Sky & Telescope - March 2022 - 31

the Seahorse knew it was impossible to reach Sumatra in time,
so they decided to stop short and set up their equipment in
Cape Town. The location was a compromise that would cost
the astronomers dearly, since the 6½-hour transit would begin
3½ hours before local sunrise on June 6th.
Having found a suitable site for their simple observatory,
Mason and Dixon took their instruments ashore on May 2nd.
They set up the pendulum clock and astronomical quadrant
in order to determine crucial data for their transit measurements:
their latitude and local time. To do so they needed
to measure the Ellicott clock's going - the rate at which the
clock was running fast or slow. They discovered it was slow by
2 minutes and 17 seconds per sidereal day. They could have
adjusted the clock's rate by altering the length of the pendulum,
but in practice simply taking the difference into account
when reducing the data would accomplish the same thing.
Between May 4th and June 5th, the pair observed on 13 nights
in order to refine their position and the clock's going.
Transit Day Arrives
On the morning of the transit, the Sun rose into thick haze
and low clouds. The scientists first spotted Venus 13 minutes
after sunrise, but because the Sun was so close to the horizon,
the view was poor. Around one hour after sunrise, Mason
began using the Dollond micrometer on one of the reflectors
at a magnification of 120×. He measured the distance
between " the Sun's farthest limb from Venus' farthest limb
(that is, the Sun's northern limb from Venus' southern limb.) "
Recording the time for each measurement was a task
that probably fell to Dixon. It's also likely that he read and
recorded the offset of the micrometer's object glasses so
Mason could remain at the telescope's eyepiece. In total, the
pair took 15 limb measurements, as well as four of the
Sun's diameter and three of Venus's diameter to determine
the apparent radii of both objects. (Mason also
reported that he did not see a Venusian satellite.) He
took his final observation of Venus's location on the
face of the Sun about 35 minutes before the planet
reached the solar limb. (You can see a representation
of the transit at the HM Nautical Almanac
Office website, which features an animation of
the event as seen from Cape Town at https://
is.gd/1761transit.)
As Venus exited the Sun's disk, both Mason and
Dixon recorded the times when the planet's leadKEEPING
TRACK Top: This page from Mason's logbook
details the attack on their ship by the French frigate L'Grande.
The loss of life and damage to the expedition's vessel, the
Seahorse, required a return to port and, ultimately, prevented the
astronomers from reaching their planned destination.
DESTINATION VENUS Bottom: The map presented here shows the
approximate route Mason and Dixon took between Portsmouth, England,
and Cape Town, South Africa, and their return from Cape Town to Portsmouth
via St. Helena. Also shown is the route to their initial destination of
Bencoolen, Sumatra.
St. Helena
Portsmouth
Bencoolen
Cape Town
sk yand tele scope .o r g * MARCH 2022 31
LOGBOOK PAGE: KISLAK CENTER FOR SPECIAL COLLECTIONS, RARE BOOKS AND
MANUSCRIPTS, UNIVERSITY OF PENNSYLVANIA; ROUTE MAP: GREGG DINDERMAN / S&T
http://www.is.gd/1761transit http://www.is.gd/1761transit http://skyandtelescope.org
Sky & Telescope - March 2022

Table of Contents for the Digital Edition of Sky & Telescope - March 2022
