|
 Nuno
da Silva, a Portuguese captain and pilot whose
merchant ship was captured by Francis Drake during
his epic circumnavigation of the globe in 1577 -
80, made the following comments on Drake's skill as
a navigator: "The first thing he did when he had
captured a vessel was to seize the charts,
astrolabes, and mariner's compasses . . . He
carries three books of navigation, one in French,
one in English, and another, the account of
Magellan's voyage, in a language I do not know . .
. He is a very skillful mariner . . ." The book in
English that Drake was very likely carrying aboard
the Golden Hind when he departed Plymouth, England,
in 1577, was either the first or second edition of
William Bourne's A Regiment for the Sea,
published in London in 1574 and 1576, respectively.
Bourne's Regiment was far and away the most current
and important English book on navigation available
at the time of Drake's departure. As such, it is
hard to conceive that Drake, the foremost English
navigator of the age, would not have been carrying
it.
"I do knowe that euery person that goes
vnto the Sea as maister of a shippe, hath not
capacitie to calculate the Sunnes declination by
the place of the Sunne [in the zodiak]
although they have the tables of declination, as
the Ephemerides, of Martin Curtyse
[Cortes]." William
Bourne
See
a comparison of the Tables of Solar Declination by
Martin Cortes with those of William
Bourne.
Though its seems to have escaped the notice of
past researchers, the fact that Drake was likely
using Bourne's Regiment (or at least its tables) to
instruct him in his navigational techniques and to
make his computations of latitude, gives us a
useful means of identifying the location of some of
his disputed landfalls. Drake's "Nova Albion"
landfall on the coast of present day California is
among them. The World Encompassed by Sir
Francis Drake (the 2nd, bart.) (1626), the most
detailed extant account of Drake's
circumnavigation, includes two latitude readings
for Nova Albion: the latitude given for the "Port
of New  Albion"
is "38 deg. 30 min."; the latitude given for
the furthest point Drake explored to the south
along the coast of Nova Albion is "38 deg."
With the instruments of the time, how would Drake
have made these determinations of latitude, and how
accurate would they be? By reference to the
Regiment, we can answer these questions with a
considerable degree of confidence.
|
 Note:
For comprehensive histories of the
publications of Drake's The World
Encompassed and Hakluyt's The
Famous Voyage, see: Kelleher, Brian
T., Drake's Bay, Unravelling
California's Great Maritime Mystery,
Day Publishing, 1997, and Thrower, Norman
J. W., (editor) Sir Francis Drake and
the Famous Voyage, 1577-1580,
University of California Press, 1984
|
The
World Encompassed covering the days addressed
in this article.
DRAKE'S INSTRUMENTS OF
NAVIGATION
In Drake's day there were three instruments that
mariners used to make celestial sightings for use
in computing their latitude: the quadrant, the
cross staff, and the astrolabe. The
sixteenth-century mariner had no means for
measuring longitude. The ability to measure
longitude practically and accurately, particularly
at sea, was150 years in the future with Harrison's
invention of the sea chronometer.
The quadrant was the first instrument
developed for use in celestial navigation, dating
back to the fifteenth century. It was a very simple
devise constructed of wood or metal in the form of
a quarter circle with degree graduations along the
arc. It had sights along one of the radial arms and
a plumb-bob suspended from the right angle. The
observer held the quadrant with the arc straight
down and looked up through the sights at the sun or
star. When his sights were aligned, he simply held
the plumb-line fast against the face of the
quadrant between his finger and thumb and read off
the altitude from the scale. In Drake's day, the
quadrant was probably most commonly employed on
land to make Pole-Star observations.
The cross-staff in use in 1579 was a simple
device that worked reasonably well for measuring
the angle of the sun above the horizon at noon. It
was fitted with one movable vane (transversary)
that, with the end of the staff placed at the eye
of the observer, was positioned so that it appeared
to touch both the horizon and the sun. The angle
was then read from a scale on the staff. In his
Regiment, Bourne admonishes the navigator against
using the cross-staff for measuring an altitude of
over 50°, the maximum angle that the sun and
the horizon could be taken in together with one
glance. The cross-staff was the method of resort on
a rocking ship since its use did not rely on
gravity. Illustrated at right is a polaris shot. It
also illustrates the problem with polaris
(see below).
