# Sun, Moon and Earth

45 44 deSIgnIng a calendar 13 months of 28 days Accurate timekeeping begins with a calendar, which ideally should relate both to the solar year and the Moons phases, divide easily into weeks, months and seasons and show us the current phase and position of the Moon, the tides and when to expect eclipses. A calendar designer soon discovers a solution based on the Moon passing the same star in the sky every 27.3 days while the Sun takes 365. Dividing one by the other suggests a calendar of 13 months each of 28 days within a 364day year, a useful representation of astronomical truth. 364 is divisible by 2, 4, 7 and 13, so we end up with a 52week year with 13 months, or four 13 week seasons, each of 91 days. All whole numbers and every year a leap year Practically, our designer could arrange 28 markers around the perimeter of a circle and arrange for a moonpole to be moved anticlockwise once a day, about 13o. A sunpole would then be moved in the same direction, only thirteen times more slowly, one hole every 13 days. This is the most simple way of providing a practical solilunar calendar, only requiring occasional resetting of the Moon place the moonpole opposite the sunpole when full. If the stars were then represented on the circumference of the circle, the date, season, state of the tides see page 18, lunar position and lunar phase could be read off at a glance. The ancient Maya were keen astronomers and calendarkeepers and also keen on the numbers 13 and 364. They came up with some extraordinary gearings between the Sun and Moon lower opposite. A basic calendar with 28 holes, each hole representing one day for the Moon, or 13 days for the Sun. Yields a 364day year and needs occasional retuning. THE MOON IN THE MAYAN CALENDAR Ritual Year, or Tzolkin 260 days 13 x 20 days Civil Year, or Tun 360 days 18 x 20 days Vague Year, or Haab 365 days, so that One Calendar Round 52 Haab 73 Tzolkin From the Dresden Codex and inscribed stones at Palenque we know 149 lunations 220 x 20 days 99.999 accurate 405 lunations 46 x 260 days, or 81 lunations 2392 days 99.999 256 lunations 21 x 360 days, or 32 lunations 945 days 99.998