rediscoveries (2) -------------------------------------------------------------------------------- CH# Date Messier NGC/IC Constel RA Dec Type Status Taki8.5 Mag -------------------------------------------------------------------------------- ___ 31/07/1783 ____ I4665 Oph 17:46.3 +05:43 OC Redisc Chart 68 4.2 13 29/09/1783 ____ N752? And 01:57.7 +37:40 OC Redisc Chart 36 5.7 ________________________________________________________________________________ -------------------------------------------------------------------------------- C. Deep Sky Objects Attributed to CH in Herschel Catalogues of 1786, 1789 Hoskin 2005 regards these as clerical errors for N2360, N205, and N189 (3) -------------------------------------------------------------------------------- CH# Date Messier NGC/IC Constel RA Dec Type Status Taki8.5 Mag -------------------------------------------------------------------------------- ___ 26/02/1783 ____ N2204 CMa 06:15.5 -18:40 OC ? Chart 103 8.6 ___ 27/08/1783 ____ N891 And 02:22.6 +42:21 Gal ? Chart 35 9.9 ___ 1783 ____ N381 Cas 01:08.4 +61:35 OC ? Chart 36 9.3 ________________________________________________________________________________ -------------------------------------------------------------------------------- D. Deep Sky Objects -- Later Shown to be Known Messier Objects (3) -------------------------------------------------------------------------------- CH# Date Messier NGC/IC Constel RA Dec Type Status Taki8.5 Mag -------------------------------------------------------------------------------- 1 26/02/1783 M93 N2447 Pup 07:44.5 -23:51 OC Known Chart 102 6.2 3 04/03/1783 M46 N2437 Pup 07:41.8 -14:49 OC Known Chart 102 6.1 6 06/04/1783 M29? N6913 Cyg 20:23.9 +38:32 OC Known? Chart 37 7.1 ________________________________________________________________________________ -------------------------------------------------------------------------------- E. Deep Sky Objects -- Observed and Listed as Messiers, to July 1783 (22) -------------------------------------------------------------------------------- Date Messier NGC/IC Constel RA Dec Type Status Taki8.5 Mag -------------------------------------------------------------------------------- 30/09/1782 M27 N6853 Vul 19:59.6 +22:43 PlaN Known Chart 42 7.3 29/10/1782 M13 N6205 Her 16:41.7 +36:28 GloC Known Chart 45 5.9 29/10/1782 M37 N2099 Aur 05:52.3 +32:33 OC Known Chart 56 5.6 13/10/1782 M36 N1960 Aur 05:36.1 +34:08 OC Known Chart 56 6.3 23/01/1783 M44 N2632 Cnc 08:40.0 +19:40 OC Known Chart 53 3.1 26/02/1783 M47 N2422 Pup 07:36.6 -14:29 OC Known Chart 102 4.4 26/02/1783 M41 N2287 CMa 06:46.0 -20:45 OC Known Chart 103 4.5 26/03/1783 M67 N2682 Cnc 08:50.8 +11:49 OC Known Chart 77 6.9 07/04/1783 M56 N6779 Lyr 19:16.6 +30:11 GloC Known Chart 42 8.3 05/05/1783 M4 N6121 Sco 16:23.6 -26:32 GloC Known Chart 93 5.9 05/05/1783 M9 N6333 Oph 17:19.2 -18:31 GloC Known Chart 92 7.9 05/05/1783 M23 N6494 Sgr 17:57.1 -18:59 OC Known Chart 92 5.5 22/05/1783 M5 N5904 Ser 15:18.6 +02:05 GloC Known Chart 70 5.8 22/05/1783 M10 N6254 Oph 16:57.1 -04:06 GloC Known Chart 69 6.6 22/05/1783 M11 N6705 Sct 18:51.1 -06:16 OC Known Chart 67 5.8 22/05/1783 M12 N6218 Oph 16:47.2 -01.57 GloC Known Chart 69 6.6 21/07/1783 M14 N6402 Oph 17:37.6 -03:15 GloC Known Chart 68 7.6 23/07/1783 M71 N6838 Sge 19:53.8 +18:47 GloC Known Chart 42 8.3 30/07/1783 M2 N7089 Aqr 21:33.5 -00:49 GloC Known Chart 64 6.5 30/07/1783 M16 N6611 Ser 18:18.8 -13:47 OC+N Known Chart 91 6.0 30/07/1783 M25 I4725 Sgr 18:31.8 -19.07 OC Known Chart 91 4.6 30/07/1783 M55 N6809 Sgr 19:40.0 -30:58 GloC Known Chart 90 7.0 ________________________________________________________________________________ -------------------------------------------------------------------------------- F. Lunar Crater "C. Herschel" named 1935 (1) -------------------------------------------------------------------------------- General Location Lat Long Diam Depth -------------------------------------------------------------------------------- Mare Imbrium 34.5N 31.2W 13.4km 1.9km ________________________________________________________________________________ ________________________________________________________________________________ Key to Abbreviations Key to Abbreviations for Constellations: for Deep Sky Object Types: And Andromeda Gal Galaxy Aqr Aquarius GloC Globular Cluster Aur Auriga OC Open Cluster Cas Cassiopeia OC+N Open Cluster with Nebulosity Cep Cepheus PlaN Planetary Nebula CMa Canis Major Cnc Cancer Cyg Cygnus Her Hercules Hya Hydra Lyr Lyra Oph Ophiuchus Pup Puppis Scl Sculptor Sco Scorpius Sct Scutum Ser Serpens Sge Sagitta Sgr Sagittarius Vul Vulpecula ------- Notes ------- 1. Sir John Frederick William Herschel, "Catalogue of Nebulae and Clusters of Stars," _Philosophical Transactions of the Royal Society of London_, v. 154 (1864), pp. 1-137, at p. 3. This is the General Catalogue (GC), available in PDF format at: . 