The Lunar Crater Furnerius and the Mystery of Schroters Dome

By Nigel Longshaw



The lunar crater Furnerius has been kept under the watchful eye of the B.A.A. lunar section for some time following the very detailed studies carried out by Keith Abineri of the relevant Lunar Orbiter frames using microscopic technique (a stereomicroscope used to examine enlarged portions of original Orbiter microfilm).The results have been published in "The New Moon", journal of the B.A.A. Lunar Section topographical sub-section, in volume 4 no. 1 June 1989, no. 2 December 1989, and no. 3 June 1990.

As the majority of members have probably not seen the articles, I thought a brief resume of the work carried out might encourage interested part to observe this fascinating feature.

The crater first attracted attention in an attempt to try and establish the true nature of the southern floor as depicted by J.H.Schroter in his "Selenotopographische Fragmente" (1791-1802), and first noted by J.H.Phillips in his paper "Lunar Domes First Observed by J.H.Schroter" from the Journal of the A.L.P.O. vol.32 nos.3-4. Secondly as a result of examination of the Orbiter data to try and observe and confirm the vast amount of interesting detail visible upon the crater floor.

At lunar longitude 60° East, 81 mile diameter Furnerius is a member of a group of large craters which lie in a North-South line and include Langrenus, the ancient flooded enclosure Vendelinus and Petavius. Sunrise over Furnerius occurs at around colongitude 300°, approximately 2/3 days after new moon. Obviously at this phase the moon presents a slim crescent lying low on the horizon.Observation under these conditions is very difficult, relief of some of the features is very subtle and soon lost to bad seeing conditions experienced at low altitudes have only one observation in my files of sunrise over Furnerius, it shows nothing of the interior details, only vague shadings.

Much more interesting and rewarding is observation under sunset condition which occurs around colongitude 112° to 119°approx, 2/3 days after full moon. At this phase the moon is much higher in the night sky and is available for a greater period allowing a fuller observing session to be carried out.

Plate 1 Plate 1

Now we come onto Schroter's observation of Furnerius (see plate 1). In fact Schrdter's drawing shows the whole region from Petavius to Furnerius, judging by the shadows shown on the inner western walls of Petavius and Snellius/Stevinus (which have their names reversed) we can assume the observation was made under lunar sunset conditions. Assuming the arc to the left of the drawing, and containing the drawing, is the position of the terminator, I estimate the suns colongitude value to be around 105°. This is probably not the best condition to observe Furnerius itself, and it is interesting to note that no internal shadows are shown inside Furnerius. It is often stated that Schroter was a poor draftsman, although he does show a feature reminiscent of a "dome" on the southern floor, something which was missed by many later observers. An indication of this "dome" can be seen in Kopals: A new photographic atlas of the moon: 1971 pl. 63 page 125, which shows a "raised" area on the southern floor retaining illumination whilst all around is in shadow.This seems to indicate a "gentle swelling" of the floor, rather akin to that seen on the floor of the crater Darwin (which is best seen 1/2 days before full moon around colongitude 72°)

Aside from "Schroter's dome" many interesting features can be observed upon the southern floor of Furnerius, these mainly comprise numerous craterlets and pits,many forming complex "chains". Under good seeing and favourable illumination many of the larger ones can be picked out with an 8 inch telescope, and under excellent conditions of seeing the whole region gives the feeling of being just beyond resolution, very reminiscent of borderline resolution given by some globular clusters. Also of interest is the central floor, which seems to show characteristics of a "lava pool'' i.e. flooding of older structures by dark basalt Material. The Orbiter images display many "pits" within this darker material, along with some minor fissures (obviously these features are beyond the resolution of terrestrial telescopes).Of further interest is the presence of a large rifle (generally regarded as collapsed lava tubes) which is known as Rima 1 or Rima Furnerius. This large feature, easily seen given correct illumination, runs from the eastern floor of Furnerius in a northerly direction, at the large crater Furnerius "B" the rifle seems to "bend" and continue its run in a more westerly direction where it disappears from view in the north west corner of Furnerius.

Keith Abineris three papers on the subject contain a much more detailed account of the many features upon the floor of Furnerius, many of which are beyond the scope of this article. Readers may be interested to know that further work is proposed in respect of the new data received from the Clementine mission, and will be published in a forthcoming issue of The New Moon.

However in the meantime members who possess medium sized telescopes in the 6 to 8 inch class will find a wealth of interesting detail visible I would not wish to discourage observers with smaller instruments and urge them to observe when conditions are favourable, it is often surprising what can be seen with limited equipment. Much of the detail requires patience, good seeing,correct illumination and high powers to detect, it would be interesting to compare what can be seen with different telescopes, I am sure some details can be observed which are not recorded on the popular charts and maps.

Plate 2 Plate 2 Plate 3 Plate 3

To assist readers in the identification of the salient features I refer to the observations reproduced on plates 2,3 and 4 made with 8 inch telescopes under favourable conditions. Plates 2 and 4 are observations by myself, and show Furnerius at early sunset and late sunset, when the formation has been almost swallowed up by shadow leaving the eastern wall visible and a small portion of the southern floor. I am indebted to Harold Hill for his kind permission to reproduce his observation of 1989 August 18th on plate 3. This is perhaps the most interesting of the three as it gives some indication of a "domelike" feature on the south floor.

Plate 4 Plate 4

I would urge interested members to observe Furnerius under a variety of lighting conditions to investigate its many subtle details, after all amateur astronomy is about pushing one's self and one's equipment to the limits, the excitement gleaned by observing last light on a large lunar feature cannot be given by a picture in a book or on a computer screen its just something you have to get outside and experience!

Nigel Longshaw


Bibliography

1. Journal of the association of lunar and planetary observers: vol.32 nos. 3-4 July 1987.
2. The New Moon (journal of B.A.A. lunar section topographical sub-sect vol. 4 no. 1 June 1989, no. 2 December 1989, no. 3 June 1990.
3. Cherrington: Exploring the Moon Through Binoculars and small Telescope, Dover publications.

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