Mars is a highly interesting subject for study, but it is also a difficult one. Both Earth and Mars orbit about the Sun, but Earth travels at a faster speed in a smaller orbit. Consequently, the distance between Earth and Mars is always changing. At intervals of about 26 months, however, the two planets come fairly close to each other, and these occasions, which are called apparitions by astronomers, provide the opportunity for studying Mars at moderately close range. The opportunities are not all equally good because the distance between Mars and Earth, even at closest approach, is not always the same.
At best, Mars can be observed for a period of a few months every 2 years or so. On the most favorable occasions, when the planet is closest to Earth, it is best observed from Earth's Southern Hemisphere. But the majority of astronomers and the most suitable telescopes are in the Northern Hemisphere. Another restriction is that at certain apparitions the north pole of Mars is directed toward Earth and then the south polar regions cannot be seen; in other years the situation is reversed. At some apparitions, however, essentially the whole surface of the planet is visible.
The rotation of Mars about its axis introduces another problem because there is a continuous change in the image seen in the telescope. The American astronomer R. S. Richardson, who has spent many years observing Mars, says that a sketch should be completed "in no more than 15 minutes,"
otherwise the surface features are "scrambled up" owing to the distortion of distances. It is true that, as a result of the rotation, the whole surface of Mars can be seen. But because the planet is visible for only a few hours at a time, a limited portion can be viewed each night.
Furthermore, because Mars takes but a little (37 minutes) over 24 hours to make a complete rotation, the parts of the planet that are visible to an observer on Earth do not change very much from one night to the next. As a general rule, about 5 to 6 weeks are required for the complete circumference of Mars to be seen. During this period, the distance between Earth and the planet will have changed significantly, and so also will its apparent size.
It should be borne in mind, too, that Mars is, at best, a "difficult" astronomical object. For example, in 1906, the Harvard astronomer William H. Pickering wrote : "To understand how the planet appears through a large telescope, we may examine the Moon some night through a small opera glass. The sharpness and amount of detail visible in the two cases will be similar. . . ."
Up to a point, the ability to discern details of the Martian surface can be improved by an increase in the aperture of the telescope, up to about 36 inches. The useful limit in this respect is determined by the condition of Earth's atmosphere. This is perhaps the most important aspect of the telescopic observation of Mars. "If the atmosphere is disturbed ..." writes R. S. Richardson, "the image of Mars in even the best telescope will be blurred and shaky. ..." The occasions of what the astronomers call good "seeing" are rare and unpredictable, and it is only when the seeing is good that there is any possibility of observing details of the Martian surface. Observatories are frequently located at high altitudes to improve seeing conditions.
Because of the limitation set by seeing, telescopes with very large apertures do not show any greater detail of the Martian surface than do smaller instruments. According to R. S. Richardson, the planet "looks as if all the color has been washed out of it" when viewed with the full aperture of the 100-inch reflector telescope of the Mount Wilson Observatory, California. "The sharpness of the image," he goes on to say, "can be considerably improved by diaphragming down, or partially covering, the secondary mirror."
Although telescopic photographs of Mars are commonly taken for purposes of record, it is recognized by astronomers that the eye can detect more detail than can be captured on film. This is one reason that photographs do not show any obvious linear features like the canals. Nevertheless, photographs are of great value in providing accurate locations of specific areas; the details can then be partially filled in by visual observation. In this way, fairly accurate maps have been prepared showing the principal features of the surface of Mars. A recent example of such a map is given in figure 2.21 at the end of chapter II.
Besides the telescope, other instruments have been used to obtain information concerning Mars. Since about the middle of the 19th century, the spectroscope has been added to the telescope to analyze the spectrum of the Martian atmosphere. As a consequence of the great distance of the planet, however, even at closest approach, and because the light from Mars must pass through the terrestrial atmosphere, most of the results have been unreliable. It is only since about 1949 that any useful data have been obtained. Other optical techniques, such as a study of the polarization and spectrum of the light reflected from the Martian surface, have also been used, although the information they provide is limited. In recent years, radar methods have been applied in the study of Mars, but the interpretation of the results may be open to question.
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