But as this made the year somewhat too long, and the error in 1582 amounted to ten days, Pope Gregory XIII., in order to bring the vernal equinox back to the 21st of March again, ordered ten days to be struck out of that year, calling the next day after the 4th of October the 15th; and, to prevent similar confusion in the future, he decreed that three leap-years should be omitted in the course of every four hundred years. This way of reckoning time is called _New Style_. It was immediately adopted by most of the European nations, but was not accepted by the English until the year 1752. The error then amounted to eleven days, which were taken from the month of September by calling the 3d of that month the 14th. The Old Style is still retained by Russia. According to the Gregorian calendar, _every year whose number is divisible by four_ is a _leap-year_, except, that, _in the case of the years whose numbers are exact hundreds, those only are leap-years which are divisible by four after cutting off the last two figures_. Thus the years 1600, 2000, 2400, etc., are leap-years; 1700, 1800, 1900, 2100, 2200, etc., are not. The error will not amount to a day in over three thousand years. 84. _The Dominical Letter._--The _dominical letter_ for any year is that which we often see placed against Sunday in the almanacs, and is always one of the first seven in the alphabet. Since a common year consists of 365 days, if this number is divided by seven (the number of days in a week), there will be a remainder of one: hence a year commonly begins one day later in the week than the preceding one did. If a year of 365 days begins on Sunday, the next will begin on Monday; if it begins on Thursday, the next will begin on Friday; and so on. If Sunday falls on the 1st of January, the _first_ letter of the alphabet, or _A_, is the _dominical letter_. If Sunday falls on the 7th of January (as it will the next year, unless the first is leap-year), the _seventh_ letter, _G_, is the dominical letter. If Sunday falls on the 6th of January (as it will the third year, unless the first or second is leap-year), the _sixth_ letter, _F_, will be the dominical letter. Thus, if there were no leap-years, the dominical letters would regularly follow a retrograde order, _G_, _F_, _E_, _D_, _C_, _B_, _A_. But _leap_-years have 366 days, which, divided by seven, leaves two remainder: hence the years following leap-years will begin two days later in the week than the leap-years did. To prevent the interruption which would hence occur in the order of the dominical letters, leap-years have _two_ dominical letters, one indicating Sunday till the 29th of February, and the other for the rest of the year.