THE PLANET MARS.

from west to east. The apparent motion from east to westcaused by the rotation of Mars being somewhat greater thanthis, it does not appear absolutely to stand still, as it would ifits period were exactly 24 hours 37 minutes ; but it has acomparatively slow movement from east to west—in fact, soslow, that about 1314 terrestrial hours are occupied in itsapparent circuit once round the Martial sky.

It may therefore be quite possible for the two, on certainoccasions, to be seen approaching each other from oppositedirections, until one may pass in front of the other and eclipseit; or, as would more often be the case, the one might passslightly above or below the other.

We cannot now repeat the calculations with regard tovarious other points of interest connected with these satellites,which we made in a course of lectures delivered at GreshamCollege shortly after their discovery. It may suffice to men-tion, that we found that Phobos altogether escapes eclipseabout once out of every twice that it is in the positionof Full Moon , owing to the tilt of its orbit to the plane ofthat of Mars, while Deimos does so four times out of everyfive. We showed that the maximum duration of an eclipseof Phobos is about 53 minutes, and of one of Deimos about 84minutes. We also stated that, owing to its slow apparentmotion across the Martial sky, Deimos may, in the intervalof about 66 hours between its rising and setting, pass threetimes through the phase of Full Moon , and thrice suffer eclipse.

An example or two of what might occur in the course of asingle night may, perhaps, not be uninteresting. Supposingsunset to take place near to such an hour as upon the Earth we should call 6 p.m., Phobos might have risen a short timepreviously in the west. After the lapse of about 3f hours, ifwe suppose the observer to be at the time in question at a placeupon the equator of Mars , and the satellites to be apparentlymoving nearly in the equator of the heavens, the Sun wouldbe about 55° below the western horizon, and Phobos about55° above its eastern point; and Phobos might now be inthe middle of an eclipse, which (having commenced at abouta quarter past 9 p.m.) might, under favourable circumstances,