arth, from the Cape of Good
Hope or from Great Britain, the positions of the stars remain absolutely
unchanged. Their distance is so stupendous that a change of place on the
earth displaces them to no appreciable extent. But the case is different
with a minor planet. It is hardly one-millionth part of the distance of
the stars, and the displacement of the planet when viewed from the Cape
and when viewed from Europe is a measurable quantity.

The magnitude we are seeking is to be elicited by comparison between the
measurements made in the northern hemisphere with those made in the
southern. The observations in the two localities must be as nearly
simultaneous as possible, due allowance being made for the motion of the
planet in whatever interval may have elapsed. Although every precaution
is taken to eliminate the errors of each observation, yet the fact
remains that we compare the measures made by observers in the northern
hemisphere with those made by different observers, using of course
different instruments, thousands of miles away. But in this respect we
are at no greater disadvantage than in observing the transit of Venus.

It is, however, possible to obviate even this objection, and thus to
give the minor planet method a supremacy over its rival which cannot be
disputed. The difficulty would be overcome if we could arrange that an
astronomer, after making a set of observations on a fine night in the
northern hemisphere, should be instantly transferred, instruments and
all, to the southern station, and there repeat the observations. An
equivalent transformation can be effected without any miraculous agency,
and in it we have undoubtedly the most perfect mode of measuring the
sun's distance with which we are acquainted. This method has already
been applied with success by Dr. Gill in the case of Juno, and there are
other members of the host of minor planets still more favourably
circumstanced.

Consider, for instance, a minor planet, which sometimes approaches to
within 70,000,000 miles of the earth. When the opposition is drawing
near, a skilled observer is to be placed at some suitable station near
the equator. The instrument he is to use should be that marvellous piece
of mechanical and optical skill known as the heliometer.[20] It can be
used to measure the angular distance between objects too far apart for
the filar micrometer. The measurements are to be made in the evening as
soon as the planet has risen hig