VENUS, in astronomy, the second of the major planets in
the order of distance from the sun, and moving next within the
orbit of the earth. Its symbol is 
. At inferior conjunction it
approaches nearer to the earth than any other major planet, but
in that position it is practically invisible. Its apparent motion
may be described as an oscillation from one side of the sun to the
other, the complete period of which is 1.6 years, and the greatest
elongation about 45° on each side of the sun. When east of the
latter it appears as the “evening star” in the west after sunset,
while near western elongation it is seen as the “morning star”
before sunrise. In these aspects it was known to the ancients
as Ἕσπερος, Hesperus, and Ἑωσφόρος or Φωσφόρος, Phosphorus.
The eccentricity of its orbit is smaller than that of any other
planet except Neptune.
Notwithstanding the near approach of Venus to the earth,
its situation relative to the sun is unfavourable to the study of its
physical constitution. Near inferior conjunction only a narrow
crescent of light is visible; and when, as the planet moves away,
this crescent becomes broader, the distance of the planet constantly
increases. When it appears as a half-moon it is at a
distance of more than two-thirds that of the sun, and nearly
double the distance of Mars in opposition. The difficulty of
reaching any conclusion on the subject of its constitution is
heightened by the seeming absence of any well-marked features
on the visible part of its brilliant surface. In the telescope it
presents much the appearance of burnished silver, without spot
or blemish. It is true that observers have from time to time
thought they could detect slight variations of shade indicating
an axial rotation. As far back as 1667 G. D. Cassini thought he
Rotation
of Venus.saw a bright spot near the southern horn, observations
of which gave a period of about 23 hours. In 1726
Francesco Bianchini (1662–1729), a papal chamberlain,
made similar observations from which he inferred a period of
more than 24 days. It was shown, however, that the observations
of Bianchini could be reconciled with those of Cassini by
supposing that, as he observed the planet night after night, it
had made one rotation and a little more. J. H. Schroeter also
found a revolution of less than 24 hours. But Sir W. Herschel,
as in the case of Mercury, was never able to detect any changes
from which a period of rotation could be determined. During
the years 1888–1890, G. Schiaparelli made an exhaustive study of
the whole subject, the results of which were summed up in five
brief notes, read to the Lombardian Academy of Sciences during
the year 1890. His general conclusion was that Venus always
presents the same face to the sun, as the moon does to the earth.
The same result has been reached by the observations at the
Lowell Observatory. The inference that the axial rotation is at
least much slower than that of the earth is strengthened by the
measures of different diameters of the planet made while it was
in transit across the disk of the sun in 1874 and 1882. These
show no measurable ellipticity of the disk, but they are not
sufficiently accurate to lead to any more precise conclusion than
that just stated. Still, the difficulty and uncertainty attending
all observations hitherto made upon the disk are such that no
conclusion respecting the time of rotation can be regarded as
established. Against the view of Schiaparelli is to be set the
great improbability that a body so distant from the sun as Venus
could be permanently so acted upon as to keep its axial rotation
in precise coincidence with its orbital motion. Only one way
seems to be open for settling the question; this is by spectroscopic
observations of the displacement of the spectral lines at
the two limbs of the planet. Attempts by this method have been
made by A. A. Belopolski at Pulkova, and by the astronomers of
the Lowell Observatory. It is, however, found that the amount
of displacement is so small that it has evaded certain detection
up to the present time. Belopolski’s measures were decidedly
in favour of an axial rotation, while the Lowell results were not.
Other observations than those we have cited show that Venus is surrounded by an atmosphere so filled with clouds that it is doubtful whether any view of the solid body of the planet can ever be obtained. The first evidence in favour of an atmosphere was found in the factAtmosphere of Venus. that, when near inferior conjunction, the visible outline of the thin crescent extended through more than 180°. Most remarkable was an observation by Chester Smith Lyman at New Haven during the conjunction of 1866, when the planet was just without the sun. A thin line of light was supposed to be seen all round the limb of the planet most distant from the sun. But as no such appearance was seen during the approach of the planet to the sun at the transits of 1874 and 1882, when the conditions were much more favourable, it seems likely that such observations are the result of an optical illusion. During the latter of the two transits the phenomena of this class observed were of an unexpected character. Not a trace of the planet could be seen until it began to impinge upon the solar disk. When about half of its diameter had entered upon the sun the outline outside the disk of the sun began to be marked by broken portions of an arc of light. This did not begin at the point A (fig. 1) farthest from the sun, as it should have done if due wholly to refraction, but immediately at the sun itself, as shown in the cut at the point B. Portions of this arc were formed one by one at various other points of the dotted outline, and when the planet was about three-fourths upon the sun the arc was completed. But there was no strengthening of the line at the middle point, as there should have been if due to refraction. Yet refraction must have played some part in the phenomenon, because otherwise no illumination could have been visible under the circumstances. The most satisfactory explanation seems to be that of H. N. Russell, whose conclusion is that the atmosphere is so permeated with fine particles of vapour up to its outer limit as to be only translucent without being fully transparent. Thus what is seen is the irregular reflection of the light at an extremely small angle from the particles of vapour.
Fig. 1.
If the orbit of Venus lay in the plane of the ecliptic, it would be
seen to pass over the disk of the sun at every inferior conjunction.But the inclination of the orbit, 3° 36′, is so large that a
transit is seen only when the earth and Venus pass a node of
the orbit at nearly the same time. The earth passes the
Transits
of Venus.
line of nodes about the 7th of June and the 7th of December of each year. The date of passage is about a day later in each successive century. Venus passes the node near enough to these dates to be seen against the sun only four times in a period of 243 years. The following list of
dates from 1518 to 2012 shows the law of recurrence.
| 1518 June 2. | 1769 June 3. |
| 1526 June 1. | 1874 December 9. |
| 1631 December 7. | 1882 December 6. |
| 1639 December 4. | 2004 June 8. |
| 1761 June 6. | 2012 June 6. |
The first of these transits actually seen was that of 1639, which was imperfectly observed by Jeremiah Horrox (1619–1641) shortly before sunset. Special interest in them was first excited by Edmund Halley a century later, who showed that the parallax of the sun could be determined by observing transits of Venus from regions of the earth's surface where the displacement by parallax was greatest. Governments, scientific organizations and individuals fitted out expeditions on a very large scale to make the necessary observations upon the four transits which have since occurred. The disappointing character of the results so far as the solar parallax is concerned are stated in the article Parallax, Solar. It may be said in a general way that the observations, even when made by experienced astronomers, exhibited irregularities and discordances several times greater than one had a right to expect. Other methods of determining the distance of the sun have been so perfected that the results of these transits now count but little. (S. N.)