Abstract
All the observations brought together in this work show that the globe of Mars is characterized by a diversity of sombre patches and bright patches fixed to the surface. Over two centuries we have had concordant results. Mars is the only planet in the Solar System whose geography can be studied. Venus, Jupiter and Saturn are constantly enveloped in clouds, and on the other planets we can see nothing with real certainty.
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Notes
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L’Astronomie, January 1889, p. 20.
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Requirements for the Observation of Mars.—While the weather on Mars is usually fair, and its clear atmosphere presents few obstacles to the observation of the surface, the differences in tone are often slight, and the outlines of the features so vague and uncertain—even after hours of exceptionally good seeing—that satisfactory results can only be obtained with very rigorous methods of observation.
The first point, naturally, is to have a good objective—or a good mirror, for a reflector. The size of the instrument is, relatively, of secondary importance. Excellent images can be obtained with small refractors of 108mm, 95mm and even 75mm aperture, while large reflectors of a metre and more in aperture can sometimes give only mediocre views, making the features almost impossible to identify. Therefore, if an instrument is to be useful, it must be really good.
The second point is to have the instrument at the same temperature as the surroundings. If the telescope is used in the open air, all is well. But with a telescope shut in a dome, the slit, windows and doors must be opened several hours before the start of observing. The waves of warm air which pass in front of the objective, and which increase in proportion to the power of the eyepieces used, constitutes a major obstacle to the clarity of the images.
Thirdly, it is important not to forget that when the first two conditions are fulfilled, the desired precision cannot be attained under our normal atmospheric conditions, because even when the air appears perfectly clear it is crossed by layers of heterogeneous density, which produce currents and disturb the view. Therefore we must wait, sometimes for several hours, for a fleeting moment of absolute calm and perfect seeing.
The atmosphere is most transparent during the hours before a thunderstorm.
Daylight hours, dawn and dusk, always seem to me to be better than the hours of night.
Also, to obtain the best views without being influenced by any preconceived ideas, it is important not to know the view of Mars which will be presented when observing begins; the greater one’s ignorance, the more one will see. It is not long before the features will be recognized. If observations are made on several consecutive days, one cannot help knowing what will be on view, no matter how much one tries to forget it. The best course is to think of nothing, and concentrate solely upon what can be seen. In this way, sketches or drawings will have their greatest possible value. If you calculate the longitude of the central meridian in advance, and have a globe or map of Mars in front of your eyes, you will see what you expect to see, and will lose the pleasure of making your own discoveries. However, it is by no means a bad idea to work out identifications immediately after you have completed an observation.
The eye will become accustomed to instrumental conditions, and experience will bring improved results. One’s first observations are not generally satisfactory. From day to day, you will see more. To make the best drawings of Mars, you must make the eye used to all aspects of the planet, over many days. Generally, one’s first drawings will be without value, and will resemble the primitive drawings made by the old observers.
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Sheehan, W. (2015). Geography of Mars, or Areography. In: Sheehan, W. (eds) Camille Flammarion's The Planet Mars. Astrophysics and Space Science Library, vol 409. Springer, Cham. https://doi.org/10.1007/978-3-319-09641-4_7
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