CHAP. XIII.] OBSERVATIONS FOR ERROR 297 



to the middle time, to reduce it to apparent noon, which cor- 

 rection is termed the " equation of equal altitudes." 



The observation of stars at equal altitudes will therefore be, Stars and 

 theoretically, the best to use, as being the simplest, and they pared, 

 will indeed give as good results as those of the sun ; but 

 practically the latter has generally been observed in marine 

 surveying for the purpose of obtaining time. In many cases 

 the inconvenience of landing, and carrying watches backwards 

 and forwards for comparison, etc., by night, besides the in- 

 creased difficulties of observing and reading instruments by 

 lamp-light, lead to the choice of day observations ; but in 

 places where clouds persistently veil the sun in forenoon or 

 afternoon, the nights are often clear, and equal altitudes of 

 stars become most valuable. 



There are two other methods of obtaining Error by stars, other 

 that are both good : the first, observing stars of corresponding yjy^stara 

 altitude on either side of the meridian, working them out as 

 absolute altitudes and meaning the results ; the other is a 

 method to wliich attention has been drawn by Captain A. M. 

 Field, R.N., of observing two different stars of nearly the same 

 declination at equal altitudes. This is published as a pamphlet 

 by the Hydrographic Office, but is briefly described on 

 p. 316. The great advantage of either of these is that the 

 complete observation is made in a very short time, thereby 

 obviating the disadvantages of star observations mentioned 

 above. 



In taking the observations of equal altitudes in the artificial Limita- 

 horizon, we are limited, as always, to altitudes between 20° observa- 

 and 60°, as the horizon will not permit us to observe a lower tions. 

 altitude than 20°, and the sextant will not measure much move 

 than 120°. These restrictions will, however, only be incon- 

 veniences, as regards the sun, in extreme latitudes, as we 

 must choose as om" time of observation, so as to minimise 

 the effects of errors of observation, the period at which its 

 motion in altitude is the greatest — i.e., when it is near the 

 prime vertical — at wliich time, in all but liigh altitudes, the 

 altitude will come between our limits. When the place of ob- 

 servation is near the Equator, and the latitude and declination 

 are nearly the same, we could observe up to a very short time 

 of noon, the sun's motion in altitude being nearly uniform 



