The mean distance of Venus from the sun is 67,269,000 miles. Her orbit
more nearly approaches the form of a circle than that of any other
planet. It is, like the orbits of the other planets, an ellipse, but
of such small eccentricity that the difference between her greatest
and least distance from the sun is scarcely more than a million miles.
Light, traveling as it does, at the rate of a little more than one
hundred and eighty-six thousand miles a second, goes from the sun to
Venus in about six minutes. It takes something more than eight minutes
for light-rays to come from the sun to us. When Venus is nearest the
earth, her silvery beams come swiftly across to us in a little more
than two minutes. When she is farthest from us, the rays of light
require a few seconds more than fourteen minutes to travel over the
distance. She is, when at her greatest distance, more than one hundred
and thirty-five million miles farther from us than when at her nearest.
This difference is due not to any great eccentricity in her orbit, or
in that of the earth, such as causes Mercury’s great variations of
distance, but to the situation of the two bodies in their orbits: they
are nearest together when they are on the same side of the sun, and
farthest apart when on opposite sides.
Usually at inferior conjunction Venus is a little more than twenty-five
million miles from the earth. At her nearest possible approach to us,
however, which takes place at inferior conjunction, when the earth
is nearest the sun and Venus is farthest from it, a situation which
occurs only once or twice in a century, the distance between us and
the planet is only a little more than twenty-three million miles. This
is nearer than any other heavenly body ever approaches us, except the
moon and, so far as we now know, one small asteroid. Also, it is nearer
than Venus comes to any other heavenly body except perhaps Mercury.
Her nearest approach to that planet is also about twenty-three million
miles.
Unfortunately, our comparative proximity to this beautiful planet does
not much aid us in learning anything about her personal peculiarities.
Shining only by reflected light, and being, like Mercury, situated
nearer to the sun than the earth is, when she comes around to the
same side of the sun on which we are, her unillumined side is turned
toward us, and at the point of very closest approach she is absolutely
invisible to the naked eye. Through a telescope, however, she can be
seen up to the very point of inferior conjunction. What we see then is
a mere curved line of light, so thin is the crescent she presents; but
it is always apparent except when the planet makes a transit. During a
transit she is actually in our line of sight with the bright disc of
the sun, and is neither above nor below it, as at the ordinary times
of inferior conjunction. The slender crescent that we ordinarily see
offers a very narrow field for observation.
If there is any one on Venus who is studying the earth, he has a
much easier task than we have in our effort to learn something about
her. The earth is not only somewhat larger than the planet, but when
the two bodies are nearest together the disc of the earth is fully
illuminated, and so must show a splendid face; and then, our atmosphere
probably interferes less with close observation than that of Venus.
This little terrestrial system would undoubtedly shine as a magnificent
pair of stars if observed from Venus. At that distance our moon would
appear considerably larger than Venus appears to us when at superior
conjunction, the earth would seem much larger than Venus ever does to
us, and the distance between them would seem to be a little more than
the apparent diameter of the full moon as we see it. The light of the
earth must cause much more of a shadow than we ever get from the light
of Venus.
It has been suggested that light from the earth is responsible for a
dusky illumination of the dark side of Venus, which is occasionally
seen, and which enables us to distinguish her entire outline even when
only the merest line of a crescent is really illuminated. It is known
to be earth-shine that causes what is apparently the same phenomenon
often seen by us on the moon; but it seems that there is no reason to
think that our earth, at its distance, would be sufficiently brilliant
to illuminate Venus even so slightly. The cause of the illumination is
not known; but it is thought that it may have some electrical origin,
probably similar to that of our aurora.
Venus has the same phases that Mercury has. She shows her full face
when at superior conjunction, and is then farthest away and smallest
to our view. As she moves toward us she first becomes gibbous, and
then, at eastern elongation, like a half-moon. As she comes nearer to
inferior conjunction, and hence nearer to us, she becomes a thinner and
thinner crescent, and as she goes from inferior to superior conjunction
these phases are repeated in reverse order. We see less than half of
her face when she is at her greatest brilliancy, a phase which usually
occurs when she is about forty degrees from the sun, as she is a few
weeks before and after inferior conjunction. A very small glass will
show the phases of Venus. They have occasionally been seen without
artificial aid to vision by an exceptionally good eye. They were
not known, however, until they were discovered by Galileo after the
invention of the telescope in 1610.
Venus would be many times brighter than she ever appears if the
entire disc of the planet could be seen when it is nearest to us. The
apparent diameter of the disc at that time is nearly seven times larger
than when we see it at the planet’s greatest distance from us. When
Venus is in superior conjunction and farthest from the earth the disc
measures only ten seconds, while at inferior conjunction its measure
is nearly sixty-seven seconds. The diameter of the moon is about 1,868
seconds, so one could string across the diameter of the moon one
hundred and eighty-six such planets as Venus appears to be when at her
smallest, and only twenty-seven of the size that she appears to be
when at her largest. Between these two extremes of size she changes
gradually, day by day, from large to small and small to large, in
ceaseless succession, as she approaches the earth and recedes from it
in her orbital journey. Apparent diameter is determined by an actual
measurement of the disc of a planet, and in the case of Venus indicates
nothing as to brightness. When the apparent diameter is largest she is
not visible to the naked eye.
[Illustration: THE LOVELY CRESCENT THAT VENUS SHOWS WHEN TO OUR VIEW
SHE IS AT HER GREATEST BRILLIANCY
This remarkable photograph was made at the Yerkes Observatory by E. E.
Barnard.]
[Illustration: RELATIVE APPARENT SIZE OF VENUS AT DIFFERENT PHASES OF
ILLUMINATION
She shows the full disc when farthest away. As she draws nearer she
shows first the half moon and then the smaller crescent. She is nearest
when she shows the larger crescent. She is brightest, though, when she
shows the smaller crescent.]