NA TURE 



[AUGU 



ST I 



1896 



factory results. There is, however, in the minds of many 

 mariners an ill-defined idea that any problem not de- 

 pending on the sim is too difficult to be meddled with. 

 lUit so great a practical authority as Captain Lecky, in 

 " Wrinkles on Navigation," says : 



" For four or five months of the year navigation in our 

 own latitudes is a much less ticklish affair when the stars 

 are brought into action. In most cases they can be 

 selected on or nearly on the prime vertical during twilight, 

 and will therefore give a very reliable longitude." 



The object of the present tables is to render Sumner's 

 method applicable to the moon and planets in all 

 •cases, and to such bright stars as lie up to 30' of de- 

 clination. The stars tabulated on page vii. of the pre- 

 face are practically all available for longitude, and nearly 

 all for latitude also. 



The moon for about two-thirds of each month lies within 

 the limits of" Burdwood and Davis," but for the remaining 

 third it lies outside ; and, as an example of a planet. 

 Mars in 1896 has a higher northern declination than 23" 

 from October 3 onwards. 



When it is considered that the simultaneous observa- 

 tion of the sun and moon, already referred to, is admitted 

 to be of great practical use even by those who are 

 sceptical as to the general utility of observations of the 

 moon, tables which permit Sumner's method to be ap- 

 plied to the moon at all times, at once establish their 

 practical utility ; and further, referring to the above- 

 quoted opinion of Captain Lecky, it is to be remarked — 



'• That it is the very stars between 23''' and 30° of de- 

 clination (same name as latitude) which are particularly 

 suitable for observation on the prime vertical, for the 

 reason that in our latitudes such bodies have at the time 

 a convenient altitude, whereas those of lower declination 

 are too near the horizon when they have a bearing due 

 east or west." 



In this case again the practical value of the tables is 

 obvious. 



A somewhat interesting illustration of the value of 

 observations of the stars is given in an article in the 

 Nautical Magazine (June i8g6;. The article treats of 

 the cross-currents said to be experienced in the Red Sea. 

 Their existence was inferred from the discrepancy often 

 noticed in the position of a ship as obtained from a.m. 

 or p.m. observations of the sun. Recently, however, it 

 has been considered that the discrepancy arises from the 

 abnormal refraction experienced in the Red Sea, by 

 means of which the position of the horizon is altered 

 sufficiently to account for large errors in longitude. 



In order to endeavour to settle the question numerous 

 observations have been made, and the results tabulated. 

 In a report on the subject by the Hydrographer of the 

 United States, giving a detailed account of what has been 

 done, the following paragraph occurs. 



" The good agreement obtained between the results 

 of the observation of dawn and twilight stars shows that 

 e.xcessive refraction is less frequent at those hours than 

 at other hours of the day.",^-- 



Tables, therefore, which tend to promote and simplify 

 observations of the stars, when a ship is traversing a sea 

 so full of dangers to navigation as the Red Sea has 

 shown itself to be, should prove to be a boon. 



In "Burdwood and Davis" the arguments are latitude, 

 declination, and time. In the present tables "altitude " 

 NO. 1398, VOL. 54] 



takes the place of " time " in the principal table. Table B 

 is of use in the case of observations made when the 

 altitude is greater than 55', or is within 2' of the meridian 

 altitude, the change in altitude being then so slow in 

 comparison with the change of azimuth that it can no 

 longer be regarded as a suitable argument. Table B is 

 also of use in Sumner's method when an observation is 

 made near the meridian, and the latitude calculated with 

 an assumed longitude. 



The use of altitude as an argument gives somewhat 

 less minute results than the four-minute intervals, but 

 there are great compensating advantages. The hour 

 angle of the sun (or ship apparent time) is very readily 

 found ; but to find the hour angle of the moon, a planet, 

 or a star, is a much more complicated matter. It in- 

 volves a knowledge of ship time, right ascension of the 

 sun, and right ascension of the body, presenting in 

 addition a somewhat puzzling variety of cases. But the 

 altitude of a star, &c., can be accurately observed in a 

 iew moments in the twilight when latitude or longitude 

 is required, and can be observed with quite sufficient 

 accuracy at any time, when visible, for the purpose of 

 obtaining the azimuth or true bearing. 



And this leads on to the second reason for the publica- 

 tion of these tables. 



The process of observing the compass bearing of a 

 bright star has been of late years very much facilitated 

 by the introduction of Lord Kelvin's compass, so that 

 the mariner has now the means of obtaining the error of 

 his compass and of checking his deviation table at any 

 time of the day or night when any of the heavenly bodies 

 are visible. Here again we may quote Captain Lecky, 

 who says : " It is perfectly wonderful how few men avail 

 themselves of the stars on a fine night to see how their 

 compasses are behaving." These tables, with the simple 

 argument of a fairly correct altitude in place of a com- 

 plicated hour angle, should render the practice of star 

 observations for compass error as frequent as they are 

 simple. 



Such observation would appear to be most useful to 

 the navigating officer of a ship of high speed, who may 

 find that the variation has altered several degrees be- 

 tween sunset and sunrise. For example, in a ship 

 steaming in the direction of New York, from latitude 

 45' N., longitude 60^ W., it would be found that the 

 variation had altered about 8' in a rim of 300 miles, a 

 distance that might very easily be traversed between 

 such times as the sun was available for observation. It 

 must therefore be of great assistance to the navigator 

 that he should have a certain means of checking his 

 compass error, not by the sun only, but by any heavenly 

 body, of suitable and easily ascertained altitude, that 

 may be visible. 



Example iii. shows very clearly the value of the pro- 

 cess (Examples i. and ii. showing the use of the tables 

 as an aid to Sumner's method in the case of the stars 

 and moon). 



The methods for determining compass error show con- 

 tinual development. Formerly it was found by amplitude 

 only, i.e. by the azimuth of the sun when its centre is on 

 the horizon. 



But, on account of refraction, the suns centre appears 

 to be about a diameter abo\c the hori/.on when it is 



