Personal Equation, or Personal Ereoe, is the error which any particular observer is found to make in his observations quite irrespective of the errors of the instrument. The time when a star crosses the meridian has to be most accurately noted; to do this the astronomer writes down the hour and minutes from the clock when the star enters the field of view of the transit instrument.
He then counts the beats from one particular second, estimating to the tenth of a second when the star crosses each line. This is written down each time, or a revolving cylinder is connected with the pendulum of a clock, and at every second the pendulum breaks an electric circuit and causes a dot to be made on the cylinder. The observer, on seeing the star crossing each line, presses a button which breaks a circuit and also causes the production of a dot. This enables the time to be measured very nearly; but it is found that some observers are late and others premature in their observations, and so it is customary to allow for this. One well-tried observer is, as it were, taken as the zero, and the observations of all the others are reduced to what they would be if he had made them. It is found that anyone's personal error remains constant over an interval of several months; hence when a comparison has once been made between any one and the standard observer the corrections are quite simple. Personal error, of course, comes into play in every sort of measurement, and the less trained the person the greater is the error, but it is practically only in astronomy that allowance is made for it.