Art. XLVIII.—Night Marching by the Stare.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 49, 1916, Page 513

Art. XLVIII.—Night Marching by the Stare. By George Hogben, M.A., F.G.S., C.M.G. [Read before the Wellington Philosophical Society, 1st November, 1916; received by Editors, 30th December, 1916; issued separately, 10th December, 1917] Marching or sailing the ocean by the stars is, of course, no new thing; in fact, owing to the use of the compass the method is less used in modern times than it was of old. But there are many occasions on which the compass is out of order, or not available; besides, only a few officers can be provided with compasses, and oftentimes it falls to the lot of the individual non-commissioned officer or private to determine his direction for himself. It is then that a little star knowledge may be most useful. The first method described below is that recommended to the members of the New Zealand Expeditionary Forces, who have been provided with copies of star maps 1 and 2, with brief directions for their use. Star map No. 1 is familiar to every one; the diagram called star map No. 2 is, I believe, new. What it enables any one to do, on any clear night, is to find the south approximately at any hour on any day of the year. I am well aware that other methods have been used or recommended for use. The chief of these were discussed in connection with a paper read before the Royal Geographical Society of London by Mr. E. A. Reeves, on the 13th April, 1916.* Geog. Journ., vol. xlvii, June 1916, pp. 440–60. The two most important are (i) by the use of Colonel Tilney's tables, which give the bearings of certain selected conspicuous stars for the local mean time; (ii) by the finding of the altitude of a known star above the horizon, and hence inferring its bearing east or west of the meridian—this method being independent of the date or hour. My second method, described below, is a modification of Colonel Tilney's, as will be seen. The first method I have described is subject to an important error due to the varying value of the equation of time at different periods of the year. Accordingly, in Table I, to be used in connection with my second method, the approximate time of meridian transit given for the first day of each month is calculated according to the average value of the equation of time for the month. The error is thus greatly reduced. The simplest way in which I can explain the methods now presented is to give the description as it would be given to an officer who wished to use these methods in the field. Star Map No. 1 (Looking North). To find the North at any Time when the Stars are Visible. The two “Pointers” in the Great Bear (Ursa Major) point very nearly to the North Pole of the sky, which is always due north of you. The distance of the nearer “Pointer” from the North Pole is about five times the distance between the two “Pointers.” (There is a small star, Polaris, near the Pole.) The two bright stars, Vega and Capella, nearly on opposite

sides of the Pole, give a check as to its direction, if the star map be turned round so as to make the position of the Great Bear on it agree with its position in the sky when you are looking north and leaning half back. Star Map No1 (Looking North) Star Map No. 2 (Looking South). 1. Note that the diagram is made for 50° north latitude. 2. Note that the time to be used is the local time of the place of observation. 3. Note that, owing to the daily rotation of the earth about its axis and to its yearly revolution round the sun, the stars appear to have two motions, the daily motion and the annual motion: i.e.,— (a.) On any given day a star moves across the sky, parallel to the Equator, at the rate of 15° an hour, from east to west;* It does not follow that the bearing of a star—east or west, parallel to the horizon—varies on account of (a) by 15° an hour, or on account of (b) by 1° a day (b.) During the year, besides the daily motion, every star has an apparent motion (parallel to the Equator) towards the west of about 30° a month, or nearly 1° a day*; that is, any star is in the meridian (due south) about two hours earlier to-day than it was on the same day last month, or nearly four minutes earlier to-day than it was yesterday. 4. During your voyage to the seat of war you should make yourself familiar, by looking at the sky every clear evening, with the stars shown on the diagram, especially with Aldebaran, Sirius, Regulus, Spica, Antares, Altair, Markab, and perhaps a Arietis. A. First Method.—To find the south approximately for any day and hour (local time). Follow the date-line diagonally up to the right until you come to the horizontal hour-line, the stars on or near the vertical line through that point are on or near the meridian—i.e., due south or nearly due south of you. Check this by the north as found by the Great Bear (star map No. 1). Example on the 1st October, at 8 p.m. The south is somewhat to the left or east of Altair; at 11 p.m. on the same evening the south will

be on the left or east of Markab. On the 8th or 9th October the stars named will be in the position just referred to at about 7.30 p.m. and 10 30 p.m. respectively. (See note 3 (b) above). B. Second Method.—The direction may be found somewhat more exactly by using Tables I and II below in conjunction with the diagram. For instance, supposing a night march is fixed to take place between 8.30 p.m. and midnight on the 24th April. From the diagram (star map No. 2) we see that the stars nearly south during that time are, first, Regulus, and afterwards Spica. Table I shows us that Regulus on the 1st May will be Star Map No 2 (Looking South) (For latitude 50° N) on the meridian about 7.30 p.m.; therefore, on the 24th April (or seven days earlier) it was on the meridian twenty-eight minutes later, say at 8 p.m. Similarly, Spica is on the meridian about 11.15 p.m. on the 24th April. At 8.30 p.m., or thirty minutes after its meridian transit, Regulus (as we see from Table II) is 12° west of south; at 9 p.m., 23° west of south; at 10 p.m., 44° west, or nearly south-west; and so on. In other words, each five minutes it alters its bearing towards the west by nearly 2°. At 8.30 p.m. Spica will have two hours and three-quarters to go before its meridian transit, and its bearing will be about 43° or 44° east of south, or nearly south-east; at 9.15 it will be 32° east of south; at 10.15, 17° east; and so on. In each quarter-hour it alters its bearing about 4°.

