This _magic_, or prophetic study of the stars, we call _astrology_ to distinguish it from the true science of _astronomy_. But mingled with it all, these priests possessed a wonderful amount of genuine scientific knowledge. Their year of three hundred and sixty days was, of course, five days too short, as they presently found out for themselves. In six years, the difference would amount to thirty days, which was exactly the length of one of their months. So they corrected the calendar very easily by doubling the month Adar once in six years. Thus, every sixth year contained thirteen months instead of twelve; that was the origin of the leap-year principle which we still use, although more accurately. It can be seen that, with all their superstition and their befooling of other people, the priests themselves were by no means ignorant; they were really keen observers. This calendar, by which we still measure the years and the seasons, is so interesting a thing that it is worth while to pause for a moment in our story in order to trace out its later development. The Babylonian calendar remained practically the same up to the time of Julius Caesar, only a few years before the Christian Epoch. The names of the months had naturally been changed into the Latin language; and the Romans, instead of doubling a whole month, had come to add the extra five days to several months, one day to each. That is the reason for some of our months having thirty-one days. When Caesar was Dictator of Rome, it had become known that the year of exactly 365 days was still a little too short. It should have been 365¼. So Caesar in reforming the calendar, provided that the first, third, fifth, seventh, ninth, and eleventh months should be given thirty-one days each, and that the others should have thirty days, except in the case of February which should have its thirtieth day only once in four years. A little later, his successor, the Emperor Augustus, after whom the month of August is named, decided that _his_ month must be as long as July, which was Julius Caesar's month. Therefore, he stole a day from February and added one to August; then he changed the following months by making September and November thirty-day months and giving thirty-one days to October and December.