SKY_September2014.pdf

28 September 2014 sky & telescope

Astronomical Experiment

Reliving

The authors observed the 2012 transit of Venus to replicate

experiments from past centuries.

Rod Pommier & Richard Smith

Thousands of people traveled great distances to observe

the fi nal transit of Venus in our lifetimes on June 5-6, 2012.

Transits of Venus are extremely rare, occurring in pairs 8

years apart followed by intervals of more than a century.

Each transit is a historical event, but they were particularly

important to astronomers of previous centuries because

they provided, in theory, a good way to measure the actual

distances between the planets and the Sun.

Kepler’s third law of planetary motion, which states that

the square of a planet’s orbital period is proportional to the

cube of its distance from the Sun, had profound implica-

tions. Knowing each planet’s orbital period, astronomers

had calculated all their relative distances, expressed as units

of the unknown mean distance from Earth to the Sun — the

astronomical unit (a.u.). The problem was that nobody knew

any of the actual distances. If astronomers knew the actual

distance between any two objects, then they could calculate

all distances in the solar system, including the a.u.

In 1716 Edmond Halley proposed observing the 1761

and 1769 transits of Venus from widely separated locations

to determine the distances to Venus and the Sun. Slight

diff erences in transit duration would permit accurate place-

ment of the apparent position of the transit lines on the

Sun’s disk. The angular separation between transit lines

could then be measured in degrees. If the distance between

observers was known precisely, simple trigonometry would

yield the distance to Venus and the a.u. The technique relied

heavily on precise timing of the moments when Venus’s

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