Edward
Wright's comments--warnings and
reservations
The mariner's astrolabe in common use by
English seamen at the time was a wheel-shaped,
cast-brass instrument of perhaps 7 or 8 inches in
diameter with a thumb ring at the top.  The
ring mount was designed to allow the instrument to
hang vertically plumb and to provide for precise
rotational control by the user. The disk was
divided into four quadrants, two or more of which
had scales divided into 90 degrees each. The
astrolabe had a rotating sighting arm (alhidada),
mounted through the center. Though the astrolabe
offered a reliable and accurate method of measuring
altitude, the mariner's ability to read the degree
scales along the rim was a limiting factor on the
precision of the observation. Since each degree
division for a 7-inch diameter instrument was only
about one-sixteenth of an inch, the mariner could
read the angle only to the nearest half degree. As
with the quadrant, the mariner's ability to make an
astrolabe sighting at sea could be completely
frustrated by movement of the ship.
More
about what constitutes a Mariner's
Astrolabe.
The astrolabe was best suited for use on land
where it could be held steady in hand or suspended
from a tripod. For his land-based readings,
however, Drake, may have been using a simple
astronomical ring or a larger astrolabe such as a
2-foot diameter instrument known to have been
produced in England by Humphrey Cole in 1575. Due
to their size and weight, these larger instruments
were not practical for use at sea. With the stable
foundation and larger scale (almost 1/4 inch per
degree for a 2-foot ring), the mariner could easily
make his altitude readings to the nearest quarter
of a degree and could at least attempt readings to
the nearest 10 minutes of arc.
DRAKE'S METHODS OF
NAVIGATION
There
were two common methods by which Elizabethan
navigators made celestial determinations of
latitude in the Northern Hemisphere: they made
sightings of the sun at noon; and they made
sightings of the Pole Star (Polaris, the North
Star) during any hour of darkness. From the
following passages in The
World Encompassed, it appears that Drake,
weather permitting, may have employed both
techniques during his California sojourn: "neither
could we [during their stay in California]
at any time in whole fourteen days together
[presumably toward the beginning of their
stay], find the air so clear as to be able to
take the height of sun or star."
In the evaluations to follow, it will be assumed
that when he actually recorded a latitude in his
log, Drake, given the choice, would always have
used his noon observations of the  sun
rather than his pole-star observations. This is
because for sixteenth-century navigators like
Drake, the noon sighting was, in modern-day
terminology, the reference method. Though it
involved more complicated calculations than the
Pole-Star method, it was the tried and true method
for obtaining latitude by which the accuracy of
other techniques such as dead reckoning and
Pole-Star readings were measured. In addition,
Drake, as a practical astronomer and practiced
navigator, would have had much experience with both
Pole Star and solar sightings. He would have made
many comparisons of his results at known positions,
and would surely have come to realize what was
formally brought to light by Edward Wright when he
published Certain Errors in Navigation at
the very end of the sixteenth century: "I must for
the present only give the mariner warning that he
not trust to it [the Pole-Star reading],
being very erroneous, and grounded on two false
positions."
Thus, to accurately measure the latitude of his
Port of Nova Albion Drake would have measured the
sun's height above the horizon at noon (meridian
altitude). Being near the summer solstice, the sun
was too high in the sky to allow him to measure the
angle with a cross-staff. Drake would use his
astrolabe.
 Whatever
sized astrolabe he used, as the sun approached
noon, Drake would begin to track its final east to
west ascent by simultaneously rotating the disk
itself along its vertical axis while adjusting the
alhidada to keep the rays of the sun passing
parallel through the two small holes on the front
and rear sights. At noon, the sun would finally
stop climbing, seem to stand still for a few
moments, and then begin to descend. With the tip of
the alhidada pointing to the position on the
quadrant scale where the day's maximum rise had
occurred, Drake would record the instrument
altitude.
