om the merits of Hipparchus
as an observing astronomer. A few words more must be said as to his
specific discoveries in this field. According to his measurement, the
tropic year consists of 365 days, 5 hours, and 49 minutes, varying thus
only 12 seconds from the true year, as the modern astronomer estimates
it. Yet more remarkable, because of the greater difficulties involved,
was Hipparchus's attempt to measure the actual distance of the moon.
Aristarchus had made a similar attempt before him. Hipparchus based
his computations on studies of the moon in eclipse, and he reached the
conclusion that the distance of the moon is equal to 59 radii of the
earth (in reality it is 60.27 radii). Here, then, was the measure of the
base-line of that famous triangle with which Aristarchus had measured
the distance of the sun. Hipparchus must have known of that measurement,
since he quotes the work of Aristarchus in other fields. Had he now but
repeated the experiment of Aristarchus, with his perfected instruments
and his perhaps greater observational skill, he was in position to
compute the actual distance of the sun in terms not merely of the moon's
distance but of the earth's radius. And now there was the experiment
of Eratosthenes to give the length of that radius in precise terms. In
other words, Hipparchus might have measured the distance of the sun in
stadia. But if he had made the attempt--and, indeed, it is more than
likely that he did so--the elements of error in his measurements would
still have kept him wide of the true figures.

The chief studies of Hipparchus were directed, as we have seen, towards
the sun and the moon, but a phenomenon that occurred in the year 134
B.C. led him for a time to give more particular attention to the fixed
stars. The phenomenon in question was the sudden outburst of a new
star; a phenomenon which has been repeated now and again, but which
is sufficiently rare and sufficiently mysterious to have excited the
unusual attention of astronomers in all generations. Modern science
offers an explanation of the phenomenon, as we shall see in due course.
We do not know that Hipparchus attempted to explain it, but he was led
to make a chart of the heavens, probably with the idea of guiding future
observers in the observation of new stars. Here again Hipparchus was not
altogether an innovator, since a chart showing the brightest stars had
been made by Eratosthenes; but the new charts were much elaborated