Light Science for Leisure Hours

judged the approaching event to be of50 such extreme importance to the science of astronomy that they presented a memorial to King George III., requesting that a

placed under the command of Captain Cook. The astronomical work entrusted to the expedition was completely successful; and thus it w

fourteen years ago, when the Astronomer-Royal began that process of laborious inquiry which a question of this sort necessarily demands. Gradually, her hints became more and more plain-spoken; insomuch that Airy-her mouthpiece in this case-stated definitely in 1868 what he thought science had a right to claim from England in this matter. When the claim came bef

ttached to them. The telescopist may see half-a-dozen such phenomena in the course of a night or two, by simply watching the satellites of Jupiter, or the passage of our moon over the stars. Even the great eclipse of 1868 did not attract so much in

little to us whether the sun is a million of miles or a thousand millions of miles from us. The subject must in any case be looked upon as an extra-parochial one. But science does occasionally attach immense interest to extra-parochial subjects. And this is neither unwise52 nor unreasonable, since we

ge the celestial depths with any feeling of assurance, unless we know the true length of that which is our sole measuring-rod. It is, in fact, our basis of measurement for the whole visible universe. In some respects, even if we knew the sun's distance exactly, i

e this very element-the sun's distance-we find that he is

accessible castle, or53 rock, or tree, or the like. We shall see presently that the ingenuity of astronomers has, in fact, suggested some other indirect methods. But clearly the most satisf

ut the solution of the famous problem. Un

cessible object-that is, he observes the direction in which it lies. It is clear that, if he were now to draw a figure on paper, laying down the base-line to some convenient scale, and drawing lines from its ends in directions corresponding to the

inaccessible object; for then, if we make the least error in observing the bearings of the object, we get an important error in the re

ugh to determine the moon's distance with such a short base-line to work from. But the moon is only about a quarter of a million of miles from us, while the sun is more than ninety millions of miles off. Thus the problem w

ow, what change of direction could be observed in the steeple by merely shifting the eye along a line of two feet? There is a ready way of answering. Invert the matter. Consider what a line of two feet long would look l

ht another set of instruments and took the bearing of the steeple from the other end of the two-feet line, is it not obvious how enormously the uncertainty of the result would be increased by such an arrangement as this? One observer would have his own peculiar powers of observation, his own peculiar weaknesses: t

stronomer must set one observer in one hemisphere, another in the other. Each observer must have his own set of instruments; an

iled totally when they attempted to determine the sun'

r off than the sun, and even those which, approach nearest to us are still far beyond the limits of distance within which the simple plan followed by surveyors could be of any servi

on from which the astronomer hopes to learn so much. To take an instance from the motions of another planet than the one we are dealing with. Mars comes sometimes so near the earth that the distance separating us from him is little more than one-third of that which separates us from the sun. Suppose that, at such a time, he is seen quite close to a fixed star. That star gives the astronomer powerful aid in determining the planet'

er than our earth to the sun. And thus there are occasions when she gets directly between the earth and the sun. At those times she is seen upon his face, and his face serves as a dial-plate by which to measure her movements. When an observer at one part of the earth sees her on one part of the sun's face, another observer at some other part of the earth will see her o

y considering the distance of V

e sun would be more than trebled in the case of Venus. But it must not be forgotten that we are to judge the motions of Venus by means of the dial-plate formed by the solar disc, and that dial-plate is itself shifted as the observe

lready mentioned that the sun's change of place is not mea

els athwart the great solar dial-plate. We are thus enabled to make a time measurement take the place of a measurement of space. If an observer in one place sees Venus cross the sun's face at a cer

ve described. It will be noticed59 that what is required for the successful application of the method is that one set of observers should be as far to the north as possible, another as far to the south, so that the path of Venus

earth does not supply observing room all over her surface, and the region where observation would be most serviceable may be covered by a widely-extended ocean. Then again, the observing parties are being rapidly s

hese, I may simply indicate the fact that the astronomer has a problem of considerabl

her way-that, in fact, instead of noticing how much longer the transit lasts in some places than in

t, and all the points which have to be attended to in the equipment and placing of the various observing parties, depend on these preliminary matters. Without attending to them-or at least to such primary points as I shall select-it would be impossibl

ly placed round a projecting corner of the course see the leading boat come into view at different times. Some one observer on the outer rim of the hemisphere would be absolutely the first to see the transit begin. Then rapidly other observers would see the phenomenon; and in the course of a few minutes61 some one observer on the outer rim of the hemisphere-almost exactly opposite the

