THE PLANET MAES.

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disadvantage is far more than compensated by the fact thatMars , being in Opposition, is at the same time in its very bestposition for observation ; that is to say, it is then seen uponthe meridian at midnight at its greatest altitude in theheavens, and it is high above the horizon during all the darkhours.

What then, let us ask, may be observed at such a time in apowerful telescope ? Certain darker markings or shadingsupon the surface of the planet are at once noticed ; which,however, occasionally appear to be of a slightly greenish hue,in contradistinction to the ruddier tint of the surrounding-parts. These markings are found to be constant, or nearlyconstant, in form, although subject at times to some amountof temporary obscuration. As observation has become morepainstaking, and telescopes have been improved, it has beenfound possible to recognize an increasing amount of detail, andto construct more or less accurate maps of the surface of theplanet. In these maps the darker markings are named oceans,seas, gulfs, bays, or inlets, on the supposition that they areportions of water; while the intermediate regions are termedcapes, continents, or islands.

Each portion of the surface is, of course, best seen when it isturned directly towards an observer, or nearly so ; i.e., when itis on, or nearly on, the centre of the planet’s apparent disc.At other times it appears foreshortened, and may be so fardistorted in its apparent shape, that it is difficult even for apractised observer to recognize it.

An approximate value of the rotation-period of Mars uponits axis may be very easily obtained by noticing the intervalwhich any small and decided mark upon the disc, or the centralpoint of any larger one, occupies in passing once round, beforeit returns to a similar position again. For a more accuratedetermination it is well to take a much longer interval, inwhich a large number of revolutions have occurred, and todivide the interval by that number; a procedure which propor-tionally lessens the effect of any slight errors of observation.For this purpose the ancient drawings of the marks seenupon the planet by the celebrated Professor of Geometry in