Averaging
a number of observations at noon. Huh?
See
this being done.
Read
William Bourne's instructions for the noon
shot.
To convert the instrument altitude into a
position of latitude, Drake first needed to correct
for any instrument error (index error) to obtain
what is called the observed altitude. The astrolabe
was easily checked for index error by rotating the
instrument 180 degrees, taking sightings from
opposite directions, halving any difference
detected, and then adding or subtracting this value
from the reading.
There were then no corrections for
parallax or dip of the horizon (dip
is not applicable with the astrolabe). There is
also no correction for semi-diameter of the
sun, because it is the center of the sun which
is projected onto the back vane. Drake would next
turn to his astronomical almanac
(ephemeris). This would contain the tables
of changing but predictable astronomical events
(ephemerides) useful to the mariner that
were then called a regiment. Bourne's 1574 Regiment
contains tables of ephemerides, including those for
the declination of the sun north or south of the
equator for four-year cycles including leap year
(year of bissextilis) and covers the years 1573
through 1592.
The year 1579 in Bourne's table falls in the
third year of the cycle. The summer solstice
occurred on June 11 (June 21 by the modern
calendar) when the sun's position in its apparent
annual orbit about the earth (ecliptic) reached the
northern limit of its drift away from the equator
(Tropic of Cancer at 23° 27' North Latitude).
Drake landed at his port of Nova Albion seven days
after the solstice on June 17 when the solar
declination (amount of drift from the equator), by
Bourne's tables, was 23° 24' at noon in
London, and departed on July 23 when the solar
declination was 18° 00'. These declinations
are to be subtracted from the true altitude, which
gives the altitude of the celestial equator at the
point where it crosses the meridian.
EVALUATION OF THE
LATITUDE EVIDENCE FOR NOVA
ALBION
There is no safe harbor at the exact "38 deg. 30
min." position The World Encompassed records
for Drake's California harbor. The closest
candidate anchorage site to the reported position
is Campbell Cove at the mouth of Bodega Harbor in
Sonoma County. Its actual latitude is 38° 18'
19". The site in Marin County's Drakes Bay where
most historians currently believe Drake made port
is located at 38° 02' 04". How well do these
candidate anchorage sites correlate with the
latitudes reported in the account?
From Drake's Bay, Unravelling
California's Great Maritime Mystery with
additions.
  Campbell
Cove/Bodega Harbor. At left is the Cove as it
appeared in the 1960s before alteration by
PG&E. - If Drake had been at Campbell Cove
(actual latitude 38° 18' 19") and made a noon
sighting on, for example, June 30, 1579 (old
calendar), modern-day reckoning tells us that the
sun would have risen to an elevation of 73°
56' above the horizon at noon. Thus, if Drake could
have read his astrolabe to the nearest minute after
corrections for index error, he would have recorded
73° 56' as his observed altitude. From
Bourne's table, he would have found the declination
for June 30th to be 22° 24'. Drake would
subtract the 22° 24' declination of the sun
from his observed altitude of 73° 56'; the
result, 51° 32'. The altitude of the equator
where it crosses the meridian is the complement of
the altitude of the pole--a latitude reading of
38° 28'. We see that if Drake could have read
the astrolabe to the nearest minute, his
measurement by reference to Bourne's tables would
have been 9 minutes high. The reasons for this
positive bias are discussed later on.
See
at Google Maps
See
Drake's the course in the Pacific from
Guatulco.
Capt.
James Colnett's 1790 plan
of his "Port Sir Francis Drake on the Coast of New
Albion," which shows the springs in the cove as
above left, and the location of the Indian
Village.