out what was the exact difference of time between their respective observations. Is it not clear that the result would at once afford the means of determining the sun's distance? It would be the simplest of all possible astronomical problems to determine over what proportion of her orbit Venus passed in the interval of time which elapsed between these observati

the transit end earliest a

at the other side of the earth. Each uses the local time of the place at which he observes, and it has been calculated that for the result to be of value there must not be an error of a single second in their estimates of local time. Now, does the reader appreciate the full force of this proviso? Each observer must know so cert

nces of which are more inconvenient. On the other hand, however, the transit is of such a nature that if once the pre63liminary difficulties are

test for observers in the north-eastern part of European Russia. Thus we see that, so far as the application of our second method is concerned, the suitable spots are not situated in the most inviting regions of the earth's surface. As the transit ha

ria, and Manchouria are suitable for the purpose. The best southern stations for this method lie unfortunately on the unexplored Antarctic continent and the islands adjacent to it; but Crozet Island, Kerguelen Land, and some other places more easy of access than the Antarctic64 continent, will serve very well

, Kerguelen Land, Mauritius, or Rodriguez is uncertain. Possibly two parties will be sent out for this purpose, and most likely Rodriguez and Mauritius will be the places selected. It had been thought until lately that the sun would be too low at some of the places when the transit begins, but a m

land or Campbell Island, not far from New Zealand. It had been thought that at the former island the sun would65 be too low; but here, again, a more

h India where this phase can be very usefully observed; and doubtless the skilful astronomers and mathematicians who are taking part in the survey of India will be invited-as at the time of th

necessary that they should be provided with instruments of the utmost delicacy, and very carefully constructed.4 They will have to remain at their several stations for a long time before the transit takes place-several months, at least-so that they may

t, and sail away the day after, with results at least as trustworthy as those which a party applying Delisle's method could obtain after several months of hard work. It is to this, rather than any other cause, that the small expense of the observations made in 1769 is to be referred. And doubtless had it been decided by our astronomical authorities to apply Halley's method solely or principally, the expense of the trans

tion of the circumstances of the transit, a French astronomer, M. Puiseux, was enabled to announce that this is not the case. Almost simultaneously I published calculations pointing to a simila

r that the quasi-failure of the observations made in 1769 must be attributed. It is true that Mr. Stone, the first-assistant at the Greenwich Observatory, has managed to remove the greater part of the doubts which clouded the results of those observatio

disc. This at least was what Halley and Delisle both suggested as desirable. Unfortunately, Venus had not68 been consulted, and when the time of the transit came

otundity of figure. She became gradually more and more pear-shaped, until at last she looked very much like a peg-top touching with its point the edge of the sun's disc. Then suddenly-'as by a lightning flash,' said one observer-the t

nged to a figure which would have touched the sun's edge if the rotundity had been perfect elsewhere? This, again, seemed unlikely, because at this moment the black band connecting Venus and the sun was quite69 wide. And, besides, if this were the true moment of contact, what eye could be trusted

d diminish the peculiarity, the observations of the transit of Mercury, in November 1868, would have sufficed to destroy that hope, for e

be expected to behave, or to misbehave herself; and the result is, that he has been able to tell the observers exactly what they will have to look for, and exactly what it is most important that they should record. In 1769, observers recorded their observations in such dou

e the error seem enormous. The sun's distance came out some four millions of miles too large, and that seems no trifling error. Then, again, the resulting estimate of the distance of Neptune c

absolute error we have to consider. A microscopist would have made a bad mistake who should over-estimate the length of a fly's proboscis by a single hair's breadth; but the astronomer had made a wonderfully successful measurement of the sun's distance who deduced it within

y deserve the ingenious processes by which the great problem has been attacked without aid from Venus. Indeed, we can but barely mention the principles on which those methods depend. But to the r

sun's distance. Secondly, a certain irregularity in the moon's motion, due to the fact that she is most disturbed by the sun when traversing that half of her path which is nearest to him, was pressed into the service with similar results. Thirdly, an irregularity in the earth's

ng about 95,000,000 miles, is little more than 91,500,000 miles. And recently a re-examination of th

unlooked-for combination of unfavourable circumstances can cause the failure of our hopes. Certainly, if we should fail in obtaining satisfactory results in 1874, the world will not say that the generosity of the English Government has been i

, October 1869.)