Hermograph Press presents...

The Planet Mercury

Drawing the Right Surface Maps of Mercury The following is 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 has never been an easy planet to observe. It never appears further from the sun than 28^o, hence it is virtually never observed in a dark sky. Although it is less difficult to locate in the sky than commonly assumed, it is always observed low in the sky and is greatly affected by air turbulence. The surface markings then have a low contrast to the sky background and are visible only at an observer's threshold of vision.

In addition, Mercury elongations vary in favorability. Indeed, for mid-northern latitudes, spring evening (eastern) elongations are the most favorable, with respect to the maximum angular distance from the sun, length of visibility (usually less than about 20 days) and height above the horizon. The same is true for the autumn morning (western) elongations; the entire situation is reversed for the southern hemisphere. The primary factor controlling favorability is the inclination of the ecliptic to the horizon.

The earliest observations of Mercurian detail were those of cusp truncation by Schroter, in 1800. Occasional truncations were observed by others but little surface detail was recorded until the 1880's, by Denning in England and Schiaparelli at Milan. Schiaparelli published no individual drawings to this writer's knowledge but did publish the first planisphere (single hemisphere) of Mercury's surface (1889). He believed the planet to rotate synchronously with a period equal to its orbital period, namely 87.969 days. This opinion was not at first universally accepted. Schroter's assistant, Harding, and the astronomer Prince concluded that the Mercurian day was approximately that of the Earth's (Sandner, 1963). Leo Brenner (1896) concluded that it was actually 33 to 35 hours. These opinions lapsed after various observers would observe a feature as motionless over a period of several hours (of date-time observing)(see, for example, McEwen, 1909).

The first third of this century found the observations of Mercury confined principally to three groups:

The English, as reported by Henry McEwen for the British Astronomical Association (BAA),
the French, principally at Meudon and the Jarry-Desloges Observatories,
and in America by Lowell.

The BAA created a Mercury observation section, which did, and still does, periodically publish drawings and analyses of observations. The observers were few in number and confined almost completely to observers in the British Isles, the principle exception being one Captain Molesworth in Ceylon. Approxmiately 50 percent of all observations from this time are to be found in the Journal of the British Astronomical Association (JBAA). The other 50 percent is to be found in Observatoires Jarry-Desloges: Observations des Surfaces Planetaries (1909-1946), perhaps the single largest and most important piece of amateur work in this century. The drawings of R. Jarry-Desloges, G. and V. Fournier, and P. Briault are among the finest ever made. In ten volumes, there are found 139 drawings of Mercury from 1909 to 1926. At the same time, at Meudon and at other sites in France, Danjon and others made occasional observations of the planet. This French research culminated in the work of E. M. Antoniadi, which is described below. During this period, the first quality photograph was taken by Quenisset (1931). The American contributions are meager. The drawings by Pickering (1892)( and Douglas) are the only useful ones. Percival Lowell made an extensive series of drawings (1898), which show linear markings like martian canali and resemble other planispheres only with the greatest stretch of the imagination. Lowell's work will be consider no further.

The most important work of the period is that of Antoniadi (1934). Working at Meudon, Antoniadi observed Mercury from 1924 to 1929, firmly established (as he thought) an 88--day rotation period, and gave nomenclature to the various bright and dark areas. Although not the first to do so, he had truly mastered the art of daylight observations. The drawings, being so excellent, if small in number, firmly entrenched various markings into Mercury observation lore, such as his famous "figure 5" (See Sandner, 1963, page 36), so much so that observational bias imitative of Antoniadi is quite strong in many later drawings.

Continue to Cartography, Part 2 | Return to Mercury in the Telescope | Return to All Things Mercury