TEACHER'S GUIDE
 From "Math and Science Across Cultures" by Maurice Bazin, Modesto Tamez, and the Exploratorium Teacher Institute

The Mayan Calendar Round:
Keeping Time

Did you know the Maya had two calendars? One was tied to the astronomical year, and had 365 days like ours. The other was shorter, 260 days, and was associated with rituals.

The Pyramid of El Castillo in Chichén Itzá was built to reflect the Mayan astronomical year. Each side has 91 steps with a final step at the top, so there are 365 steps total. The Pyramid was constructed so that it marks the equinoxes—the two days of the year when there are equal amounts of day and night. On these days, the sun casts an image of a snake that appears to move down the pyramid. You can see it happen by watching our webcast from Chichén Itzá.

The cycles of time were central to Mayan life, so the times when the two calendars came together were considered special events. This activity will help you understand how the Maya figured out when those coinciding days would occur.

History and Nature of Science: History of Science

History and Nature of Science: Historical Perspectives

• Principles and Standards for School Mathematics
http://illuminations.nctm.org/info/standards.asp

Numbers and Operations

Key Concepts
Gear ratios, smallest common multiple, prime factors, prime numbers

Background Science
"How Gear Ratios Work"
from How Stuff Works
http://science.howstuffworks.com/gear-ratio.htm

History and information about the Mayan calendar
From Web Exhibits, by the Institute for Dynamic Education Advancement
http://webexhibits.org/calendars/calendar-mayan.html

Ideas for Discussion
• One event that happens for you annually is your birthday. What if you were born on February 29? That "leap day" occurs only once every four years. How often could you celebrate your real birthday? How "old" would you be now if you counted only that date as your birthday? How "old" would the oldest person in your family be?

• What role do cycles of time play for us in the modern world? Often students have defined schedules at school and after school, and parents have regular schedules as well. Can you think of a time in your life when you've had to look through two different schedules to find coinciding dates? How did you do it?

Going Further
• Most calendars have been developed in relation to astronomical events and the change of seasons. What events do you think our current calendar is based on? What things in the sky change, and how long do they take to go through their cycles?

• Many cultures have their own calendars. What other number systems have been used for calendars? Do you know any holidays or events that follow a different calendar than the one you usually use? You can read about different calendars on the Web Exhibits Web site:
http://webexhibits.org/calendars/calendar.html