Drake, however, could not read instrument
altitude to the nearest minute of arc. If we assume
he used either the standard 7-inch astrolabe or
Cole's 2-foot astrolabe, did not make a sighting
error, and corrected for any index error, his best
estimate of the observed altitude in reading the
astrolabe's scale to either the nearest half or
quarter degree, would have been 74°. After
subtracting the solar declination 22° 24' from
74° and subtracting the result from 90°,
he would have arrived at a latitude of 38°
24'.
To determine the latitude of his landfall to the
best of his ability, Drake, as weather permitted,
would have made additional sightings during the
course of his stay to refine his determination. As
a skilled navigator, Drake would have recognized
that his calculated atitude readings would change
somewhat daily due to his inability to read the
ever-changing meridian altitude of the sun to any
closer than half or quarter degree. Repeating the
sightings would have allowed him to average not
only these daily variations, but also any
non-biased sighting error. To assess the
variability of Drake's readings, we must know the
sun's altitude for each of the thirty-seven days of
Drake's California sojourn.
 This
can be determined by creating a table of solor
declinations for 1579. Current tables must be
adjusted backwards by the rule for the obliquity
of the ecliptic--a factor of 468.15 seconds of
arc per thousand years. Adding those declinations,
and a small amount of solar parallax and refraction
to the complement of the latitude being examined,
establishes just where the sun would have been seen
in the sky in that place at noon on the day in
question.
tracking a
virtual sun
In Tables 1 and 2 (appended
below) the true altitudes of the sun at
Campbell Cove and Drakes Estero have been
calculated for each of the days between June 17 and
July 23, 1579. Though we do not know on what days
Drake actually made his sightings, by reference to
these tables we can make some useful observations.
For example, if Drake took sightings at Campbell
Cove (38° 18' 19") with the 7-inch
mariner's astrolabe on the day he landed (June 21),
and repeated the observations weekly (on June 28,
July 5, 12, 19), the results for Campbell Cove
would have looked like this:
38° 13', 38° 37', 38° 13',
38° 30', and 38° 30';
the average is
38° 26'.
If Drake had used a larger instrument and
recorded the altitude to the nearest quarter
degree, the result for the same five days would
be:
38° 28', 38° 22', 38° 28',
38° 30', and 38° 30';
the average is
38° 28'.
For the entire thirty-seven days of Drake's
California sojourn, the average of all Campbell
Cove readings made to the nearest half degree is
38° 26', and for the nearest quarter degree is
38° 28'. If we assume that the two-week period
the Elizabethans could not make a sighting due to
fog was at the beginning of the sojourn, the
averages are a little higher: 38° 33' and
38° 30'. Though the averages for the smaller
and larger instruments are close, the advantage of
the larger instrument in reducing the range of the
extremes is apparent.
Because of the difficulty of doing division by
the galley, or scratch method of dividing numbers
in the sixteenth century, Drake would have
estimated an average from his results, which in
this example, he very likely
would have called "38 deg. 30 min." for either the
larger or smaller instrument. Thus, based on the
results of this evaluation, there is certainly very
good reason to believe that Campbell Cove is indeed
the location where Drake logged the "38 deg. 30
min." reported in The World Encompassed.
reading
the scale.
 Drake's
Cove/Drakes Bay - The estero at Drake's Bay has
long been the favored candidate anchorage site, as
evidenced by its name on modern maps. The mouth of
the estero (Drake's Cove) is at a latitude
of 38° 02' 04". If Drake had been there
during his thirty-seven day California sojourn, the
weekly series of readings described above would
look like this for readings made to the nearest
half degree:
38° 13', 38° 07', 38° 13',
38° 00', 38° 00';
the average is
38°
07'.
for readings made to the nearest quarter degree
38° 13',
37° 58', 38° 07', 38° 13',
38° 15', 38° 15';
the average is
38°
09'.
The average for all the thirty-seven days for
the estero when read to either half or quarter
degrees would probably have recorded the latitude
either as "38 deg." or "38 deg. 10 min.", depending
on the size of the instrument. He certainly would
not have logged the "38 deg. 30 min." reading
reported in The World Encompassed.
