MABS AND HIS MOONS. 87 



junctions with the inner moon in the course of its lunar day. It is 

 likewise evident that the outer satellite will frequently be above the 

 horizon of Mars more than sixty hours, during which period six con- 

 junctions with the inner may occur. Moreover, as the outer moon will 

 go through its cycle of phases in a little more than thirty hours, all of 

 these changes may be accomplished while it is above the horizon of 

 the observer on the surface of Mars. 



The apparent diameter of Mars, as seen by an observer on the in- 

 ner satellite, would be no less than 41 '8°, or about seventy-eight and a 

 half times the apparent diameter of the sun as seen from the earth ; 

 and from the outer moon the diameter of Mars would subtend an angle 

 of 16"T°, or about SI'S times the apparent diameter of the sun as seen 

 by us. Of course the apparent areas of the disk of Mars, as seen 

 from his two satellites, would be in the ratio of the squares 6i these 

 numbers, that is, the apparent area of the disk of Mars, as seen from 

 his inner moon, would be 6,167, and from the outer 980 times the ap- 

 parent area of the solar disk, as seen from the earth. 



From the innermost satellite of Saturn, the diameter of the primary 

 would subtend an angle of 35*8° ; from the nearest satellite of Jupi- 

 ter, the diameter of that planet would subtend an angle of 18-6° ; and 

 from our moon the earth's diameter would subtend an angle of less 

 than 2°. 



Astronomers are, as yet, ignorant of the real magnitude of the 

 Martial satellites ; but, assuming each of them to be one hundred 

 miles in diameter, it is easy to calculate their apparent magnitudes as 

 seen by an observer on Mars.* The inner moon being 5,733 miles 

 distant from the center of Mars, would, when in the zenith of the ob- 

 server, be only 3,633 miles distant from the surface of the planet. 

 Hence it appears that, when this satellite is seen in the horizon of the 

 observer on the surface of Mars, its diameter would subtend an angle 

 of about 60', or nearly twice the apparent diameter which our moon 

 presents to us ; but, when it is in the zenith of the observer, it would 

 subtend an angle of 94-3', or more than three times the apparent 

 diameter presented by our moon. In other tei'ms, in rising from the 

 western horizon to the zenith, the apparent diameter of this moon 

 would be increased nearly in the ratio of two to three ; and, of course, 

 its apparent area would be augmented nearly in the ratio of four to 

 nine. 



* Professor E. C. Pickering, of the Harvard College Observatory, has attempted to 

 determine the real magnitude of the satellites of Mars, by comparing the intensity of 

 the light reflected from the primary with that reflected from each of his satellites. He is 

 thus led to estimate the diameter of tlie inner satellite to be about seven miles, and that 

 of the outer one to be about six miles ! (" Annual Report of the Director of Harvard 

 College Observatory," November, 1877, page 17.) It is very questionable whether esti- 

 mates, founded on photomctrical comparisons in which the relative reflecting powers of the 

 bodies compared are unknown, can inspire the confidence of astronomers in relation to 

 the accuracy of the deduced diameters. 



