Not long after graduation in June, 1895, I joined the Atlanta at the New York Navy Yard, and the end of that month found me at sea in a ship with a speed of about 8 knots, bound for Port-au-Prince.
My experience in practical navigation began with that cruise and continued, more or less intermittently, until my seagoing ended April 1, 1935. During my 2-year cruise I was assistant navigator for nearly a year, and for a time I had the same duty as ensign. I was navigator of two ships for short times and navigator of a battleship from July, 1909, to June, 1912. The foregoing is mentioned solely to show that I had fully average experience in practical navigation.
The forenoon meridian altitude of Venus was an old friend, but in the summer of 1933 I learned of a sight that was new to me. I have asked many experienced navigators about this sight, but not one had ever heard of it. One officer did write me that he had heard of it, but I have never talked to him about it. Very recently I asked the navigator of one of our new cruisers about this sight and he knew nothing about it. Because of the apparent lack of knowledge of the sight, I am writing this article in the hope that it may be of assistance to navigators.
In 1933 I was temporary custodian of a new, and very fine, sextant. While at anchor off the south end of San Clemente Island, I went on deck after sunset, but many minutes before dark, to try the new sextant. The gyrocompass was not running, so I made no effort to determine exactly the north point of the horizon. I did not compute the altitude of Polaris, but set the sextant at an angle equal to the latitude of the anchorage.
Moving the sextant sufficiently to insure covering true North, I eventually saw in the horizon glass an image that was so bright I thought it must be a light on shore, as there was no very bright star in the part of the sky I was observing. I could see no light on shore, so I continued to observe the reflected image at short intervals of time. Finally Polaris became visible to my eye, and the image in the sextant was unquestionably the reflected image of that star.
After this first observation, I continued to work over the matter and finally worked out the following procedure which was satisfactory to me and which I invariably followed in taking the sight:
Determine the most probable position of the ship for a time 15 minutes after sunset. Using this time and position, compute the altitude of Polaris, and set the sextant for this altitude. Fifteen minutes after sunset begin looking for the reflected image, taking observations about once a minute until the reflected image is seen.[1] When the image is in plain sight and the horizon is still perfectly clear, bring the reflected image to the horizon and record the reading of the sextant and time of observation.
To check the sight, decrease the sextant angle by one minute and observe the apparent distance the image is above the horizon; then shift the reading to one minute more than the observed altitude and observe the distance the image seems below the horizon. To me, the apparent distance above the horizon was about twice the apparent distance below it. This was my personal equation, and others might have different personal equations. But this difference bears out what a fine marksman told me about keeping the sights of a rifle below the black of the bull’s-eye. There is ample time for this checking, as the altitude changes slowly, if at all, and there should be sufficient daylight for this purpose.
With the latitude accurately known, sights of two stars, one to the eastward, the other to the westward, should give a very accurate “fix” and should save several sights to the average navigator whom I have observed taking star sights.
In the morning, no computation is necessary, as the approximate altitude may be observed while the star is in sight; then accurate observations may be taken as soon as the horizon is clear.
On one occasion while I was in the Battle Force, I took this sight with excellent result, and I found the dead-reckoning latitude about 5 miles in error. Not long afterwards the navigator appeared to take sights and found the northern sky obscured, while much of the remainder was partly cloudy. I gave him an accurate latitude, something he could not obtain.
A description of this sight was issued in a memorandum to the Battle Force, with copies to the Hydrographic Office and various Force Commanders. Later, I wrote the Hydrographer, asking him to publish it for the information of navigators in general. I was told the sight would be described in the new edition of the American Practical Navigator.
After many years of experience, I consider this sight the most useful sight there is, always excepting the meridian altitude of the sun.
“Understond wet that the latitude of any place in a regioun is verreyly the space by-twixt the senith of hem that dwellen there and the equinoxial cercle north or southe, taking the mesure in the meridional lyne, as sheweth in the almikanteras of thyne astrolabie.”—Chaucer, as quoted in The Romance of Navigation, Captain W. B. Whall.
[1] I once saw the reflected image 15 minutes after sunset. This was in the winter, and I generally saw the image from 18 to 20 minutes after sunset.