ionysius was carried out with
regard to the numbers by which the years were to be named and called.
Thus the year which had been known as 754 became, under the new system,
the year 1. And the succession of years from that year 1 is called the
Christian era. To get the numbers of its years you have only to subtract
753 from the years in the Roman numbering."

"If we add 753," said Joe, "to 1880, will we get the number of years
since old Rome was founded by Romulus and Remus?"

"Yes," said the Professor; "the rule works both ways. There is, however,
some uncertainty as to whether the Romans themselves were correct in
regard to the age of their city. Very early dates are hard to settle."

"Where did the months get their names?" asked May, "and how did months
come to be thought of at all?"

"The months were suggested by the moon. In most languages the word
_month_ is very nearly like _moon_, as you see it is in ours. From new
moon around to new moon again is about twenty-nine days, which is nearly
the length of a month. The exact time between two new moons is a very
puzzling problem. It always involves a troublesome fraction of a day,
and is, in fact, never twice alike. So it was found convenient to divide
the year into twelve parts, nearly equal, and to call each one a month."

"Why didn't they make them just equal?" asked Gus.

"To do so would have made it necessary to split up some of the days,
which would have been awkward. If you divide the 365 days of the year by
twelve, there will be five remaining."

"How was it found out that the year had 365 days in it?" asked Joe.

"It took the astronomers to do that," said the Professor; "and until
nations became civilized enough to study astronomy accurately, they did
not know the number of days in the year. This, however, did not prevent
them from being able to count the years, because they could know that
every time summer or winter came, a year had passed since the last
summer or winter. But now the length of the year--that is, the time
occupied by the earth in going completely round the sun--is known within
a fraction of a second."

"Was it worth while to go into it so precisely?" asked May. "Would it
not have been enough to know the number of the days?"

"By no means," said the Professor. "For then the calendar could not have
been regulated so that the months and festivals would keep pace with the
seasons. If 365 days had been constantly taken for a year, Christm