For example, the variable may lie two thirds of the way in brightness from the fainter comparison star to the brighter. Using a denominator greater than 5 becomes difficult. The observations are later reduced by applying the ratio to the catalog brightness of the two comparison stars. This method may at first appear easier than the step method, but appearances are often deceptive. In addition to a well-trained eye, careful record keeping is required for useful observations.
Some variables change brightness slowly over periods of weeks or months.
Useful light curves can be built up only from carefully recorded observations that include the observing conditions and the comparison method used. There are several worldwide organizations dedicated to collecting and utilizing amateur observations of variables stars. Both of these organizations publish stars charts specifically designed for variable star observing, with comparison stars clearly marked. Amateurs considering embarking on a variable star observing program would do well to consult one of the many books on the subject. The Webb Society Deep-Sky Observer's Handbook, vol.