Observatory at Greenwich. If in the latitude of London (51 deg. 30' N.),
we proceed 10 miles and 1,383 yards either in an easterly or westerly
direction, we find that the local time is respectively either one minute
faster or one minute slower than it was at our initial point. Let us
try to understand the reason of this. If we fix a tube rigidly at any
station on the earth's surface, pointing to that part of the sky in
which any bright star is situated when such star is due south (or, as it
is technically called, "on the meridian"), and note by a good clock the
hour, minute, and second at which it crosses a wire stretched vertically
across the tube, then after a lapse of 23 h. 56 m. 4.09 s., will that
star be again threaded on the wire. If the earth were stationary--or,
rather, if she had no motion but that round her axis--this would be the
length of our day. But, as is well known, she is revolving round the sun
from left to right; and, as a necessary consequence, the sun seems to be
revolving round her from right to left; so that if we suppose the sun
and our star to be both on the wire together to-day, to-morrow the sun
will appear to have traveled to the left of the star in the sky; and the
earth will have that piece more to turn upon her axis before our tube
comes up with him again. This apparent motion of the sun in the sky is
not an equable one. Sometimes it is faster, sometimes slower; sometimes
more slanting, sometimes more horizontal. Thus it comes to pass that
solar days, or the intervals elapsing between one return of the sun to
the meridian and another, are by no means equal. So a mean of their
lengths is taken by adding them up for a year, and dividing by 365;
and the quantity to be divided to or subtracted from the instant of
"apparent noon" (when the sun dial shows 12 o'clock), is set down in the
almanac under the heading of "The Equation of Time." We may, however,
here conceive that it is noon everywhere in the northern hemisphere when
the sun is due south. Now the earth turns on her axis from west to east,
and occupies 24 h. in doing so. As all circles are conceived to be