The Farallons - According toThe World
Encompassed after departing the port of Nova
Albion on July 23, 1579, Drake arrived at certain
islands the next day out, July 24. From there he
set sail on July 25 en route across the Pacific for
the Mollucas. Assuming Drake did indeed harbor in
the San Francisco area, these certain islands had
to have been today's Farallon Islands, the largest
of which stands at about 37° 42'. The
Farallons thus mark the southern extent of Drake's
exploration of the California Coast which The
World Encompassed reports to have been "38
deg." By modern calculation the altitude of the sun
at noon July 24, 1579 was 68° 50'. If Drake
had taken an astrolabe sighting for the Farallons
that day, he probably would have measured the
altitude of the sun as 70° even and,
after reduction, recorded the latitude as "38 deg."
Given the relatively short distance from port,
Drake may also have approximated the reported "38
deg." latitude by dead reckoning. If so, this would
be consistent with a July 23 departure from
Campbell Cove which is roughly a half degree north
of the Farallons. Such is not the case for Drakes
Estero, however, which is only about 1/4 degree
north of the Farallons.
SOURCES
OF ERROR IN DRAKES
MEASUREMENTS
Pole-Star Readings - The false positions to
which Wright refers to with respect to Pole-Star
readings concern the corrections that must be
applied to account for its angular distance from
the pole. Polaris, in our century, rotates within
about plus or minus 1 degree of arc about the pole
every 24 hours. Thus, the observed altitude of
Polaris today is within about a degree of the
latitude. In the sixteenth century, however, the
distance of Polaris from the pole was significantly
greater (about 3 degrees), and not accurately
determined until the very end of the century.
Various authorities placed the distance at from
4° 9' (Johann Werner in 1541) to 3° 30'
(Martin Cortes in 1551). Bourne in 1574 also put
the angle at 3° 30' minutes, which seems to
have been the most accepted figure until 1599, when
Edward Wright corrected this to 2° 52'. Thus,
assuming Drake was using Bourne's value of 3°
30', his Pole-Star readings, depending on the time
of day of the observation and the relative
positions of the stars, were subject to an error
from this source alone that ranged from zero to
plus or minus just over a half degree.
There were other sources of error. After taking
a sighting of the altitude of Polaris for purposes
of latitude determination, it was necessary to
correct for the distance of the star above or below
the pole at the time of time of the observation. To
do this Drake may have been using Bourne's
Regiment of the North Star which gave the
approximate degrees of correction to apply based on
the relative position of Polaris to the Guard Stars
of Ursa Minor (Little Bear or Little Dipper) which
rotates about the pole approximately once each day.
To obtain a more refined value for the angle of
correction, Drake could have employed a simple
astrolabe-derived device similar to a nocturnal in
conjunction with an associated table. Either way,
however, the correction for altitude was an
approximation that was apt to increase the error of
the Pole-Star latitude measurement. Sighting error
was another problem due to the small size of the
star and the lack of optical instruments.
Additional errors associated with cross-staff
observations are discussed below.
Sighting
problem.
Read
about changes in polaris since Drake's
time
Solar Readings - It was fortunate for our
purposes that Drake was compelled to use the
astrolabe to make solar observations during his
California sojourn, because the astrolabe provided
several advantages over the cross-staff that
navigators were not aware of at the time.
The problems with the cross staff were first
comprehensively addressed by Edward Wright in
Certain Errors in Navigation in 1599. The
principle problem was instrument parallax. If the
end of the staff was not placed so that the top
surface was in the exact center of the eye, and the
scale (because of the length of the staff) was not
an exact distance from the focal plane of the eye,
the angle would not be determined correctly. Bourne
noted this problem, but offered no practical
solution. Because there is no solution. The error
can be quite large, a whole degree of arc or
more.
Improvements
in the cross staff.
