1996/97 Mars Sketch (13)
from CMO #210 (25 December 1998)

-- HUYGENS versus DOLLFUS --

Japanese here


t may look extraordinary to cite a Mars drawing from another kind of journal, but we here refer to the April-May 1998 issue of l'Astronomie published by la Société Astronomique de France (SAF) and cite an interesting drawing made by Audouin DOLLFUS on 17 April 1997. This drawing was obtained by the use of a newly reconstructed astroscope à la HUYGENS.
  In April 1995, a congress of the SAF decided to build a telescope without tube in almost the same way as Christiaan HUYGENS (1629 - 1695) did three hundred years before from a view point of historical, scientific and educational interest. In addition to A DOLLFUS, such persons as P BACCHUS, F BIRAUD, R BOTTARD,
B DAVERSIN, G FARRONI, J FORT, J-M LECLEIRE, P MOATHY, G PHILIPPON, and A THIOT joined this project. The issue is a special number having feature articles on Ch HUYGENS: DOLLFUS himself wrote at least two articles, one was on the Huygens brothers and some long refractors without tube, and the other on a report of the practical use of the reconstructed astroscope.


(Fig 1) From the rear cover of l'Astronomie avril-mai 1998

  We try to cite some figures from the Journal, and hope the copies will be helpful to understand the system of the astroscope. Roughly speaking, the astroscope first needs a tall pole or a mast (like the one used on a Holland ship) on which an objective lens is put. Then it is connected by a cord to the mount and the eyepiece (Fig 1). The lens is balanced and pulled by the cord to face to the eyepiece part (just like by the use of a means to pull the strings of a lute). Problem is that if a star is caught, it will soon move out from the eye-field because of a diurnal motion. Hence a lozenge keeps the star longer: The plan of the lozenge is originally à la HUYGENS, but here this seems to have been designed by Prof DOLLFUS (Fig 2). The feet must play a delicate role. Originally, the light axis must have been checked by the use of a flame of a candle. As easily supposed, the system, mast and string, must have been weak against the wind.
  The objective glass was polished this time by LECLEIRE. Originally HUYGENS' objective had an aperture 4.6
pouces=11.6cm, but the present one 13cm. The focal length was originally 34 pieds=10.5m, while the lens produced here had a 7m. The eyepiece was also constructed: its focal length was 91.5mm (with the aperture 89mm; big!) so that one can obtain a magnification of 76. In DOLLFUS' case, however, he describes the astroscope of 10cm, and uses higher magnifications, and hence the eyepieces must be various including Plössl.
  Since the focal length is 7m, the mast must be over 8m, and so they needed a wider place to put the astroscope. It was eventually settled in a Star Park of the Triel Observatory, Trier-sur-Seine, Yvelines (westward of Paris). For example, the astroscope was open to the public when a partial eclipse occurred on 12 Oct 1996, and about 300 persons gathered.

(Fig 2) Lozenge à la HUYGENS designed by Prof DOLLFUS


  The Mars Sketch here by DOLLFUS was made on 17 April 1997 22:00 TU (106°Ls). Used 233×10cm Astroscope (Fig 3). The planet was going away and the apparent diameter was 12.7 arcsecs. The phase angle was 23.5 degrees, and DOLLFUS detected a defect of illumination. The LCM was 025°W, and M Acidalium was evident. The other dark marking at upper left must includes Aurora S. The season was 106°Ls, and hence the area of the north polar cap (2"×1" in angle) did not well show up, just light obscure.

  Professor DOLLFUS also made a sketch of Mars by 100×10cm, and interestingly compares it with a drawing of Ganymede (of 1.2" arcsecs) anotherly made by the use of 1000×100cm at the Pic du Midi. Apart from the contrast, the images are quite similar.

left : (Fig 3) DOLLFUS' Drawing on 17 Apr 1997 at LMC=025°W
right : (Fig 4) HUYGENS' Drawing on 7 Apr 1683

  The drawing by DOLLFUS reminds us of a sketch by Ch HUYGENS made in 1683 (Fig 4). HUYGENS obtained the drawing cited here on 7 April 1683 at 9.5h by the use of a 36 pieds refractor. The refractor used by him was not any astroscope without tube, because the idea of the latter was introduced after August 1683. Although F TERBY made a different interpretation, we may say that HUYGENS also saw M Acidalium if we compare both drawings.

  As detailed in CMO #106 p909 by the present writer, the year difference 284 is one of the best recurrence year to produce a similar Mars season as well as the apparent diameter (that is, both planets meet at the similar points on their orbits). Since 1683+284=1967, HUYGENS' Mars was similar to the planet in 1967. In 1967, Mars was at opposition on 15 April with the season 120°Ls, and so the north polar cap must have been smaller than that in the case of DOLLFUS. This implies in turn that it was hard for HUYGENS to detect the npc also. (We add that the 1999 Mars is in another sense a returned planet of the 1683 Mars since 4×79+1683=1999, where 79 is another -not best- recurrence year.)

  It is described by DOLLFUS that it was because the diameter of the south polar cap was of the dimension of 6 arcsecs that HUYGENS did discover the polar cap on 13 August 1672. Yes, 1672+284=1956, and the 1956 Mars showed us the south polar cap clearly as well as the great dust storm. At another page, Professor DOLLFUS compares his detailed drawing of Mars in 1956 with the famous one by HUYGENS made in 1659 (made on 28 Nov 1659) both of which describe Syrtis Major and alludes to the decisive progress made during the 300 years. Prof. DOLLFUS is right, but one thing to be noticed is that there is a natural reason why HUYGENS did not write the polar cap in 1659, and in this point HUYGENS' 1659 drawing should not be compared with any drawing in 1956. The reason is simple:1659+284=1943 and hence HUYGENS' Mars corresponds to the 1943 Mars if we refer to the planet at hand. Mars was closest to the Earth on 29 Nov 1943! when the season was 345°Ls and φ(=DE)=5°S, and therefore we can say in 1659 nobody could have detected the south polar cap if a superior telescope could have been used.

  N.B.: We should incidentally note that at p 444 of the April 1992 issue of the Sky & Telescope, Alan BINDER wrote about his experience on an application of his 7.5 cm Hevelius-type telescope to the planet Mars in 1988. The south polar cap was detected as well as some dark markings by the 5.2-metre-long Hevelius. (Hevelius himself used a 45-metre-long telescope.)

(Mn : Masatsugu MINAMI)

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