(from μετα, post, and πτωσις, cado, "I fall.") a term in chronology, expressing the solar equation, necessary to prevent the new moon from happening a day too late. By which it stands contradistinguished from proemptosis, which signifies the lunar equation, necessary to prevent the new moon from happening a day too soon.

The new moons running a little backwards, that is, coming a day too soon at the end of 312 years and a half; by the proemptosis, a day is added every 300 years, and another every 2400 years: on the other hand, by the metemptosis, a bixfextile is suppressed each 134 years; that is, three times in 400 years. These alterations are never made but at the end of each century; that period being very remarkable, and rendering the practice of the calendar easy.

There are three rules for making this addition or suppression of the bixfextile day, and, by consequence, for changing the index of the epacts. 1. When there is a metemptosis without a proemptosis, the next following, or lower index, must be taken. 2. When there is a proemptosis without a metemptosis, the next preceding or superior index is to be taken. 3. When there are both a metemptosis and a proemptosis, or when there is neither the one nor the other, the same index is preserved. Thus, in 1600, we had D: in 1700, by reason of the metemptosis, C was taken: in 1800, there was both a proemptosis and a metemptosis; so the same index was retained. In 1900, there will be a metemptosis again, when B will be taken: which will be preserved in 2000, because there will then be neither the one nor the other. This is as far as we need compute for it: But Clavius has calculated a cycle of 301,800 years; at the end of which period, the same indices return in the same order.

See FACT.