the earth. It should here be noted that the
telescope was not invented until nearly seventy years after the death of
Copernicus.

The apparent swing of an inferior planet from side to side of the sun,
at one time on the east side, then passing into and lost in the sun's
rays to appear once more on the west side, is the explanation of what is
meant when we speak of an _evening_ or a _morning star_. An inferior
planet is called an evening star when it is at its eastern elongation,
that is to say, on the left-hand of the sun; for, being then on the
eastern side, it will set after the sun sets, as both sink in their turn
below the western horizon at the close of day. Similarly, when such a
planet is at its western elongation, that is to say, to the right-hand
of the sun, it will go in advance of him, and so will rise above the
eastern horizon before the sun rises, receiving therefore the
designation of morning star. In very early times, however, before any
definite ideas had been come to with regard to the celestial motions, it
was generally believed that the morning and evening stars were quite
distinct bodies. Thus Venus, when a morning star, was known to the
ancients under the name of Phosphorus, or Lucifer; whereas they called
it Hesperus when it was an evening star.

Since an inferior planet circulates between us and the sun, one would be
inclined to expect that such a body, each time it passed on the side
nearest to the earth, should be seen as a black spot against the bright
solar disc. Now this would most certainly be the case were the orbit of
an inferior planet in the same plane with the orbit of the earth. But we
have already seen how the orbits in the solar system, whether those of
planets or of satellites, are by no means in the one plane; and that it
is for this very reason that the moon is able to pass time after time in
the direction of the sun, at the epoch known as new moon, and yet not to
eclipse him save after the lapse of several such passages. Transits,
then, as the passages of an inferior planet across the sun's disc are
called, take place, for the same reason, only after certain regular
lapses of time; and, as regards the circumstances of their occurrence,
are on a par with eclipses of the sun. The latter, however, happen much
more frequently, because the moon passes in the neighbourhood of the
sun, roughly speaking, once a month, whereas Venus comes to each
inferior conjunction at intervals so lon