Venus 9

VENUS ( 9) is the second of the planets in order of distance from the sun. It revolves in an orbit which has the smallest eccen tricity (0.007) in the planetary system, and an inclination to the ecliptic of 3° 24'. Its mean distance from the sun is 67,200,000 miles; but, whereas at inferior conjunction it is less than 26, 000,000 miles from the earth, at superior conjunction it is 160, 000,000 miles. The time it takes Venus to complete a revolution in its orbit is 225 days, but its synodic period, or the period of its phases, is 584 days. At its maximum elongations it recedes about or 48° from the sun, so that in middle latitudes it can set or rise over 3 hours after or before the sun. When seen in the western sky in the evenings, i.e., at its eastern elongations, it was called by the Ancients (Hesperus), and when visible in the mornings, i.e., at its western elongations clkoccOpos (Phos phorus). In volume and mass Venus is slightly smaller than the earth, its diameter being about 7,700 miles and its mass (deduced from its action on the earth and Mercury) 0.81 that of the earth. At superior conjunction its angular diameter is about Io", but at inferior conjunction it exceeds 6o".

Like the earth Venus is enveloped in an atmosphere. This is shown by the fact that, near inferior conjunction, the extremely thin crescent of the visible portion of the illuminated hemisphere has often been observed to exceed 180°, while at the time of actual entry on the sun's disc during the transit of 1882, as soon as about of the planet's body was in front of the sun, the remaining por tion was completely outlined by a narrow border of light. This atmosphere of Venus is apparently heavily cloud-laden, and, as the intensity of the solar radiation is almost exactly twice what it is at the earth's distance, the planet shines with a dazzling lustre, its stellar magnitude varying from —3.3 to —4.4. Its greatest bright ness is attained at about 36 days on either side of inferior con junction, its elongation from the sun then being 39°, and its phase similar to that of a 5 days old moon. When suitably situated the planet is easily visible at noonday with the naked eye, and after dark it readily casts a shadow.

As a telescopic object Venus is disappointing, since apart from the beauty of its phases it presents but few features of a definite nature. Its surface appears permanently screened from view by its cloud-laden atmosphere, and many observers have failed to detect any markings at all upon it beyond the general fading of light near the terminator and a brightness at the cusps or other features which appear to be merely phase effects. Occasionally

diffuse faint markings of a dusky character or bright areas are seen, but these are probably nothing more than inequalities in the cloudy stratum. On Feb. 13, 1913, a very definite indentation in the terminator, or line bounding the illuminated part of the disc, was observed simultaneously by McEwen of Glasgow, and Sargent at the Durham university observatory, and similar irregu larities have been recorded by previous observers.

The Planet's Rotation.-In

view of what has been said as to the elusive nature of the surface features, it is not surprising that the planet has been able to preserve the character of its rotation a secret to the present day. It was concluded by some of the earlier telescopic observers such as G. D. Cassini, Bianchini and Schroeter that its period is in the neighbourhood of 24 hours; but Schiaparelli (189o), after a careful study of the available material including his own observations, formed the conviction that the rotation is very slow and that it probably takes the same time as the planet's orbital revolution. This last conclusion was also arrived at by Lowell at Flagstaff. Flammarion in his review of the recorded observations considered that no reliable deduction could be drawn from them. A slow rotation would seem to be indicated by the absence of any observable ellipticity of the planet during its transits of the sun, as well as by the failure of certain spectroscopic observations to show any definite differential radial velocity at opposite sides of the visible disc. On the other hand the radiometric observations at the Mt. Wilson and Flagstaff ob servatories in 1922, showing a considerable amount of heat to be emitted by the dark part of the planet's disc, favour a quick rota tion, as also do photographs taken in ultra-violet light by Ross at the Mt. Wilson observatory on which dusky belts are shown perpendicular to the terminator and varying from night to night. It is, however, typical of the mystery enveloping this planet that on June 26, 1927, a dark marking was photographed at Mt. Wilson which apparently remained stationary for an hour.