Page:Popular Science Monthly Volume 10.djvu/425

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amount which varies with the size of the telescope, the perfection of its lenses, and the tint and brightness of the sun's image. The edge of the planet's image is also rendered slightly hazy and indistinct.

The planet's atmosphere also causes its disk to be surrounded by a narrow ring of light, which becomes visible long before the planet touches the sun, and at the moment of internal contact produces an appearance of which the accompanying figure is intended to give an

PSM V10 D425 Extrapolating sun distance by passing shadow object.jpg
Fig. 4.

idea though on an exaggerated scale. The planet moves so slowly as. to occupy more than twenty minutes in crossing the sun's limb; so that, even if the planet's edge were perfectly sharp and definite, and the sun's limb undistorted, it would be very difficult to determine the precise second at which contact occurs; but as things are, observers, with precisely similar telescopes, and side by side, often differ from each other five or six seconds; and where the telescopes are not similar the differences and uncertainties are much greater. The contact observations of the last transit in 1874 do not appear to be much more accordant than those of 1769, notwithstanding the great improvement in telescopes; and astronomers at present are pretty much agreed that such observations can be of little value in removing the remaining uncertainty of the solar parallax, and are disposed to put more reliance upon the micrometric and photographic methods, which are free from these peculiar difficulties, though of course beset with others; which, however, it is hoped will prove less formidable.

The micrometric method requires the use of a peculiar instrument known as the heliometer, an instrument common only in Germany, and requiring much skill and practice in its use in order to obtain with it accurate measures. At the late transit a single English party, two or three of the Russian parties, and all five of the German, were equipped with these instruments, and at some of the stations extensive series of measures were made. None of the results, however, have appeared as yet, so that it is impossible to say how greatly, if at all, this method will have the advantage in precision over the contact observations.

The Americans and French placed their main reliance upon the photographic method, while the English and Germans also provided