ERROR ILat. error, from errare, to wander). In observations of every kind errors are un avoidable. As in astronomy correctness in the result of instrumental measurements is of the first eonseonence, it is the constant care of the observer to defect and make allowances for errors. The three principal sources from which they may arise are: (1) External or incidental causes, such as fluctuations of weather, which disturb the amount of refraction; changes of temperature, affecting the form and position of instruments, etc. (2) Errors of observation, be ing such as arise from inexpertness, defective vision, slowness in seizing the exact instant of an occurrence, atmospheric indistinctness. etc.; and such errors as arise from slips in clamping and momentary derangements of the instrument. (3) Instrumental defects, owing to errors in work manship, and such as arise from the instrument not being properly placed ('errors of adjust ment'). The first two classes of errors, so far as they cannot be reduced to known laws, alter the results of observations to their full extent; but being accidental, they necessarily sometimes di minish and sometimes increase them. Bence, by
taking numerous observations under various cir cumstances, and by taking the mean, or average, of the results obtained, these errors may be made to counterbalance one another partially, and to that extent they may be rendered harmless. With regard to the third class, it is the peculiarity of astronomical observations to be the ultimate means of detection of all defects of workmanship and adjustment of instruments, which, by their minuteness, elude every other mode of detection, and such errors, when found out, can almost in variably be removed. It may be mentioned, how ever, that the method of subduing errors of the first two classes by the law of average is not applicable in all cases. In certain cases, recourse must be had to a system of reduction or calcula tion, known as the method of least squares. See