Sky & Telescope - March 2022 - 33

of Venus couldn't be calculated with just one set of measurements
- good results from at least one other geographic
position were required. Although the event was observed from
some 150 locations, amateurs using poor equipment and who
failed to accurately determine the local time and location provided
most of the data, rendering most of the results useless.
A parallax estimate was eventually obtained by pairing
Mason and Dixon's measurements with the best from the
Northern Hemisphere, but the results had a large scatter. The
root of the problem was that none of the methods utilized
proved sufficiently accurate. This was the case not only for
the 1761 Venus transit, but also for those in 1769, 1874,
and 1882. Using parallax to determine a good value for the
astronomical unit had to wait until the 20th century, when
the asteroid Eros passed within 0.27 a.u. of Earth during its
opposition of 1900-1901, and within 0.17 a.u. during its
opposition of 1930-1931.
A Final Transit
Although the Venus transits failed to yield the desired outcome,
the efforts of Mason and Dixon didn't go unnoticed. A
little more than a year and a half after returning to England,
the pair signed the contract that would keep them in America
for four years and ten months, between 1763 and 1768, to
do the work that would eventually bear their names. (A full
account of the survey of the Mason-Dixon Line can be found
in Edwin Danson's book Drawing the Line).
Mason and Dixon returned to England from America in
October 1768, and, early in 1769, the Royal Society contracted
them to observe the June 6, 1769, transit of Venus. This time,
however, they would not team up. Mason was sent to Cavan,
Ireland, while Dixon voyaged above the Arctic Circle to Hammerfest,
Norway. Mason was able to observe the transit, but
Dixon was clouded out.
After the 1769 transit, Dixon moved back to Cockfield,
northeastern England, where he was born. He died there at
the age of 45 on January 22, 1779. Mason returned to the
Royal Greenwich Observatory to continue unfinished work
from before the 1761 transit on German astronomer Tobias
A MATTER OF OFFSET Mason
used a Dollond divided objectglass
micrometer mounted on the
front his telescope. The left half of
this diagram shows Venus (small
black dot) on the face of the Sun as
it would have appeared with zero
offset of the object-glass halves.
The right pair of illustraions shows
the farthest northern limb of the
Sun superimposed on the farthest
southern limb of Venus by adjusting
the offset. Mason could then read
the separation off the micrometer
scale to gauge the planet's progress
across the face of the Sun.
sk yand tele scope .o r g * MARCH 2022 33
PENNSYLVANIA
OHIO
Mason-Dixon line
MD
WEST
VIRGINIA
VIRGINIA
DE
NJ
DRAWING A LINE As their reward for their work on the transit of
Venus, Mason and Dixon travelled to America to survey a disputed territorial
boundary. The result of their efforts became known as the MasonDixon
Line, as shown in the map above.
Mayer's lunar tables. Mason used the tables to determine
longitude by the lunar distance method, and the Board of
Longitude rewarded him £1,317 for his work. However, by
1786, he felt his scientific efforts were underappreciated, particularly
by Maskelyne, who by then had become the Astronomer
Royal. So, Mason departed England for America in July
1786, accompanied by his wife and eight children. He died on
October 25th of that year at the age of 58, only a few weeks
after arriving in Philadelphia.
Although Mason and Dixon's Venus expeditions failed to
produce astronomically useful results, their efforts and willingness
to risk their lives in pursuit of science ultimately led
them to America, where they surveyed the border for which
they will be forever linked and remembered.
¢ TED RAFFERTY is a retired astronomer who worked for the
U.S. Naval Observatory for 32 years.
OBJECT GLASS USE DIAGRAM: GREGG DINDERMAN / S&T, SOURCE:
AUTHOR ORIGINAL; MASON-DIXON LINE: GREGG DINDERMAN / S&T
http://skyandtelescope.org
Sky & Telescope - March 2022

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