Categorized | Experiments

Make a perpetual calendar

calendarYou will need

* Scissors
* Split pin or drawing pin and eraser
* Printout of the two discs, on separate pieces of paper. You can download them here: download

To make the calendar

1. Cut out the smaller disc.
2. Cut out the thick dark window on the smaller disc.
3. Put the smaller disc on top of the larger one, and push a split pin through the middle of both discs. If you don’t have a split pin, put an eraser underneath both discs and push a drawing pin through the centre of both discs and into the eraser.

To use the calendar

1. First, find the year on the outside disc.
2. Then, find the month on the inside disc. Rotate the two discs so the year and month line up.
3. The calendar on the smaller disc will tell you the days of the week for the month and year you have chosen.
4. Remember that some months have fewer than 31 days. Only read the dates that exist!

What’s happening?

This little device contains calendars for every month from January 2000 to December 2027. If you work out the patterns for where the years are written, you can use it to find the calendar for any year!

Our calendar is based on the Julian calendar, started by Julius Caesar in 45 BCE. The Julian calendar has a leap year every 4 years, and it repeats every 28 years. For any two days 28 years apart (say 28 February 2012 and 28 February 2040) it will be the same day of the week (Tuesday).

The calendar we currently use – the Gregorian calendar – occasionally skips leap years on years ending in 00. As a result, sometimes it doesn’t repeat every 28 years. However, it will repeat every 28 years until the end of the century.

Our current calendar repeats every 400 years, so 28 February 2412 will also be a Tuesday!

Although the days of the week will be the same whenever the calendar repeats, the date for Easter each year depends on the position of the Moon. As a result, the date for Easter changes each year, and doesn’t repeat on a 28 year or a 400 year cycle.

experiment 1 What you will need.
experiment 2 Cut out the window on the smaller disc.
experiment 3 Line up the year and month (indicated in orange) and read the calendar (blue).
experiment 4 If you don’t have a split pin, you can use a drawing pin and eraser.

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