Will be evening star throughout the month, and continue his retrograde
motion from a point about twenty minutes west of _Præsepe_ on the 1st, to
7 hours 48 minutes 35 seconds right ascension on the 29th. He will rise
on the 1st at 3:56; on the 15th at 2:53; and on the 29th at 1:52 p. m.,
and will set on the 2d at 6:30; on the 16th at 5:29; and on March 1st
at 4:30 a. m. On the 9th, at 5:39 a. m., he will be 5° 45′ north of the
moon. Of the four satellites, or moons, revolving around Jupiter, three
are so near as to be eclipsed by him at each revolution. Roemer, a Danish
astronomer, observed, however, that when the earth and Jupiter were on
opposite sides of the sun, these eclipses occurred, as he estimated,
about twenty-two minutes later than the time predicted by the tables. As
the earth in this position was some one hundred and eighty-six millions
of miles farther away from Jupiter than when Jupiter and the earth were
on the same side of the sun, the discovery was made that the discrepancy
in time was occasioned by the fact that light must have time to travel;
and later and more accurate investigations afford us the truth that it
takes light sixteen minutes and forty seconds to cross the earth’s orbit,
or eight minutes and twenty seconds to come from the sun to the earth;
and hence, that it travels about 180,000 miles per second. These eclipses
occur frequently every month, and can be observed with telescopes of
quite moderate power.