Origins of the Calendar
There are three natural divisions of time on Earth. The most obvious is the
alternation of night and day, and all calendars are organized in terms of the 24-hour
day, which is the approximate average length. However, not all calendars are orga-
nized in terms of hours with a fixed length. Our ability to measure seconds and even
minutes accurately was achieved only in modern times. A medieval sundial dividedCalendar
[108] THEASTROLOGYBOOK
The famous Aztec calendar can be read in terms of cycles covering thousands of years.
Reproduced by permission of Fortean Picture Library.