Unlike the cross staff, the astrolabe or
quadrant were not subject to errors of instrument
parallax. Because it is referenced by gravity to
the center of the earth, a visible horizon was not
needed (a distinct advantage on the foggy
California coast) and the sighting needed no
correction for dip of the horizon. The error due to
dip (not addressable in 1579) would have amounted
to 3 to 4 minutes of arc from a sighting made from
the deck of a ship of the time, or from that same
elevation on land. Furthermore, because the rays of
the sun passing through a hole in the front sight
of the alhidada pass through another hole in the
back sight, both limbs of the sun are the
reference, and there is no necessary correction for
semi-diameter of the sun. The errors of solar
refraction and solar parallax for altitude
correction that would apply to the time and
location of Drake's Nova Albion sighting amount to
only 17 seconds of arc, and while not addressed
here, are too small to be considered in sighting
with the relatively course instruments of the
time.
Notwithstanding the advantages of the astrolabe
over the cross staff, Drake's latitude
determinations for the thirty-seven days in
California would have been high by 4 to 11 minutes
of arc with the error increasing with time. There
are two causes for this increasing positive bias in
the readings during those particular days.
First, Bourne's declination tables published in
1574 contained errors which are due in part to the
apparent slow and swift motions of the sun between
the solstices and equinoxes.
Edward
Wright's comments
These slow and swift motions were not fully
explained until Kepler's laws of planetary
motion.
Copernicus
Bourne's declination table puts the sun on the
June 11th solstice as 23° 28' (for the
longitude of London). The correct figure is two
minutes lower at 23° 26'. As the days progress
forward, however, away from the solstice, the error
in Bourne's tables increases, so that by the time
Drake left the coast, the error resulting from
Bourne's tables would have been about 6 minutes of
arc.
Secondly, Bourne's declination tables, which
were derived for London, had to be corrected for
longitude when used anywhere significantly east or
west of London. Bourne notes in Regiment that his
solar declinations "will serve for all Europe,
without much error, or any other country or place
that has our longitude, as the most parts of
Africa, as Ginnie..." Mariners in 1579, however,
with no way to measure their longitude presumably
ignored the problem .
We know for certain that Drake did not
correct his solar declinations for longitude.
Because after having sailed 360° west and
returned to Plymouth, he found his own reckoning of
the date to be off by one whole day. According to
the last paragraph of The World Encompassed,
the day of his return was "Monday in the just and
ordinary reckoning of those that had stayed at home
in one place or country, but in our computation was
the Lord's Day, or Sunday."
|
 Circumnavigation,
H.M.S. Beagle, Captain Robert
FitzRoy.
Charles Darwin, Tahiti, November,
1835:
16th: "This was our Sunday, but the
Monday of Tahiti: if the case had been
reversed, we should not have received a
single visit, for the injunction [of
the missionaries] not to launch a
single canoe on the Sabbath is rigidly
obeyed.
 17th:
"This day is reckoned in the log-book as
Tuesday the 17th, instead of Monday the
16th, owing to our, so far, successful
chase of the sun."
|
By the time Drake had reached the coast of
California, he had traveled one third of the way
around the world from London (W120° or eight
hours, although Drake would have called it
east 240°). At noon in London on the
17th of June, the north declination of the sun by
Bourne's tables, was 23° 24'. Adjusted eight
hours from London, however, Drake, had he known,
should have corrected 23° 24' to 23° 22'
for June 17, a difference of 2 minutes of arc. When
he departed California, longitudinal error had
increased to 5 minutes of arc as the rate of change
in declination increased. By July 23, 1579, the
combined errors from the use of Bourne's tables,
and the fact that he did not correct for longitude,
resulted in a total positive bias of about 11
minutes of arc.
|
 Latitude:
Interpolation between
dates.
In the example at right, on the 25th
the declination is 4° 30'.
Twenty-four hours later, on the 26th, it
is 4° 52'.
To be able to determine the correct
latitude at a place at 123 West longitude,
about 8 hours of time must be added to the
declination of the 25th by
interpolation.
Even if one understood the necessity,
and had a good approximation of their
longitude, interpolation between days was
difficult.
