The latter differs from the other only in that the spectrum of the
star is photographed, and the observer makes his measures on the
negative. This method was first extensively applied at the Potsdam
Observatory in Germany, and has lately become one of the
specialties of the Lick Observatory, where Professor Campbell has
brought it to its present degree of perfection. The Yerkes
Observatory is also beginning work in the same line, where
Professor Frost is already rivalling the Lick Observatory in the
precision of his measures.
Let us now go back to our own little colony and see what is being
done to advance our knowledge of the solar system. This consists
of planets, on one of which we dwell, moons revolving around them,
comets, and meteoric bodies. The principal national observatories
keep up a more or less orderly system of observations of the
positions of the planets and their satellites in order to
determine the laws of their motion. As in the case of the stars,
it is necessary to continue these observations through long
periods of time in order that everything possible to learn may be
discovered.
Our own moon is one of the enigmas of the mathematical astronomer.
Observations show that she is deviating from her predicted place,
and that this deviation continues to increase.
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