Is morning star, rising at the following times: 1st, at 3:55 a. m.; 15th,
at 3:16 a. m.; 30th, at 2:32 a. m. Its motion is direct, and equals about
5° 45′ 14″ of arc. Its diameter increases one second, being on the 30th
31″. On the 16th, at 8:30 p. m. is 4° 55′ north of the moon.
The satellites of Jupiter, four in number and designated as 1, 2, 3, 4,
outwardly from the planet, are frequently used to find the longitude. To
do this, however, requires the use of a telescope. By observing the time
at which one of these satellites passes into or emerges from the shadow
of its primary, and comparing this time with the recorded time of the
same event in Washington City, for example, one can determine whether
he is east or west of this city, and how many degrees. On the 14th No.
1 enters the shadow of Jupiter at 4:46 a. m., Washington mean time.
Suppose the observer at Allegheny Observatory should note the same event
as occurring at exactly 57 minutes 50.84 seconds after four, Allegheny
Observatory time. He would find the difference of the two times to be
11 m. 50.84 s., which reduced to longitude by multiplying by 15 (since
one hour of time equals 15° of arc) gives the difference of longitude 2°
57′ 40.3″. And since the ingress occurred at Allegheny Observatory at
an earlier hour (by its local time) than by Washington local time, it
follows that the latter place is 2° 57′ 40.3″ east of the former.