Here
an example of interpolation as taught by
Eden in his translation of Martin
Cortes.
D. W. Waters, The Art of
Navigation in England in Elizabethan and
Early Stuart Times, Yale University
Press, 1958:
Accuracy [of
determinations] depended in great part
upon the correct calculation of
declination. This in turn depended upon
the conversion into time of the difference
in longitude between the position and the
meridian for which the declination tables
were calculated, a problem skated over so
far [1594] by all the manuals.
As late as 1705, Pére
Labat would write, "I only report the
longitude [of Hispaniola] to warn
the reader that nothing is more uncertain,
and that no method used up to the present
to find longitude has produced anything
fixed and certain."
After Drake's
circumnavigation:
Richard Madox, Diary, 1582.
I considered that all ephemerides
which are calculated according to the
latitude o f
any place have certainty of truth no wher
but in the same longitude wher the
observation was taken, which is a note
ether not heeded or not as I knoe by
any yet published.
Edward Wright, Certaine Errors in
Navigation, 1599: Seamen do take
the sun's declination out of their
regiments without any equation by addition
or subtraction of the part proportional
agreeable to the difference of longitude
of the place where they are . . . so may
be deceived sometimes 10 or 12 minutes in
taking the sun's declination.
Edward Wright
further illustrates the problem and
presents the solution in Certain Errors in
Navigation, 1599.
Longitude=Time:
how easy it is today to carry the correct
time about.
|
The bias in Drake's latitude determinations
resulting from his use of Bourne's declination
tables was not always positive. The error for a
given longitude is dependent on the season relative
to the solstices and equinoxes. For example, for
the longitude of Drake's Nova Albion landfall, for
the plus or minus 91 days from the summer solstice
to the fall and spring equinoxes, the error would
range from plus 15 minutes to minus 13 minutes of
arc.
CONCLUSIONS
The error in Drake's reported determinations
come from three components: reading error (+/-),
his longitude west of London (+), and errors in
Bourne's tables of solar declination (+/-). The
first and last of these will be either additive or
subtractive depending on the day and the season of
the year. It is possible for some errors to
compensate for others. Therefore,
there is no constant error; every
determination must be looked at on an individual
basis.
Though the 4 to 11 minutes of positive bias in
Drake's astrolabe readings for this particular
range of days in the 1579 calendar is relatively
small, it appears to have considerable significance
with regard to the long-standing debate about the
location of Drake's port of Nova Albion. The
results of this analyses demonstrate that if Drake
had harbored at Sonoma County's Campbell Cove (true
latitude 38° 18' 19") in June 1579 and made a
competent measurement of the latitude using the
instruments and methods available at the time, the
value
he most likely would have recorded would have been
"38 deg. 30 min.," just as reported in the most
detailed account of the voyage. The results of this
analysis further show that the "38 deg." latitude
The World Encompassed reports for Drake's
furthest exploration south is consistent with what
Drake would have recorded at the Farallon Islands
(actual latitude 37° 42'). The reported
latitude of the Farallons is also consistent with a
July 23 departure from Campbell Cove since the
Farallons are located roughly a day's sail and a
half degree south of this port. Conversely, the
latitude evidence in The World Encompassed
does not support a sojourn at today's Drakes Bay or
points south.
|
Due
to its comprehensiveness, what follows is
quite a lot of information. If you are
curious enough to proceed, it will cover
the following:
reducing
a celestial observation to a
position.
This
article vailable as pdf
download
(27 pages, 756KB)
Dear Bob,
For
a look at an experiment to test the conclusions of the
above, click
the astrolabe.
email interest, comments, or questions
Circumnavigation.
Tom Chaffin, the most recent of Frémont's
biographers (Pathfinder: John Charles
Frémont and the Course of American
Empire) and long-time friend
of this website, has recently had his new book
Sea of Gray released. Here
Tom visits Frémont's Long Camp site.
Winter
2006: Sea
of Gray : The
Around-the-World Odyssey of the Confederate Raider
Shenandoah
