sing her orbit, moved round to the opposite side of the Sun,
thereby bringing Jupiter into conjunction, an eclipse occurred sixteen
minutes twenty-six seconds later than it did when Jupiter was in
opposition or nearest to the Earth. As there existed an impression that
light travelled instantaneously, it was believed that an eclipse
occurred at the moment it was perceived in the telescope. This, however,
was not so. Roemer, after a long series of observations, concluded that
the discrepancies were due to the fact that light travels with a
measurable velocity, and that it requires a greater length of time,
upwards of sixteen minutes, to traverse the additional distance--the
diameter of the Earth's orbit--which intervenes between the Earth and
Jupiter, when the planet is in conjunction, as compared with the
distance between the Earth and Jupiter, when the latter is in
opposition. This discovery of Roemer's was the means of enabling the
velocity of light to be ascertained, which, according to recent
calculations, is about 187,000 miles a second. As an acknowledgment of
the importance of his communication, Roemer was awarded a seat in the
Academy, and apartments were assigned to him at the Royal Observatory,
where he carried on his astronomical studies.

In 1681 Roemer returned to Denmark, and was appointed Professor of
Mathematics in the University of Copenhagen; he was also entrusted with
the care of the city observatory--a duty which his reputation as an
astronomer eminently qualified him to undertake. The transit
instrument--a mechanism of much importance to astronomers--was invented
by Roemer in 1690; it consists of a telescope fixed to a horizontal
axis, and adjusted so as to revolve in the plane of the meridian. It is
employed in observing the passage of the heavenly bodies across the
observer's meridian. To note accurately by means of the astronomical
clock the exact instant of time at which a celestial body crosses the
centre of the field of view is the essential part of a transit
observation. Small transit instruments are employed for taking the time
and for regulating the observatory clock, but large instruments are
used for delicate and exact observations of Right Ascensions and
Declinations of stars of different magnitudes. Meridian, and altitude
and azimuth circles, are important astronomical appliances, which owe
their existence to the inventive skill of this distinguished astronomer.

Roemer resided for many