# Advent calendar 2015: week one Inspire article

Welcome to the Science in School Advent calendar for 2015.

For each week of Advent we will be sharing some inspiring teaching ideas for Christmas, winter and the end of term. To begin with we thought we should look at what a calendar is and how we will be counting down the days until the end of term (and beyond).

A year is how we measure one revolution of our planet, Earth, around the central star in our system, the Sun. Every revolution takes just over 365 days and so every four years, to stop our seasons starting to drift, we add an extra day to our modern Gregorian calendar. We call these leap years.

According to the Gregorian calendar, there are three rules used to determine if a year is leap year or not a leap year.

• Rule 1: Leap year is divisible by four
• Rule 2: Exception to Rule 1, any year divisible by 100 such as 1900 or 1800 is not a leap year
• Rule 3: Exception to Rule 2, any year divisible by 400 is a leap year such as 2000

Putting 2016 into these sums easily shows that next year will be a leap year.

The other planets in our solar system rotate at different speeds and so have different length days and years. The Nasa Mars site can help you explore the days and years of Mars and your students could try to figure out a suitable calendar for the Martian year. And if you think the year and day on Mars are strange, be sure to check out Mercury’s orbit data from European Southern Observatory.

Over on the Exploratorium, you can even calculate your age on other planets and on Pluto. Pluto, of course, is no longer considered a true planet. In 2011, Mike Brown explained to us how he unintentionally sealed Pluto’s fate and triggered our new understanding of what is – and what is not – a planet.

Of course the Gregorian calendar that we use today was not the first calendar system to use leap years, and many ancient civilisations also had extremely accurate calendar systems based on astronomical observations and mathematics. The Maya from Mesoamerica, for example, had several different calendar systems and contemporary Maya farmers still use astronomy and their knowledge of the cycles of the Sun and other stars to determine the production cycle of maize. The Smithsonian National Museum of the American Indian in the USA has teaching activities on Mayan maths and astronomical measurements in both English and Spanish.

And if we wanted to travel back in time to see the ancient Maya in action, or just find a little extra time for our Christmas shopping, could we do it? Einstein’s theory of relativity tells us that space and time are connected and that in theory time travel is possible but in 2009 Jim Al-Khalili bet that time travel to the past will soon be shown to be impossible even in theory. This activity from the American PBS NOVA site helps explore some of the concepts raised by Al-Khalili, including the grandfather paradox.

So if we can’t make extra time through exploiting Einstein, perhaps the way to get ahead is to combine lessons and Christmas chores. For example, home-made cards are always appreciated, but these are illustrated using geometrical designs.

Meanwhile, the maths department of the University of Sheffield, UK, has created a series of equations to help make a mathematically perfect Christmas tree. They do provide a handy calculator on the site but you might like to get your students to put the numbers in individually, or suggest changes to the formulae. There’s lots of maths you can do if you assume that your Christmas tree is a perfect cone.

Well that’s it until next week, but if you still want some more, we’ve also collected together the advent calendars we posted in 2010 and 2012. Be sure to check out all the links there as well!

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