Mars Through The Telescope Early Telescopic Observations of Mars

The telescope, invented in 1608 and developed as an astronomical instrument by Galileo Galilei in Italy, in the following year, provided support for the Copernican system. If the planets did indeed move in orbits about the Sun, then Venus, which is between Earth and the Sun, should show distinct crescent, gibbous, and full phases like the Moon. Because these phases could not be seen, at least by the unaided eye, it was argued that Venus does not travel around the Sun. When Galileo turned his telescope toward Venus, however, he was able to observe the different phases, eliminating the objection to the view that Venus orbits about the Sun.

The apparent shape of Mars was- also expected to exhibit changes, but because this planet is farther from the Sun than Earth, such changes are relatively minor. Galileo looked for them and at the end of 1610 he wrote in a letter to a friend: "I dare not affirm that I was able to observe the phases of Mars; nevertheless, if I am not mistaken, I believe I have seen that it [Mars] is not perfectly round."

The earliest known drawing of Mars as seen in the telescope is shown in figure 2.10; it was made by Francisco Fontana in Italy in 1636. He said that "the disk of Mars is not uniform in color (discolor), but it appears fiery in the concave [sic] part." The nonuniform appearance of the surface may have been caused by details that Fontana was unable to resolve. The sketch is of historical interest only, and must not be taken seriously. The dark spot in the center, for example, was apparently caused by a fault in Fontana's telescope, as a similar spot appears in his drawing of the planet Venus. Furthermore, the ring near the edge of the disk has no counterpart in later telescopic observations of Mars.

FIGURE 2.10. Earliest known drawing of Mars; made by Fontana in 1636. (From C. Flammarion, "La Planète Mars," Vol. 1.)

Fontana's drawings are of interest, however, because one made in August 1638, at the next apparition of Mars, shows a definite gibbous phase (fig. 2.11), as required by the Copernican (and Keplerian) theory. The dark central spot and the outer ring are, however, still present. Actually, the departure from the circular shape in the sketch is somewhat greater than it should be, but there seems little doubt that Fontana did observe and record the expected change in the shape of the visible part of the Martian disk.

In 1659, the Dutch mathematician and physicist Christian Huygens, who had improved the telescope by the use of better lenses, made the first recognizable drawing of Mars (fig. 2.12). It shows a conspicuous dark, triangular area, which was at one time called the Hourglass Sea (Mer du Sablier), since it looks like half of an hourglass. It is now known as Syrtis Major (fig. 2.21).

Before proceeding further with a review of the early studies of Mars, a short digression is necessary. The image of a celestial object as seen in a telescope is always inverted. It has been the universal practice among astron-

FIGURE 2.11. Fontana's drawing showing gibbous phase of Mars. (From C. Flammarion, "La Planète Mars," Vol 1.)

FIGURE 2.12. Earliest known drawing of Mars showing identifiable surface feature; made by

Huygens in 1659. (From C. Flammarion, "La

FIGURE 2.11. Fontana's drawing showing gibbous phase of Mars. (From C. Flammarion, "La Planète Mars," Vol 1.)

FIGURE 2.12. Earliest known drawing of Mars showing identifiable surface feature; made by

Huygens in 1659. (From C. Flammarion, "La

omers since the early 17th century to show sketches and photographs of such images in the way they are seen in the telescope; that is, in the inverted form. Such is the case in Huygens' drawing in figure 2.12, and also in nearly all the sketches and pictures in this book. The south of all illustrations of Mars is at the top and north at the bottom! The current practice is to refer to the right as west and the left as east, but this has apparently not always been so (fig. 2.13).

Because he could observe a particular distinctive feature on the Martian surface, Huygens was able to show that the planet rotates, like Earth, about a north-south axis. He found the period of rotation, that is, the length of a day on Mars, to be close to 24 terrestrial hours. Some 7 years later, in 1666, the Italian astronomer Giovanni D. Cassini, who is said to have discovered the rotation independently, made more precise observations and found the length of the Martian day to be 24 hours 40 minutes. This is about 2 J/2 minutes longer than the value accepted today (p. 64).

Cassini is also supposed to have observed the polar caps of Mars, as indicated by the sketch in figure 2.13, where the letter M (Latin, meridies) stands for south and S (septentrio) for north. The drawing also shows white (or light-colored) areas on the equator, which may represent some of the bright regions on the Martian surface. According to the Greek-born French astronomer, Eugenios M. Antoniadi, the upper and lower areas of the dark dumbbell-shaped feature which crosses the equator are the regions now known as Sinus Meridianii and Mare Acidalium; the line joining them is said to be Gehon, a so-called canal, but this seems questionable.

A more convincing indication of the south polar cap is seen in the somewhat crude sketch made by Huygens in 1672. This drawing (fig. 2.14) also shows the triangular Syrtis Major. A somewhat similar representation was made in 1719 by Cassini's nephew Giacomo F. Maraldi; he drew attention, to the fact that the center of the cap did not coincide exactly with the apparent Martian pole. It may be noted that Earth's arctic and antarctic icecaps are also not centered on the poles. Maraldi reported that the appearance of the Martian surface changed while he was observing the planet in the telescope. W. Ley has suggested that this may have been caused by one of the occasional duststorms that occur on the planet (ch. VII).

Extensive reports with several sketches, based on observations of Mars made in 1777

Prima Marris iacies

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