We should write beforehand, say on a small card, for use if necessary during the march, the bearings of these stars at quarter-hour or half-hour intervals. Such a card would show entries somewhat as follows:— From South Regulus Spica 8.30 p.m 12° W. 44° E. 9.0 " 23° W. 36° E. 9.30 " 34° W. 29° E 10.0 " 44° W 21° E. 10.30 " 53° W 13° E. 11.0 " 60° W. 4° E From South Spica. Antares 11 30 p.m. 4° W. 36° E. 12 (midnight) 13° W. 30° E. 12 30 a m. 21° W. 24° E 1 0 " 29° W. 17° E. 1 30 " 36° W 11° E 2 0 " 44° W. 4° E 2 30 " 4° W. 3.0 " 11° W. 3 30 " 17° W. 4 0 " 24° W. 4.30 " 30° W. The bearings of Antares are added to our card as a precaution, in case the march takes longer than was expected. Let us suppose that at 9.30 p.m. we have reached a point whence our map shows that we have to advance in a direction 20° west of south. Regulus is then 34° west of south; so that we are to take the direction 14° on the east or left side of Regulus. This angle can be taken off beforehand on our small card, and found on the field by three pins stuck in the card so as to show the angle—the card being held horizontally.* 14° is very nearly the angle subtended by the line joining the tip of the thumb and the tip of the middle finger of the left hand stretched at full length (palm outwards) when looked at with the right eye. Other bodily measurements may be similarly used for determining angles roughly; or even distances between points on the rifle, &c. But, generally speaking, it is well for every man, or, at all events, every officer and non-commissioned officer, to be able to ascertain the general direction; therefore the most important thing is to find the north-and-south line first, and to determine the line of march from that—which may be easily done by placing the map or plan flat on the ground in its true position. In conclusion, I may say that officers at the front have borne testimony to the practical value of the first method described; I trust that the second method (perhaps in conjunction with the first) may prove as useful. If it saves one man's life, I shall feel myself a thousandfold repaid for all the thought spent upon it.

Showing the Approximate (local) Times at which certain Stars are on the Meridian (due South) on the Dates named Table I Star 1 Jan 1 Feb 3 March 2 April 1 May 1 June 1 July 2 Aug 2 Sept 1 Oct 1 Nov. 1 Dec p m p m p m a m a.m a.m. p m. Aldebaran 9 50 7 50 5.50 5 45 3.50 1.50 11.50 a.m. Betelgeux 11 10 9 10 7 10 5.10 3 10 1.10 a m p m. Sirius 12 0 10 0 8 0 6 0 6.0 4.5 2.0 a m p m. Regulus 3 25 1 25 11 25 9 25 7 30 7.25 5.25 a m a m p m p m Spica 6 40 4 40 2 40 12 40 10 45 8 45 6 45 8 40 Arcturus 7 30 5 30 3 30 1 30 11 35 9 35 7 35 a m p m Antares 7 40 5 40 3 40 1 45 11 45 9 45 7 40 a.m. a m p m. p.m. p m. Altair 5 10 3 10 1 10 11 5 9.0 7.5 5 5 a m a m p.m. Markab 4 25 2 20 12.15 10 20 8 20 6.20 At London Sunrise (a m.) 8 8 7.42 6.48 5.35 4 34 3.50 3.49 4.25 5.14 6.2 6.55 7.45 Sunset (p m) 3 58 4 45 5.38 6 32 7 20 8.5 8 19 7.47 6.46 5.38 4.33 3.53 Note—Each star is on the meridian about four minutes earlier on any night than it was the night before. Showing the Approximate Bearing of certain Stars in Degrees East from South at Intervals named before West from South at Intervals named after the Time when on the Meridian (due South); also Meridian Altitudes of the same Stars Table II. Star Bearing in Degrees East or West of South at Times named before or after Meridian Transit. Altitude of Star when on Meridian. 30 mm. 1hr 1hr 30m. 2hr. 2hr. 30 m 3hr. Aldebaran 13 25 36 45 55 63 56° Betelgeux. 11 22 31 41 49 57 47° Sirius 8 16 24 31 38 45 23° Regulus 12 23 34 44 53 60 52° Spica. 8 17 25 32 40 47 29° Arcturus 14 27 39 49 58 66 60° Antares 7 14 20 27 33 39 14° Altair 11 22 32 41 50 58 49° Markab 12 24 35 45 54 62 55°