SKY_September2014.pdf

SkyandTelescope.com September 2014 29

Histor y

Earth

N

Seen from Washington

Seen from Maui

S

Sun Venus

Not to scale

Edmond Halley conceived this geometric method

for measuring the astronomical unit (a.u.). The

authors observed the June 5-6 transit of Venus

from widely separated locations on Earth. They

were able to detect the planet’s subtle shift

(parallax) as seen on the Sun. After using GPS

to calculate their baseline distance, they applied

basic trigonometry to calculate the distance to

Venus and the a.u.

Facing page: Coauthor Rod Pommier recorded this sequence of

Venus’s transit across the Sun on June 5, 2012, from the summit

of Haleakala- on the Hawaiian island of Maui.

disk, silhouetted on the Sun, just touched the solar limb

at ingress and egress, known as second and third contact.

The 18th-Century’s Holy Grail

Observing the transits of Venus to determine the a.u.

became the holy grail of 18th-century astronomy. The

transits spawned some of the fi rst great scientifi c expedi-

tions, with observers traveling to remote destinations car-

rying crates of fragile telescopes and instruments. Astron-

omers would compare their observations with those in

other locations. Unfortunately, making the observations

was not as easy as Halley had imagined and astronomers

encountered a multitude of problems, including clouds,

warfare, and disease.

Because of various diffi culties, only a handful of suc-

cessful observations were made of the 1761 transit. They

yielded a.u. values ranging from 77.86 to 96.19 million

miles (125.30 to 154.80 million km).

For the 1769 transit, Britain dispatched Captain James

Cook to Tahiti, but his party’s observations were thwarted

by the black-drop eff ect. Venus’s disk did not appear to

touch the inside of the Sun’s limb for only an instant.

Rather, Cook and his companions saw a thin, dark bridge

between Venus and the Sun’s limb that persisted for

about a minute. The eff ect prevented his group from tim-

ing the exact moments of internal contact to any degree of

certainty. Still, more groups observed the 1769 transit, and

they narrowed the a.u. to between 92.03 and 96.16 million

miles. But the 4-million-mile range was still too impre-

cise to nail down important planetary parameters.

For the transit of 1874, many more nations joined the

quest. Although visual timings of second and third con-

tact were still the goal of most expeditions, some relied

heavily on the new technology of photography. Astrono-

mers hoped photography would produce objective records

that were not subject to optical illusions, which was sus-

pected to be the cause of the black-drop eff ect. Unfortu-

nately, the black-drop eff ect was plainly evident on photos,

rendering scores of images useless. Ultimately, estimates

of the a.u. from the 1874 transit were scarcely better than

those from the 18th century. By the time of the 1882 tran-

sit, Scottish astronomer David Gill had nailed down the

a.u. by measuring the parallax of Mars.

2012: Amateurs Join the Fray

Given the interesting history of using transits of Venus to

measure the a.u., many amateur astronomers wanted to

attempt their own measurement during the 2012 transit.

There were plentiful opportunities for observers of all

skill levels. For example, a NASA website off ered three

ways for the public to estimate the a.u. The simplest was

to measure and upload the diameters of the Sun and

Venus in images taken during the transit. At the high

end, amateurs could replicate Halley’s method by record-

S&T:

GREGG DINDERMAN

Transit_Experiment.indd 29 6/23/14 12:17 PM