Hermograph Press presents...

The Planet Mercury

Drawing the Right Surface Maps of Mercury, Part II The following continues a revised excerpt from Larry Krumenaker's (c) 1976 MS Thesis in Astronomy at Case Western Reserve University, A Computer Analysis of Visual Observations of Mercury.

Mercury observation became sporadic after Antoniadi's work. Only a rather well-observed elongation in 1936 (McEwen, 1936) was reported in the 1930's plus a few French contributions (Danjon 1937, Quenisset 1931). In 1942 and 1944 Lyot and Camichel at Pic du Midi Observatory observed Mercury and made the first good photographic series of observations (see Dollfus, 1953). In 1950, Dollfus also obtained a very fine series of drawings (1953) using filters for increasing the visibility of features. The French efforts effectively ended, in terms of published material, at this time and the JBAA has published a scant nine observations for the 1950's and 1960's.

Most of the post-World War II drawings are to be found in the American journal, The Strolling Astronomer, of the Association of the Lunar and Planetary Observers. Created by Walter Haas, who originally published in the JBAA, this journal also has a Mercury section, which publishes selected drawings for each year. Unfortunately, many, if not most, of the drawings are inaccurate and unusable.

A short discussion of the Mercury observations is appropriate here. As noted previously, the markings are at the limit of visual perception, due somewhat to their own soft nature but more to the observing constraints. The personal equations (personal styles and tendancies) of the observers are quite significant. Often, general features agree between observers observing the same apparition but the detail and placement of the markings often show little, if any, agreement (see, for example, the drawings of the Fournier brothers (Jarry-Desloges, 1910)). Once the 88-day rotation period was "confirmed" by Antoniadi, it became rather easy for an observer to "see" Antoniadi's markings in the fleeting images of steady seeing.

Still, discrepancies were reported from time to time. Major dark areas disappeared, bright areas of the disk and limb would appear, even polar caps were recorded on both poles. Some Mercury observers, especially Haas (1947), ascribed these occurences to clouds and haze in a Mercurian atmosphere. Most of such phenomena may be ascribed to the personal equation of the observer (where or not he was Antoniadi-biased), others imply poor observing and/or poor observer. Some short-lived phenomena have been ascribed possibly to luminescence (Cruikshank 1966), but the aforementioned factors may still play a role. Any apparent motion of the features was considered as librations, a rocking of about 23 degrees amplitude in longitude and about seven degrees in latitude. But the major cause of discrepancies is the true nonsynchronous rotation of Mercury, so that observers were really viewing more than just the one hemisphere they thought they were seeing!

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