t two hundred years have from very insufficient data
concluded that Venus rotated in about twenty-three hours. Schiaparelli,
of Milan, turned his attention to this planet in 1877 and noticed a dark
shade and two bright spots, all situated not far from the southern end
of the crescent. This most painstaking astronomer watched these
markings for three months, and found that there was no change
perceptible in the position which they occupied. This was particularly
the case when he continued his watch for some consecutive hours. This
fact seemed to show conclusively that Venus could not rotate in
twenty-three hours nor in any other short period. Week after week the
spots remained unaltered, until Schiaparelli felt convinced that his
observations could only be reconciled with a period of rotation between
six and nine months. He naturally concluded that the period was 225
days--that is to say, the period which Venus takes to complete one
revolution round the sun; in other words, Venus always turns the same
face to the sun.

This remarkable result was confirmed by observations made at Nice; but
it has been vigorously assailed by several observers, who maintain that
their own drawings can only agree with a period about equal to that of
the rotation of our own earth. Schiaparelli's result is, however, well
supported by the letters of Mr. Lowell. He has published a number of
drawings of Venus made with his 24-inch refractor, and he finds that the
rotation is performed in the same time as the planet's orbital
revolution, the axis of rotation being perpendicular to the plane of the
orbit. The markings seen by Mr. Lowell were long and streaky, and they
were always visible whenever his own atmospheric conditions were fairly
good.

We have seen that the moon revolves so as to keep the same face always
turned towards the earth. We have now seen that the planets Venus and
Mercury each appear to revolve in such a way that they keep the same
face towards the sun. All these phenomena are of profound interest in
the higher departments of astronomical research. They are not mere
coincidences. They arise from the operation of the tides, in a manner
that will be explained in a later chapter.

It happens that our earth and Venus are very nearly equal in bulk. The
difference is hardly perceptible, but the earth has a diameter a few
miles greater than that of Venus. There are indications of the existence
of an atmosphere around Venus, and the