52
See also: Nicolaus Copernicus 34–39 ■ Johannes Kepler 40–41P
lanetary transits offered
an opportunity to test the
first of Johannes Kepler’s
three laws of planetary motion—
that the planets orbit the Sun in an
elliptical path. The brief passages
by Venus and Mercury across
the disk of the Sun—at the times
predicted by Kepler’s Rudolphine
Tables—would reveal whether the
underlying theory was correct.
The first test—a 1631 transit
of Mercury observed by French
astronomer Pierre Gassendi—
proved encouraging. However,
his attempt to spot the transit of
Venus a month later failed due
to inaccuracies in Kepler’s figures.
These same figures predicted a
“near miss” for Venus and the Sun
in 1639, but English astronomer
Jeremiah Horrocks calculated that
a transit would in fact occur.
At sunrise on December 4, 1639,
Horrocks set up his best telescope,
focusing the Sun’s disk onto a piece
of card. Around 3:15 pm, the clouds
cleared, revealing a “spot of unusual
magnitude”—Venus—edging
across the Sun. While Horrocksmarked its progress on the card,
timing each interval, a friend
measured the transit in another
location. By using the two sets of
measurements from the different
viewpoints, and by recalculating
the diameter of Venus relative to the
Sun, Horrocks could then estimate
Earth’s distance from the Sun more
accurately than ever before. ■THE FIRST
OBSERVATION OF A
TRANSIT OF VENUS
JEREMIAH HORROCKS (1618–1641)
IN CONTEXT
BRANCH
AstronomyBEFORE
1543 Nicolaus Copernicus
makes the first complete
argument for a Sun-centered
(heliocentric) universe.1609 Johannes Kepler
proposes a system of
elliptical orbits—the first
complete description of
planetary motion.AFTER
1663 Scottish mathematician
James Gregory devises a way
to measure the exact distance
from Earth to the Sun using
observations of the transits of
Venus in 1631 and 1639.1769 British explorer Captain
James Cook observes and
records the transit of Venus
in Tahiti in the South Pacific.2012 Astronomers observe
the last transit of Venus
of the 21st century.I received my first
intimation of the remarkable
conjunction of Venus and the
Sun...it induced me, in
expectation of so grand a
spectacle, to observe with
increased attention.
Jeremiah Horrocks