THE MYSTERY OF MARS — WILKINS 237 



period which is less than 6 hours longer than that of Mars itself, and 

 to the Martians must seem to move very slowly across the sky. Deimos 

 rises in the east and sets in the west, in the same manner as the sun 

 and the stars. As seen from Mars it would look like a bright star; 

 its disk would be rather difficult to detect with the naked eye. 



Both these moons are totally eclipsed at every "full moon," and 

 also they frequently eclipse the sun. The solar eclipses are never total, 

 for even the nearer and larger moon, Phobos, can only appear about 

 a third of the diameter of the sun. There are no total eclipses of 

 the sun for the Martians; they are either partial or annular. The 

 shadow cones which the moons cast from the side turned away from 

 the sun never reach the surface of Mars. There are also eclipses of 

 Deimos by Phobos, but the latter moon moves so quickly that it only 

 eclipses the sun or occults Deimos for a short time, in both cases 

 measured in seconds. As luminaries they must cut a poor show, for 

 the total amount of light they afford during the night is far inferior 

 to that afforded by our single moon. 



Owing to their small dimensions and their proximity to Mars, the 

 moons are difficult telescopic objects. The period of visibility is a 

 couple of months either side of the date of opposition, amounting to 

 three or four months at intervals of two years. Even the largest 

 telescopes fail to show any detail on their tiny disks; indeed, it 

 requires considerable attention to make out their disks and to dis- 

 tinguish them from stars. 



In 1952 Mars was farther from the earth than in 1954, but higher 

 in the northern sky, and thus better placed for observers in Britain. 

 On May 17 the writer and Patrick Moore, observing together with 

 the writer's lS^-inch reflector, saw Deimos clearly for more than half 

 an hour, following it until it had drawn so close to Mars as to be 

 lost in the glare. Moore has also caught Phobos with certainty with 

 his 12!/2-inch reflector, describing it on April 28, 1952, as "unmistakably 

 seen, but excessively faint even with Mars outside the telescopic field ; 

 on the very limit of visibility with this instrument, at least to my 

 eyes." 



These moons add to the mystery of the red planet. Perhaps they 

 were two of the hundreds of minor planets which revolve around the 

 sun between Mars and Jupiter, captured by Mars during a close ap- 

 proach in the past. They are mere lumps of rock; we are not even 

 sure that they are globes, and it is not impossible that they may have 

 a somewhat irregular shape, as some of the minor planets seem to 

 have. Eros is an example of a small body, about the size of the 

 moons of Mars and perhaps slightly larger, which comes quite near 

 to the earth at certain times and varies greatly in brightness. It has 

 been suggested that this may be caused by Eros being a rough and 

 practically shapeless rocky mass. 



