Cassini's Bright Spot
Cassini's Bright Spot(glossary entry for an informal name)
Mario Weigand This photo shows the region from dark-floored Pitatus (in the upper left), to dark-haloed Tycho (at the bottom) as it appears near Full Moon. Cassini's Bright Spot is the area around the intense bright patch in the upper right. Depending on the exposure it can look much larger. The dark-floored crater surrounded by a bright ring to the west of the Bright Spot is Hell. The core of the bright patch is itself mostly to the northeast of a small crater currently called Hell Q.
In its modern usage, this term is used to refer to a bright patch near the crater Hell Q. Indeed, the IAU name Hell Q seems to have originally referred to this albedo feature, rather than to a crater. The name derives from reports of a transient "white cloud" in this vicinity by the 17th century astronomer G. D. Cassini. But as with nearly all old reports, especially without access to Cassini's actual words, it is very difficult to be sure the current feature is actually the one Cassini was talking about.
- The association of Cassini's "white cloud" with the Bright Spot named after him would seem (and probably is) too obvious to question. Yet there are several odd aspects to the story. Especially odd is his report that two years later the cloud had disappeared and been replaced by a crater.
- The feature we now call Cassini's Bright Spot is a very prominent feature that does not require sophisticated instrumentation to observe. Dinsmore Alter, in his Lunar Atlas, devotes a number of pages to the Tycho ray system, and shows that, although most striking near Full Moon, the Bright Spot is a prominent landmark throughout nearly the part of the lunar cycle in which that area is in sunlight.
- Whitaker (p. 208), believes the "very bright nimbus at Deslandres QA" (a typo for Hell QA?) was designated Mons Tabor on Hevelius' maps from circa 1647, which would presumbably have been known to Cassini.
- At least in later years, G. D. Cassini was a very competent and experienced lunar observer. The depiction of the Deslandres region in his celebrated 1679 map clearly shows the craters Hell, Hell A, Hell B, and Hell C. It seem inconceivable that he could have seen that level of detail and not been able to re-locate the present day Bright Spot in those years.
- Schröter's account based on editorial notes found accompanying a new edition of Cassini's map.
- The most readily available account of Cassini's observation is that retold by Sky and Telescope’s Joseph Ashbrook, who says he is translating from Beer and Mädler:
- On October 21, 1671, M. Cassini noticed near Gauricus a kind of whitish cloud, of which some remnants remained on the 25th. On November 12th of that year, the same cloud reappeared in the same place.
- October 18, 1673. New large crater between Pitatus and Walter in exactly the spot where the whitish cloud was seen in 1671.
- Beer and Mädler (according to Ashbrook) felt the "cloud" must be a reference to "the surface feature that can be seen every month for about 10 days around the date of Full Moon," and that the "new crater" seen in 1673 was Hell or Hell B.
- Elger (1893) recounts the story slightly differently. He puts the first appearance of the "large white cloud" in November, 1671; and feels it must have appeared over the 20-km crater just outside the north rim of Lexell. He adds that Cassini described the cloud on subsequent occasions as resembling "a lightish spot with star-like rays."
- Since early astronomers commenced their "days" at noon, 12 hours before our present changing of the civil day at midnight, Cassini's October 21, 1671 observation was presumably made on what we would now call the night (in Paris) of Oct. 20/21. This would have been well past the most recent Full Moon (which occurred at 04:00 UT on October 18). Although the modern Bright Spot is quite visible at this phase, what is odd is that the crater Gauricus would have been used as a landmark. The sun angle there would have been 50° above the horizon, and Gauricus is nearly invisible under those conditions, and it is only slightly closer to the modern Bright Spot than the much more easily recognized Pitatus (or even Tycho). Possibly lacking our modern atlases Cassini mistook Pitatus for Gauricus?
- The recurrence of the cloud on November 12, 1671 is consistent with the behavior of the modern Bright Spot.
- The October 18, 1673 date for the discovery of a crater at the former location of the cloud seems to be in error (unless Ashbrook has corrected it to the modern day numbering convention). On the evening of October 17, 1673 (which a Parisian astronomer of the day would have called the start of October 18), the Moon would just have been at First Quarter. The Deslandres area, where the modern Bright Spot is located, would not have been in sunlight until the next evening. On that evening the sun angle would have been similar to that on the morning of October 25, 1671 (when traces of the spot were said to be still visible), but from the opposite direction.
