Venus 9

It may be that the harmonizing of many of the discordances referred to will ultimately be found in the theory of Professor W. H. Pickering. Observing in Jamaica in 1921, he reported observations of dusky markings indicating a rotation in approxi mately 68 hours about an axis which is nearly in the plane of the orbit and in line with the radius vector in heliocentric longitude 7'. This result has received general support from McEwen, and it has been pointed out that the failure of the Flagstaff spec troscopic observations in 1903 to indicate rotation is explained by the fact that, on Pickering's hypothesis, the planet's pole was at that time directed towards the Earth, and that the rotation of the surface markings was accordingly almost in the plane of vision. It is to be noted that the earlier spectroscopic observations of Belopolsky at Pulkowa made under different conditions had given distinct evidence of rotation.

Habitability.-There

is a point which is of considerable im portance as regards the question whether Venus is fitted to be the abode of animate life. If oxygen and water vapour exist in any large quantity, we might expect their presence to be revealed by absorption lines in the spectrum of the sunlight reflected by the planet's surface. St. John, however, has found no evidence of such lines, and has concluded that the amount of oxygen above the visible surface is less than one thousandth part of the quantity in the atmosphere of the earth. It must, however, be remembered that the visible surface of Venus is apparently only that of the upper layer of a stratum of cloud, and that, although the quantities of oxygen and water vapour above this layer are apparently small, there may be considerable amounts below it. In the absence of any certain knowledge as to the planet's rotation and other important data it is not possible to form conclusions concerning its habit ability, but the resemblance of Venus to the earth in size and mass, coupled with its possession of a dense atmosphere, would suggest the probability that it supports life of some kind.

Supposed Satellite.-It

was at one time thought that Venus possessed a satellite, several observers in the 17th and 18th centuries reporting that they had seen it, though others searched the neighbourhood of the planet for it in vain. Observations with

more perfect instruments, however, eventually demonstrated the non-existence of any such object, and it is evident that what was seen must have been the appearance of a "ghost," caused by some fault in the construction or adjustment of the instruments used.

Transits of Venus.-As

is the case with Mercury, Venus, revolving round the sun inside the earth's orbit, sometimes transits the sun's face, and is seen projected on it as a small black disc. Were the planet's orbit plane coincident with that of the earth, these transits would, of course, occur at each inferior conjunction, but owing to its inclination a transit can only happen when the two planets pass near one of the nodes of Venus at about the same time, which is possible only at present in June and December. Actually a transit happens but four times in 243 years, and the intervals between transits are successively 8, 1211, 8, 1°51, 8, 1211 years et seq, as illustrated in the following table of dates of these phenomena: 1518, June 2, 1769, June 3, 1526, June I, 1874, Dec. 9, 1631, Dec. 7, 1882, Dec. 6, 1639, Dec. 4, 2004, June 8, 1761, June 6, 2012, June 6.

The first transit to be actually observed was that of 1639, the occurrence of the event having been calculated by Jeremiah Horrox, a young clergyman who was curate of Hoole near Preston in Lancashire. Dec. 4 in that year happened to be a Sunday, and Horrox missed seeing the beginning of the transit through having to take a service in church that afternoon, but on returning home he found to his great delight the black body of the planet clearly projected on the sun's disc.

Following on the suggestions of Edmund Halley a century later, transits of Venus were utilized for the determination of the solar parallax which gives the distance of the sun-a quantity of funda mental importance to the astronomer. Practical difficulties, how ever, in the observations, arising from the effect of irradiation in introducing uncertainties as to the precise moments of the internal contacts between the limbs of the sun and planet, rendered the method unsatisfactory, and far more effective ways of attacking the problem are now available for the purpose. (T. E. R. P.)