Royal United Services Institution. Journal ISSN: 0035-9289 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/rusi19 Astronomy for the Army Lieutenant E. W. Cox R.E. To cite this article: Lieutenant E. W. Cox R.E. (1911) Astronomy for the Army, Royal United Services Institution. Journal, 55:397, 314-338, DOI: 10.1080/03071841109418035 To link to this article: http://dx.doi.org/10.1080/03071841109418035 Published online: 11 Sep 2009. Submit your article to this journal Article views: 5 View related articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=rusi20 Download by: [University of California, San Diego] Date: 21 June 2016, At: 05:51 ASTRONOMY FOR THE ARhIY l3y Lieutenant E. IV. COX, R.E., zst Field Troop, Aldcrshot. IT is of no practical use ta a soldier to be able to determine his latitude, his longitude, the error of his watch, or the true bearin of any natural feature he can see, with the aid of scientigc instruments. On service he will have only his eyes, a compass and a watch to help him, and it is fair to asume that both compass and watch may be lost or broken. The object of this paper is to show how the sun and stars .can be used to replace either a compass or a watch, and, under certain conditions, to replace both. This practical use of the sun and stars is open to every soldier, however unfamiliar with the science of astronomy he may be. Night operations tend to become more frequent with every campaign, and there is no reason why the stars should not be as helpful to the men of a strategic patrol as they are to the staff officer who directs a large column. It is possible, with the help of a few simple tables, to calculate on any day, and at any place- (a) The time of sunrise and sunset‘, (b) The true bearing, and height above the horizon, of the sun at any lbur of the day, (c) The true bearing of any bright star at rising and setting, and (d) The number of hours that such a star is above the horizon. Wiih these facts, and a knowledge of the apparent movements of the sun and stars in the heavens,it is possible, except when clouds entirely obscure the sky, to use the sun and stars instead Downloaded by [University of California, San Diego] at 05:51 21 June 2016 of landmarks, and after a little practice, instead of watches. As it is obvious that all the movements of celestial bodies, which are referred to in this paper, are apparent movements, the term will not be applied to them again. Before any calculations are attempted, a few astronomical terms must be explained. The meridian of a place is an imaginary circle on the surface of the earth, passing through both the North and South poles, and that place. When the sun is due *N.or S. of a place it is said to be “on the meridian” of that place. In Fig. I are shown the meridians of Greenwich and Calcutta. ASTRONOJIY FOK THE ARXT. 315 The latitude of a place is its distance N. or S. of the equator, measured in degrees. A degree is about 70 land miles, and there are go degrees betmeen either pole and the equator. A place which is nearer either pole than the equator is said to be in a “ high latitude,” while a place which is near the equator is said to be in a “ low latitude.” s In Eig. 2 the thick lines show the latitudes of Greenwich, 51’ N. (high), and of Rio Janeiro, 23’ S. (low). The hgilude of a place is its distance East or West of the meridian of Greenwich, measured in degrees or hqurs. A degree is 4 minutes of time, and, as the earth rotates once in 24 hours, it is clear that there must be 360 degrees of longi- Downloaded by [University of California, San Diego] at 05:51 21 June 2016 tude, 180 East and 180 West of Greenwich. Longitude &n lie measured from any meridian, and in French maps the meridi& of Paris is taken as the “ standard meridian.” In all British maps the meridian of Greenwich is the standard. In Figs. I and 2 the dotted lines show the longitudes of Calcutta, 88” E., and of Rio Janeiro, 43’ W. 316 ASTRONOMY FOR THE ARMY. The celestial sphe7e is an imaginary sphere, infinitely large, with the earth as centre, The heavenly bodies appear to move on the surface of this sphere. The celestial equator is an imaginary line on the surface of the celestial sphere, marking where the latter is cut by the plane of the equator of the earth. To an observer standing on the equator the celestial equator divides the heavens into two equal parts, passing vertically over his head, and meeting the horizon at points due E. and W. of him. To an observer who is not on the equator the celestial equator meets the horizon at points due E. and W. of him, but is tilted towards the N. or S. according as he is South or North of the equator. The N. and S. poles of the heavens are two imaginary Downloaded by [University of California, San Diego] at 05:51 21 June 2016 -points on the celestial sphere vertically above the N. and S. poles of the earth. The sphere of the earth is, as it were, enclosed by the celestial sphere, whose N. and S. poles and equator correspond exactly to the poles and equator .of the earth. In Fig. 3 NQSE is the celestial sphere, N and S are the North and South poles of the heavens, and EQ is the celestial equator, The declination of a heavenly body is ik distance N. or S. of the celestial equator, measured in degrees. ASTRONOMY FOR THE ARMY. 317 In Fig. 4 the thick lines show the declinations of Arciurus (a Bootis), 20' N. and of Sirius ( Canis Majoris), 17' S. Downloaded by [University of California, San Diego] at 05:51 21 June 2016 The declination of the sun is constantly changing, but the declinations of the stars are always the same. The stars appear $18 ASTRONOMY FOR THE ARMY. to revolve round the N. and S. poles of the heavens always at the same distance from the celestial equator. It is clear from Fig. 5 that a star whose declination is, for example, 30's. will, at the highest point of its path, pass directly over the head of an observer in latitude30' S. This connection between declination and latitude is useful mhen the paths of the sun and stars are being traced in the heavens. An observer who wishes to learn the movements of the sun and stars in the heavens must be able to imagine the correct position of the celestial equator. In order to do this he must know his latitude and the direction of the true North. The latitude of any place in civilised countries can be obtained from maps, and even in uncivilised countries an estimate can rarely be 5 degrees wrong. Methods of finding latitude and the direction of true North, where neither a map nor a compass is available, are given below in "Bearings by Stars." An observer, knowing his North point, can imagine,the points,- due E., and W. of him, where the celestial equator meets the horizon. Subtracting Ks latitude from goo he obtains the height above the horizon of the highest point of the celestial equator. This point will be on his meridian, that is due S. of him if he is N. of the equator, and due N. of him if he is S. of the equator. Fig. 6 explains this. FI~6 z H- R Downloaded by [University of California, San Diego] at 05:51 21 June 2016 0 is a place on the earth's surface whose latitude is 50" N. e.g., Dinant, in Belgium). ZNRQSHE is the celestial sphere. I is the imaginary point vertically over 0, called the zenith, N is the North pole of the heavens, EQ is the celestial equator, 'and HR is the horizon. ASTRONOMY FOR THE ARMY.. 31 9 The latitude of 0 is 50" N., that is, the angle EOZ is 50° As the angle ZOH is goo, the angle EOH must be 40°, or, in other words, E, the highest point 'of the celestial equator, is wominus 50" (the latitude) above the horizon. E is also on the line NZS, that is, on the meridian of 0, and therefore, as 0 is North of the equator, E; is due South of 0. To an observer who remains in the same latitude the path of a star through the heavens is always the same, from the time it rises above the horizon to the -time of its setting, although It does not pass across the heavens at the same time every night. If the observer .moves to a place in a different latitude he will notice that the star, though rising and setting at approxi- mately the same points as before, swings up to a different height on the meridian, because the celestial equator is tilted up at a different angle. An observer who knows the declination of a star and his own latitude can, at any moment when the star is above the horizon, trace the rest of its path through the heavens.