By far the most important of all the phenomena which astronomers are now expecting is the transit of Venus, which will take place on December 8, 1874. Even the eclipses of the last few years, though they have attracted so much attention, and have been observed so carefully,have in reality been regarded as altogether less important than the next transit of Venus. Total eclipses are almost every-year phenomena, but transits of Venus occur only at average intervals of more than half a century. The last took place in 1769, and after the transit of 1882 none will occur till 2004. Apart from this circumstance, a transit of Venus is of extreme importance in the science of astronomy. It admittedly affords the most satisfactory means of determining the distance of the sun-in other words, the dimensions of the solar system itself. And such determination of the scale on which our system is constructed affords the only means we possess of measuring the vast spaces which separate us from the fixed stars. So that the observations which are to be made in December, 1874, and renewed (but under somewhat different conditions) in December, 1882, bear directly on the fundamental problem of astronomy, so far as astronomy relates to the determination of the distances and the magnitude of the celestial bodies.*
I propose here, after inquiring briefly into the general question of the determination of the sun's distance, to describe the nature of the opportunities which will be afforded during the transit of 1874, and to discuss the preparations which are being made by this country to take her part in the work of observation. It will be seen, as I proceed, that this discussion of the
* I venture to quote here the appeal made by Halley (when Astronomer Royal) forty-five years before the transit of 1761, the earlier of the pair of transits then looked forward to. It will show that, in dealing with a transit 21 months before the date of its occurrence, I am not looking forward so inordinately as might be supposed by those unfamiliar with the nature of these inquiries. I should remark, however, that since Halley's day other methods for determining the sun's distance have been devised and employed. Six methods are described in my treatise on the ' Sun,' and a seventh has, within the last few months, been suggested by the great French astronomer Leverrier. Thus, then, wrote Halley in 1716:- 'I could wish, indeed, that observations of the transit should be undertaken by many persons in different places : first, because of the greater confidence which could be placed in well-according observations; and, secondly, lest a single observer should, by the intervention of clouds, be deprived of that spectacle which, so far as I know, will not be visible again to the men of this and the next century, and on which depends the certain and sufficient solution of a most noble and otherwise intractable problem. I therefore again and again urge upon those inquiring observers of the celestial bodies, who, when I have departed this life, will be reserved to observe these things, that, mindful of my counsel, they should devote themselves strenuously and with all their energies to conduct the observation; I desire and pray that they may be favoured in every way, and especially that they may not be deprived of that most desirable spectacle by the inopportune darkness of a clouded sky; and that, finally, the magnitudes of the celestial bodies, forced into narrower limits (of exactness), may, as it were, make submission-to the glory and eternal fame of those observers.'
These hopes were not fulfilled, so far as the transit of 1761 was concerned; but the transit of 1769 was observed with great care at no less than seventy-four stations, fifty of which, however, were in Europe.
subject does not labour under the fault of being premature. On the contrary, the time is now at hand when a final decision must be made as to the course which this country is to pursue; and inasmuch as my purpose is not solely to describe what is being done, but to point out what (in my opinion) should be done, the present is the proper time to speak.
A surveyor, who wishes to determine the distance of an inaccessible object, measures a convenient base-line and observes the direction of the object as seen from either end of the line. He thus has the base and the two base-angles of a triangle; and the simplest geometrical considerations teach that the other two sides of the triangle can thence be determined. These sides are, of course, the distances of the inaccessible object from the two ends of the base-line. Now this is the fundamental method employed by astronomers to determine the distances of the celestial bodies. It is applied directly to the moon. An observer at Greenwich (let us say), notes the direction of the moon when at her highest, or due south; another at Cape Town (let us say), does the like; then a line joining Greenwich and Cape Town is a base-line of known length, and the two directions give the base-angles. The triangle is a very long one, its vertical angle (that is, the angle opposite the base) being one of about a degree and a half, or about the angle swept out by the hand of a clock or watch during a quarter of a minute; but such a triangle is quite within the methods of treatment available to astronomers.
In applying this method to the sun, a serious difficulty comes in. He is so far off that, instead of a triangle with a respectable vertical angle, there is a triangle having a vertical angle of about the 240th part of a degree (under the most favourable conditions which can be conveniently obtained). To know how small such an angle is, let the reader note the minute hand of a clock or watch, and observe how little it shifts around its centre in a single second of time; yet this angular shift is twenty-four times as great as that we have mentioned.
It must not be forgotten that, in all such cases, the question is not whether the astronomer can recognise such and such an effect, but whether he can measure it It is not the whole quantity about which astronomers are troubled. Unquestionably the observer at Greenwich can recognise the depression of the mid-day sun,1 due to the fact that Greenwich lies above (or north of) the earth's centre. For this depression is an element which he has to take into account in his observations. The corresponding depression, even in the case of bodies far more distant than the sun, as the planets Jupiter and Saturn, is announced systematically in our national astronomical almanac. But the direct measurement of the depression is altogether out of the question.