2. Charles Messier's _Catalogue des Nébuleuses et des Amas des Étoiles_ or _Catalogue of Nebulae and Star Clusters_ was used by Caroline and William Herschel in its 1781 edition with 103 objects listed, as published in _Connoissance des Temps, ou Connoissance des Mouvemens Célestes, pour l'Année bissextile 1784 (Paris 1781), pp. 227-272. An identical reprint from the _Connoissance des Temps_ published in 1784 for use in 1787, is available in a PDF facsimile including the pages listing and describing all 103 objects: , with a helpful and convenient English translation also featuring Messier's handwritten annotations and including the seven additional objects making up the present "Messier 110," available at , by Harmut Frommert and Christine Kronberg. It is safest to say that there are "about 110" Messier objects because there has been a controversy over whether M102 is a distinct object in its own right, NGC 5866, or in fact identical to M101 (NGC 5457). For a summary of this historical "M102 controversy," see Frommert and Kronberg, ; these authors favor the view that there is "sufficient evidence" to associate M102 with NGC 5866, "quite an easy, beautiful object." 3. The "Herschel 2500" set of objects, actually totalling in one modern enumeration to 2514 reported objects, were published in three successive catalogues, all now available in PDF format on the Internet. Thus see Herschel, William, "Catalogue of One Thousand new Nebulae and Clusters of Stars," _Philosophical Transactions of the Royal Society of London_, v. 76 (1786), pp. 457-499, at ; "Catalogue of a second Thousand of new Nebulae and Clusters of Stars; with a few introductory Remarks on the Construction of the Heavens," _Philosophical Transactions of the Royal Society of London_, v. 79 (1789), pp. 212-254, at ; and "Catalogue of 500 new Nebulae, nebulous Stars, planetary Nebulae, and Clusters of Stars; with Remarks on the Construction of the Heavens," Philosophical Transactions of the Royal Society of London, v. 92 (1802), pp. 477-528, available in a reprinted version at . A few of these observations cannot be identified with any object observable now, or might be so identified only problematically. For a convenient listing of all 2500-plus objects, see Harmut Frommert and Christine Kronberg, "William Herschel's Deep Sky Catalog," at . The "Herschel 400" list, a subset developed by the Ancient City Astronomy Club of St. Augustine, Florida, USA serving as the basis for an observing program and award by the Astronomical League, is available at , with more information about the program and its award requirements at . As a follow-on to this program, Rose City Astronomers of Portland, Oregon, USA developed a list of another 400 Herschel objects presenting overall a greater level of challenge to amateur astronomers, and known accordingly as "Herschel II." For general information, see ; for one version of this Herschel II list in ASCII text format, see . 4. See Herschel, 1786 (n. 3 above), pp. 465-466. The passage deserves quotation in full: "When the diurnal motion of the earth was first maintained, it could not but greatly add to the reception of this opinion when the telescope exposed to our view Jupiter, Mars, and Venus, revolving on their axes*; and if these instances of the similar condition of other planets support the doctrine of the diurnal motion, the view of so many sidereal systems, some of which we may discern to be of a most surprising extent and grandeur, will in like manner add credit to what I have proposed with regard to the condition of our situation within a system of stars: for, to the inhabitants of the nebulae of the present catalogue, our sidereal system must appear as either a small nebulous patch; an extended streak of milky light; a large resolvable nebula; a very condensed cluster of minute stars hardly discernable; or an immense collection of large scattered stars of various sizes. And either of these appearances will take place with them according as their own situation is more or less remote from ours." In the footnote marked with an asterisk, he adds: "To these may now also be added Saturn, on whose body I have, in the year 1780, seen several belts, with spots which changed their situation in the course of a few nights." 