tracks to be calculated. The lengths being known, their
places on the circular disc of the sun are determined, and hence the
amount of displacement of Venus in transit is ascertained. Thus it is
that the distance of Venus is measured, and the scale of the solar
system is known.

[Illustration: Fig. 47.--To Illustrate the Observation of the Transit of
Venus from Two Localities, A and B, on the Earth.]

The two transits to which Halley's memorable researches referred
occurred in the years 1761 and 1769. The results of the first were not
very successful, in spite of the arduous labours of those who undertook
the observations. The transit of 1769 is of particular interest, not
only for the determination of the sun's distance, but also because it
gave rise to the first of the celebrated voyages of Captain Cook. It was
to see the transit of Venus that Captain Cook was commissioned to sail
to Otaheite, and there, on the 3rd of June, on a splendid day in that
exquisite climate, the phenomenon was carefully observed and measured by
different observers. Simultaneously with these observations others were
obtained in Europe and elsewhere, and from the combination of all the
observations an approximate knowledge of the sun's distance was gained.
The most complete discussion of these observations did not, however,
take place for some time. It was not until the year 1824 that the
illustrious Encke computed the distance of the sun, and gave as the
definite result 95,000,000 miles.

For many years this number was invariably adopted, and many of the
present generation will remember how they were taught in their
school-days that the sun was 95,000,000 miles away. At length doubts
began to be whispered as to the accuracy of this result. The doubts
arose in different quarters, and were presented with different degrees
of importance; but they all pointed in one direction, they all indicated
that the distance of the sun was not really so great as the result which
Encke had obtained. It must be remembered that there are several ways of
finding the distance of the sun, and it will be our duty to allude to
some other methods later on. It has been ascertained that the result
obtained by Encke from the observations made in 1761 and 1769, with
instruments inferior to our modern ones, was too great, and that the
distance of the sun may probably be now stated at 92,000,000 miles.

I venture to record our personal experience of the last transit of