se of the
other until the nineteenth century was well past its meridian. It is
singular how often errors conspire to lead conviction astray.

Hansen's note of alarm in 1854 was echoed by Leverrier in 1858.[757] He
found that an apparent monthly oscillation of the sun which reflects a
real monthly movement of the earth round its common centre of gravity
with the moon, and which depends for its amount solely on the mass of
the moon and the distance of the sun, required a diminution in the
admitted value of that distance by fully four million miles. Three years
later he pointed out that certain perplexing discrepancies between the
observed and computed places both of Venus and Mars, would vanish on the
adoption of a similar measure.[758] Moreover, a favourable opposition of
Mars gave the opportunity in 1862 for fresh observations, which,
separately worked out by Stone and Winnecke, agreed with all the newer
investigations in fixing the great unit at slightly over 91 million
miles. In Newcomb's hands they gave 92-1/2 million.[759] The
accumulating evidence in favour of a large reduction in the sun's
distance was just then reinforced by an auxiliary result of a totally
different and unexpected kind.

The discovery that light does not travel instantaneously from point to
point, but takes some short time in transmission, was made by Olaus
Roemer in 1675, through observing that the eclipses of Jupiter's
satellites invariably occurred later, when the earth was on the far
side, than when it was on the near side of its orbit. Half the
difference, or the time spent by a luminous vibration in crossing the
"mean radius" of the earth's orbit, is called the "light-equation"; and
the determination of its precise value has claimed the minute care
distinctive of modern astronomy. Delambre in 1792 made it 493 seconds.
Glasenapp, a Russian astronomer, raised the estimate in 1874 to 501,
Professor Harkness adopts a safe medium value of 498 seconds. Hence, if
we had any independent means of ascertaining how fast light travels, we
could tell at once how far off the sun is.

There is yet another way by which knowledge of the swiftness of light
would lead us straight to the goal. The heavenly bodies are perceived,
when carefully watched and measured, to be pushed forward out of their
true places, in the direction of the earth's motion, by a very minute
quantity. This effect (already adverted to) has been known since
Bradley's time as "aberrat