5. See Michael Hoskin, with an Appendix by Brian Marsden, 2005, "Caroline Herschel as Observer," _Journal for the History of Astronomy_, v. 36, part 4, no. 125 (November 2005), pp. 373-406, at pp. 382-383. As Hoskin explains, there was in this period no established method for accurately measuring possible changes in a star's relative magnitude. However, in the early 1780's, Edward Pigott and John Goodricke had arrived at a technique of observing what might be described as the _comparative_ brightness of a possible variable star in relation to an established "sequence of stars arranged in order of descending brightness and including the supposed variable." Thus on 31 July 1783, Caroline Herschel wrote an observation at to how the striking double star ß Cygni ranked in brightness with other stars of the constellation Cygnus; and in August, "she determined sequences of the stars of six different constellations," ibid. at 382-383. 6. More precisely, Leavitt's discovery of the period-luminosity relationship for the Cepheids provided a direct yardstick for measuring distances of up to about 10-12 million light years using terrestrial-based telescopes, thus encompassing the region of our own galaxy and our nearest intergalactic neighbors, and served as a vital link in a chain of measures soon extended to the realm of more distant galaxies also. During the 1920's, Edwin Hubble played a central role both in applying Leavitt's Cepheid yardstick to show that nebulae such as M31 in Andromeda must be independent galaxies outside our own Milky Way, and in finding extended yardsticks such as the red shift which could be used to measure larger intergalactic distances. The Hubble Space Telescope has extended the direct reach of the Cepheid yardstick by roughly an order of magnitude or factor of ten, with Cepheid variables detected in a galaxy as distant as NGC 4603 (discovered by John Herschel, nephew of Caroline) in the Centaurus cluster, measured at about 108 million light years from our Milky Way. In a pattern somewhat like that of the 1920's, astronomers such as Wendy L. Freedman have applied this data both to refine the Cepheid yardstick itself and to recalibrate other cosmic yardsticks or parameters relevant to more remote distances, for example the Hubble Constant or rate of expansion of the universe. 7. Ibid., p. 382. 8. Ibid., p. 376. See also n. 2 above for more information about the Messier catalogue with links to a 1784 reprint in PDF format substantially identical to the 1781 edition used by the Herschels, and an English translation. 9. Ibid., p. 382. Hoskin, ibid. at p. 373, notes that this Astronomer Royal saluted Caroline Herschel as "Dear Sister Astronomer," one indication of the esteem she enjoyed among her scientific colleagues. 10. Ibid. 11. Ibid. This is the "small sweeper" to be described in the next section of this article, "A note on telescopes and magnitudes": a reflecting telescope with an aperture of 4.2" and a focal length of 27". For a history of this telescope including a sketch of the convenient alt-azimuth design and a photograph of the surviving optics, see Michael Hoskin, "Caroline Herschel's `Small' Sweeper," _Journal for the History of Astronomy_, v. 36, part 1, no. 122 (February 2005), pp.28-30. 12. Jane Houston Jones, 2002, "The Caroline Herschel Objects," _Sky and Telescope_, v. 104, no. 5 (November 2002), pp. 107-111 13. Jones, ibid.; and Hoskin, n. 5 above, p. 399. Harmut Frommert and Christine Kronberg, "Carole Herschel's Deep Sky Objects" , also discuss these possible misnumbered attributions in the Herschel catalogues. 