Noon-interval tables

United States. Hydrographic Office

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O. No. 202R
NOON-INTERVAL TABLES
PUBUSHED BY THE HYDROGRAPHIC OFFICE
UNDER THE AUTHORHT OF THE
SECRETARY OF THE NAVY
WASHINGTON
COVERNMENT PRINTING OFFICB
1920No. 202H. O.
NOON-INTERVAL TABLES
FIRST EDITION
PUBLISHED AND SOLD BY THE HYDROGRAPHIC OFFICE
UNDER THE AUTHORITY OF THE
SECRETARY OF THE NAVY
PRICE 90 CENTS
WASHINGTON
GOVERNMENT PRINTING OFFICE
1920AUTHORIZATIONSTATUTES OF
Navigation in theHydrographic Office attached to the Bureau ofThere shall be a
navigating safely the vesselsfor the improvement of the means forNavy Department,
providing, under the authority of theof the mercantile marine, byof the Navy and
nautical charts, sailing directions, navigators,the Navy, accurate and cheapSecretary of
all vessels of the United States, and for themanuals of instructions for the use ofand
(R.benefit and use of navigators generally. S. 431.)
is authorized to cause to be prepared, at the Hydro-The Secretary of the Navy
Bureau of Navigation in the Navy Department, maps,graphic Office attached to the
and to publish andbooks relating to and required in navigation,charts, and nautical
and to purchase the platesnavigators at the cost of printing and paper,furnish them to
sailing directions, and instruc-of such existing maps, charts, navigators,and copyrights
may deem it expedient to do so, andmay consider necessary, and when hetions, as he
may prescribe. (R. S.such regulations and instructions as he 432.)under^*
NOON-INTERVAL TABLES
The result computed from the morning time-sight of the sun, usually stated as the
local apparent time, gives also the angle at the pole, or difference of longitude expressed
in hours, minutes, and seconds, between the meridian of the observer and the meridian
passing through the geographical position of the sun, or that place on the surface of
the globe which has the sun in its zenith at the instant of observation. When the
remains stationary,observer the interval of time that must elapse before the sun crosses
his meridian, or the arrival of apparent isnoon, therefore given by the number of hours,
seconds in this angleminutes, and at the pole, or hour-angle, which interval is equal to
remainder found by subtracting fromthe 12 hours the local apparent time. But when
observer is changing his place, if his movementthe has a component of change of place
to the eastward or to the westward, that is, a component of change of longitude, the
interval to noon will be shortened or lengthened according as his change of place causes
him to move to the eastward to meet the sun, or to the westward to be overtaken by
the sun.
The rate of change of longitude of the sun's geographical position in its diurnal
path from east to west is 15° or 900' per hour, and if to this be added the hourly change
longitude of the observer when his change of place is to the eastward, or ifin from it be
subtracted his hourly change in longitude when his change of place is to the westward,
the result will be the rate of approach per hour of the meridian of the sun toward the
of the observer, expressed in minutes of arc ofmeridian longitude. And this rate of
divided into the number of minutesapproach being of arc expressing the difference of
which separates the meridians of the sunlongitude and the observer at the instant of
morning time-sight, will give the interval to, noon expressedthe in hours and fractions
hour, provided the course and speed of the vessel remainof an uniform or, at least,
the rate of change of longitude continues the same throughout.
With the aim of obviating the necessity on the part of navigators of arithmetical
computations in finding the interval to noon for use in setting the ship's clock to the
time of apparent noon and in ascertaining the run of the vessel from the morning time-
sight until noon, the midshipmen of the class of 192 1 of the United States Naval Academy,
serving in the U. S. S. Wisconsin of the Midshipmen Practice Squadron, during the
summer of worked out values extending throughout the scope of the present1919, tables,
under the direction of Lieut. Commander W. D. La Mont, United States Navy, by pro-
to formulaceeding according the deduced by Midshipman (now Lieut. Commander)
of class ofChapman Coleman Todd, jr., the of 1913 the United States Naval Academy.
subsequent revision of this work in the Hydrographic Office theBy resultsnow presented
in the tables were obtained.
Side by side in each double column of the tables the interval to noon is stated in
hours and decimals of an hour to the fourth decimal place, and also in hours, minutes,
and seconds, reckoned from the local apparent time stated at the head of the column.
424441:
TOINTERVAL NOON4
rangingcorresponding to values of the hourly change of longitude of the observerand
and the westward. The local apparent times forfrom i' to 40', both to the eastward to
are tabulated extend from a. m. to 10 a. m. everywhich the intervals to noon 7 30
seconds.
of illustration the following example is givenFor purposes
66°navigator of a vessel steaming on a course (true) finds from an a. m.The
8^ 00" 8^observation of the sun, taken at \V. T. 02". that the L. A. T. is 17™ 30^,5,
38° 03. continues coursecorresponding to the latitude by D, R. '2 N. The vessel on this
until Find the interval that will have elapsed whenat a speed of 11.7 knots noon.
to noon, and the change that will necessary in the settingnoon arrives, also the run be
tliat it will indicate 12 o'clock when noon arrives.of the watch so
From the Traverse Tables the departure corresponding to the distance 11.7 on
course 66°, is which, in latitude 38°, is equivalent to a difference of longitude of10.69,
3.
'6. Entering the Noon-Interval Tables in the column whose heading is L. A. T.1
gh j^m 13'2cf^ and interpolating between the tabulated values for hourly change of
14'longitude to the eastw^ard and hourly change of longitude to the eastward, the
13. hourly of to willinterval to noon corresponding to '6 change the eastward
3^ 39"" 11^. givesbe found to be 3''.653i, or Multiplying 11.7 by 3.6531 42.7 nautical
8^ 3*^miles as run to noon. of noon would be 00" plus 1",the The W. T. 02^.5 39™ 1
or 11^ 39™ hence the watch would have to be set ahead 20™ 46^.5 in order to13^.5;
indicate 12 o'clock upon the arrival of apparent noon.