VENUS AS A DAY SIGHT
By Lieutenant R. B. Carney, U. S. Navy
The problem of accurately determining the ship's position during the day is one that presents many difficulties and varying errors. For the most part, it seldom occurs that proper cross-bearings can be obtained and the navigator is obliged to fall back on that rather uncertain quantity, the run to local apparent noon. On ships whose missions permit of steady steaming on an uninterrupted course, the calculation of the correct interval to noon presents few real difficulties and no real disadvantages, but there are many times in the course of naval work where it is well-nigh impossible to properly run the ante-meridian sight up to local apparent noon with any degree of certainty. The navigator cannot be considered in the matter of maneuvers, and this very question of maneuvers gives rise to an error that varies in amount with the character of the evolutions performed, and with the nature of the ship performing them.
Particularly in latitudes where the range of solar azimuths is small, that is to say where at no time is the sun near the prime vertical, the greatest accuracy is necessary in order that a good noon fix can be secured; this is evident from a consideration of the intersection of Hues inclined at a small angle, and it is also evident that a small mistake in the matter of the ante-meridian line will result in a magnified error when the line is run up to local apparent noon. This accuracy of advancing the line is a hopeless task where there are numerous changes of course such as are encountered in destroyers or other craft engaged in screen, scout, maneuvering, or attack work, and it was this contingency that led the writer to consider the feasibility of using Venus in conjunction with the sun during daylight hours, as a method of determining position without reference to a sun sight previously taken and run up.
Venus is generally visible to the naked eye some time before sunset as the evening star, and this very fact seemed to indicate that it was only a matter of locating the planet at any time during the day in order to make use of it. This use of Venus is by no means original with the writer; it has been used for years by older navigators, but its general use does not seem to be known among junior members of the craft and it is for their information that this paper has been prepared.
The problem, then, is the location of Venus during daylight hours when it is not discernible to the naked eye, and its solution is so simple as to make it practical, quick, and accurate. It becomes purely a matter of working backward with the star identification tables. Let us first consider the general method involved before taking up a concrete example.
A glance at the chronometer will give an approximate idea of the G. M. T. Apply the data from page two of the Nautical Almanac to obtain the G. S. T. and to this apply the D. R. longitude to determine the L. S. T. From the ephemeris of Venus take out the right ascension and declination roughly, applying the R. A. to the local sidereal time to determine the hour angle. The hour angle and declination, together with the D. R. latitude, are now known— the necessary arguments for entering the star identification tables.
Turning to the identification table with the latitude nearest to that of the D. R. position, find by inspection that part of the table where the tabulated hour angle and declination correspond to those predetermined for Venus; pick out the azimuth and altitude corresponding to these arguments, having due regard to the direction in which the tabulated azimuth must be applied, and also to the fact that should the declination be marked minus ( — ) it is to be named opposite to the name of the latitude. It is unnecessary to interpolate more closely than is possible by inspection.
With this altitude and azimuth set the sextant and pelorus accordingly. Owing to the fact that these calculations are all made by inspection and are not exactly accurate it has been found advisable for purposes of practical work to use the solar telescope attached to the sextant. This telescope facilitates the location of the star by virtue of its magnification but at the same time offers little hindrance owing to its relatively large field. Venus, although invisible to the naked eye, shows up surprisingly plain when viewed through the solar telescope in this manner.
The steps enumerated sound rather complicated on the face of it, but in actual practice it is not more than a matter of two or three minutes' work to obtain the necessary information to properly set the sextant and pelorus. An example will serve to illustrate the method more fully.
On February 8, 1921, in about latitude 21 30 00 South, longitude 76 30 00 West, destroyer maneuvers in conjunction with the battle fleet made it impossible to accurately run up the antemeridian line to local apparent noon. The latitude was easily determined by catching the sun on the dip, but the longitude could not be set down with any degree of certainty. So at about 12.55 p. m., the chronometer was seen to read 6.02 ; allowing five minutes for working out the data, and knowing that the C. C. was 2 m. 24 s., it was evident that five minutes hence the G. M. T. would be roughly 6 04 36. Applying the correction for G. M. T found on page two (2) Nautical Almanac, the G. S. T. was found to be 3 17 47.6. Subtracting the longitude (5h. 06m. 00s. West) gave the L. S. T. By inspection the R. A. of Venus was seen to be about oh. 21m. The hour angle was therefore 22 11 47 and the declination was roughly taken as 3 25 00 North.
Chro. (allowing five minutes 6 07 00
C.C (-) 2 24
G.M.T. 6 04 36
21 12 11.8
59.8
G.S.T. 3 17 47.6
Long. (D.R.) 5 06 00
L.S.T. 22 11 47.6
R.A. (Approx.) 21 00
H.A. Venus 21 50 46.6
Or 2 09 12.4 East
Dec. Venus 3 25 00 North
Entering the star identification tables, it was seen that the above hour angle and declination were closely approximated in juxtaposition where the altitude was 52 00 00 and the azimuth South 128 East or 52 True. The sextant and pelorus were set accordingly and Venus was at once picked up near the horizon close to the calculated bearings. The actual altitude and azimuth were found to be 50 31 00 and 50 00 00 respectively when the sight was taken.
The ease with which Venus may be located in this manner is surprising and the advantage of its use in connection with the sun is self-evident. Numerous methods of application of Venus as a day sight will suggest themselves to the navigator. For example, the altitude may be predicted for local apparent noon and a cross obtained, the meridian transit of Venus may be predetermined and simultaneously taken with the sun. In point of fact only the roughest assumptions need be made in regard to the selected G. M. T.; if the H. A. is worked to minutes by inspection no difficulty is experienced in locating the planet.
Jupiter offers almost the same advantages at certain times, depending upon its determining coordinates, and the writer has even at times been able to make use of Sirius before sunset, although in the case of the latter more exact calculations are necessary.
The use of Venus and Jupiter, however, is commended to all navigators as a most simple and efficient method of eliminating the uncertainty entering into all calculations that necessitate the advancing of one line to another through a period that may cover hours of steaming through unknown currents or doubtful errors due to service conditions.