14. Hoskin, ibid., pp. 374-378. A delightful episode in Caroline's astronomical apprenticeship with this small refractor took place on 30 September 1782, when "she came across what proved to be" M27 -- which her elder brother and mentor William, albeit already famed for his discovery the previous year of the planet which came to be called Uranus, evidently had not seen! Hoskin concludes that he "was no doubt impressed" with the sight that his younger sister had to share with him, ibid. at p. 376. This beautiful planetary nebula is sometimes now known as the "Dumbbell Nebula." 15. Ibid., pp. 378, 397. Messier's catalogue published in 1781 for use in 1784, and reprinted in 1784 for use in 1787, see n. 2 above, gives a declination for M48 in degrees, minutes, and seconds of "1.16.42 A" -- that is, 1°16'42" _Australis_ or south, or in modern terms -1°16'42", with the negative sign used to express declinations south of the celestial equator, and the positive sign to express those north of it, or Messier's "B" for _Borealis_. The actual declination for M48 is around -5°48' -- a slip on Messier's part giving Caroline the opportunity for her independent rediscovery of his "missing" object. 16. Ibid., pp. 378-380; see also Hoskin, n. 11 above. 17. Ibid., p. 375, Table I, "Caroline Herschel's Telescopes," and n. 95 for the estimated limited magnitudes in David W. Hughes, "Caroline Lucretia Herschel -- comet huntress," _Journal of the British Astronomical Association_, v. 109 (1999), pp. 78-85 at p. 78. 18. Hoskin, ibid., pp. 386-391. 19. Ibid., p. 390. 20. Ibid. 21. Ibid., p. 394. 22. Ibid., p. 382. 23. Michael Hoskin, "In Caroline Herschel's Footsteps: Can you replicate the observations of a pioneering 18th-century deep-sky astronomer?" _Sky and Telescope_ (August 2007), pp. 59-62. 24. Tony Flanders, "See Caroline's Objects as She Saw Them," _Sky and Telescope_ (August 2007), p. 62. 25. Tony Flanders, "In Caroline Herschel's Footsteps" (May 9, 2007), . For a project to observe the Caroline Herschel objects with astronomical binoculars (e.g. 20X100 or 25X100) which draws on this article by Flanders, see Paolo Morini, "Gli oggetti di Caroline Herschel: un progetto di osservazione al binoculo," . 26. Ibid. 27. Ibid. Specifically, Flanders suggests that Caroline may have discovered NGC 663, a large and bright open cluster in Cassiopeia, writing a description which William matched with the view in a reflector of much greater aperture of the nearby NGC 659, the latter easily resolvable into stars in his telescope, but not necessarily in hers; see also below. 28. Ibid. One might add that comparable observations might ideally be made with a telescope roughly equivalent in aperture or resolving ability to Caroline's small sweeper _under dark sky conditions_. Observing with a larger instrument under light polluted urban conditions may make for a very interesting and challenging situation, but hardly for a reasonably close replication of her original observations. 29. Ibid. 30. Ibid. 31. Ibid., p. 378, on the similar experiences of sister and brother; and p. 399, item (18), for Caroline's initial observation of 4 March 1783 which seemed "not in Mess. catalogue," and her later recognition on 19 November that "My 3d nebula is Mess. 46th." 32. Ibid., p. 398, item (11) on NGC 752, finding Owen Gingerich's opinion that this is what she saw on 29 September 1783 as "highly likely"; and p. 399, item (19), concluding of her relevant observation on 6 April 1783 that her description of the object's position and configuration (which she sketches in a diagram) "makes it almost certain that was in fact M 29." Flanders, n. 25 above, while questioning identifications of certain other objects (specifically NGC 189 and 659), finds "no such doubt" about NGC 752, on which he agrees with Hoskin. 33. Michael Hoskin, "Caroline Herschel's Catalogue of Nebulae," _Journal for the History of Astronomy_, v. 37, part 3, no. 128 (August 2006), pp. 251-255 at 255. Flanders, n. 25 above, agrees that "IC 4665 is pinpointed precisely by Caroline's notes." Here the possible element of doubt involves whether Caroline or William saw it first, since each sibling credits the other for the discovery! See Hoskin, ibid., who argues persuasively that their respective recorded itineraries of observation on the night in question (31 July 1783) make Caroline the likelier candidate. Margo Schulter Sacramento, California, USA mschulter@calweb.com 29 January 2009 CHAT list version 0.002