- It is unclear from any of these descriptions if Cassini observed this region of the Moon on other nights. When he saw the cloud replaced by a crater in 1673, had he actually been looking at the Moon for two years without seeing either a white spot or a crater at the former location?
- In summary, the most inconsistent part of the story is the impression given that in later years Cassini saw only a crater at the location where he had earlier seen a bright cloud. The unlikely possibility exists that he had seen some sort of transient event in 1671 and that although he must have frequently seen the modern Bright Spot in later years he could never again re-find this separate and distinct cloud from 1671. Much more probably he saw the modern Bright Spot many more times (just as modern amateurs do) and recognized it as his 1671 cloud, but failed to correct the impression left by his earlier journal entries, which the modern re-tellers of his story have relied on.
The Modern-day Feature
- For photos of Cassini's Bright Spot, see the Hell Q page.
- Dinsmore Alter appears to associate Cassini's Bright Spot with the ray system of Tycho, rather than seeing it as a separate pattern of ejecta.
- Cherrington describes the spot as being located on the eastern floor of Deslandres, near the outer wall of Walter.
- Chong, Lim and Ang in their Photographic Atlas of the Moon (p. 40), place Cassini’s Bright Spot "adjacent to Lexell on the south wall" (of Deslandres?). This is seems to be a slightly garbled inversion of Cherrington's description of the location of Lexell as being "in the south wall (of Deslandres), adjacent to the bright spot."
- Peter Grego's The Moon and How to Observe It (p. 107) identifies Cassini’s Bright Spot as being equivalent to "Deslandres HA" and says it is the brightest feature on the Moon. "Deslandres HA" seems to be a misreading of Hell HA on the System of Lunar Craters charts. In any event, Hell Q seems a better locator than Hell HA, and Aristarchus would certainly seem a contender for brightness (although it is unclear if Grego is talking about brightness per unit area or brightness totaled over the area of the feature).
- From his study of photos taken at small phase angles, Robert Wildey believes Aristarchus and Cassini’s Bright Spot have the strongest opposition surges of any of the features on the Moon's nearside.
- According to former Sky and Telescope columnist Stephen James O’Meara, Cassini’s Bright Spot is readily visible to the unaided eye (at least for those with good eyesight) as “an intense splash of light” in the Moon’s southern hemisphere near Full Moon. However, he says, this fact is not well known because the naked eye bright spot has for many years been erroneously attributed to the crater Tycho. Through binoculars (or a small telescope), he says, Tycho is much more striking and draws our attention because of the contrast between its bright core and the dark halo that surrounds it. But with the naked eye, the bright and dark parts of Tycho tend to cancel out, and it is Cassini’s Bright Spot (to its north) that is seen. According to O’Meara, this difference can be demonstrated by noting the shift in the apparent location of the eye-catching bright spot in the naked eye versus the binocular view. It can also be seen by looking at a Full Moon photo from a distance or with unfocussed eyes. If it is true that Cassini’s Bright Spot has a greater opposition surge than other features, then presumably this shift should be most striking at the fullest of Full Moons (those with very small phase angles).
What? More Tycho? (shows an enhanced-color view of Tycho and surroundings, with Cassini's Bright Spot at upper right, photographed by the Minsk Miracle Imagers).
- Dinsmore Alter. 1956. "Cassini's bright spot": Strolling Astronomer, v. 10, no. 5-6, pp. 54-57.
- Dinsmore Alter. 1964. Lunar atlas. [Downey, Calif.]: North American Aviation
- Joseph Ashbrook. 1965. "The 'Long Night' of Selenography". Sky and Telescope (February issue), p. 92. Reprinted as Chapter 47 in the Astronomical Scrapbook (1984).
- Ernest H. Cherrington. Exploring the Moon through Binoculars and Small Telescopes.
- S. M. Chong, Albert Lim, and P. S. Ang. 2002. Photographic atlas of the moon. New York: Cambridge University Press
- Thomas Gwyn Elger. 1893. Lexell and its Surroundings. The Observatory 16: 355-356
- Peter Grego. 2005. The Moon and How to Observe It. London: Springer-Verlag.
- Stephen J. O’Meara. 2006. "Scattered Pearls". Sky and Telescope (February issue), p. 66.
- Robert Wildey. 1978. The Moon in Heiligenschein. Science 200: pp. 1265-1267.