Sky & Telescope - March 2020 - 61

LE AH TISCIONE / S&T

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users to understand the underlying mathematics but only
to follow a number of well-deﬁned steps, looking up the
necessary astronomical data from the Nautical Almanac and
logarithm values from another book.
By the time of Cook's second voyage (1772 to 1775), he
brought along a few chronometers that were copies of Harrison's H4 clock. Only one proved reliable, the one made by
Larcum Kendall and referred to as K1, though Cook always
called it "the watch." The chronometers were set to Greenwich Time and were meant to eliminate the need for universal clocks based on astronomical phenomena. But astronomical observations were still necessary for determining local
time. At ﬁrst, Cook was reluctant to trust the timekeeping
capabilities of chronometers and continued to use lunar distances to determine longitude.
To overcome the uncertainties that crept into the K1
watch's accuracy over long timespans, Cook revisited two
locations whose longitudes he'd determined during his ﬁrst
voyage. These were Venus Point in Tahiti (where he observed
the June 3, 1769, transit of Venus) and Ship Cove, on the
northern tip of the South Island of New Zealand, where he
spent an extended period repairing his ship.
Over the course of his three voyages, Cook visited Venus
Point ﬁve times and Ship Cove six times. Knowing the
longitudes of both locations, he could reset the time of the
Kendall watch. He discovered that it worked very well indeed.
Cook departed Ship Cove in November 1773 and returned 11
months later after sailing a counterclockwise route around
the southern Paciﬁc from latitude 71°S near Antarctica to
9°S in the tropics. The time kept by the Kendall watch (after
applying its known correction rate) was found to be off by
just 19 minutes and 31 seconds. Although not as accurate
as the lunar distance method, it was much faster and easier
to use, and it eliminated the possibility of observational and
computational error. Cook was so impressed that he took the
K1 on his third voyage as well.
The astronomical equipment that Cook carried included
sextants, a telescopic quadrant, telescopes (both refractor and
reﬂector), and an astronomical pendulum clock. Although
he and his assistants could use the sextants both at sea and
on land, the other instruments were useful only on land.
Each time Cook dropped anchor for fresh water and food,
ﬁrewood, ship repairs, or to rest his crew, he took the opportunity to set up a tent observatory. Weather permitting, he'd
make observations in order to determine a more accurate latitude and longitude than he could obtain at sea. Professional
astronomers accompanied Cook on all his voyages to oversee
the observations and computations, though his own observations often proved at least as accurate as theirs.

Charles Green (1734-1771) was Cook's astronomer for his
ﬁrst voyage on the ship Endeavour. The British Navy supplied
two ships for Cook's second voyage: the Resolution (with
Cook as captain) and the Adventure (with Tobias Furneaux
as captain). William Wales (1734-1798) was the astronomer on the Resolution and William Bayly (1737-1810) on
the Adventure. Two ships also went on the third voyage: the
Resolution (with Cook as captain) and the Discovery (with
Charles Clerke as captain). Bayly served as the astronomer
on the Discovery, while Cook himself took on the duties of
astronomer aboard the Resolution.

Longitude and Time
15° longitude =
1 solar hour =
1.003 sidereal hours

Sunlight

Noon
10 a.m.

2 p.m.

0° N,
30° W
0° N,
60° W

Direction of
Earth's rotation

0° N,
0° W

Longitude and Time
Your orientation in space is determined by your latitude
and the local time, so those values are relatively easy to
measure astronomically. For instance, it's local noon by
definition when the Sun is at its highest. And the Sun's
zenith distance at that moment is equal to your latitude
minus the Sun's declination, which can be found for
any given day in an astronomical almanac.
Longitude is much harder because all people at any
given latitude go through the same cycle of space orientations, regardless of their longitude. Two hours from
now, a person 30° directly west of you will see the Sun
at the same spot in the sky that you do right now. But
if you have a clock (either mechanical or astronomical) that tells you the time in Greenwich, at longitude
0°, you can compute your longitude by subtracting
Greenwich Time from local time and dividing by 15.
Conversely, if you are at a spot with known longitude,
you can compute Greenwich Time from local time.

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Sky & Telescope - March 2020

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