Published May 2008,February 2011.
It is important to distinguish the difference between the
astronomical movements of the earth, due to the influence of the
moon and the sun, from the choices made by different cultures about
the division of the day into 'hours' and the year into 'months' or
'seasons'. From the earliest times, people had difficulties in
matching the motion of the moon (the lunar months) with the
apparent duration of the year as measured by the equinoxes.
While we divide the year into 12 months, and use this for our
international calendar, other cultures have had (and still have)
different ways of organising the year. It would be interesting to
explore the Chinese, Indian, Hebrew and Islamic calendars that are
still in use today, and the Mayan calendar, where they calculated a
slightly better value for the length of the year than the Gregorian
Calendar For more information, see http://astro.nmsu.edu/~lhuber/leaphist.html
While we base much of our payment of bills and salaries on the
calendar month, the state pension is paid into bank accounts every
Lunar Month (so pensioners receive 13 instalments each year). What
other transactions are managed in a similar way? Which cultures
traditionally have different dates for commercial transactions?
What are the advantages and disadvantages of analogue and
digital representations of time? Pupils often see sports events on
TV with a timer on the screen. Which sports display times in
minutes, in seconds, in tenths of a second, (or even smaller) and
why is it important? Where do pupils regularly meet time
represented as a 24 hour system? Many questions like this occur as
soon as we start talking about the nature of time and our use of
time measurement, rather than just the arithmetic of 'telling the
Sun time and clock time are only the same at four dates during
the year. The variation has two causes; the plane of the Earth's
equator is inclined to the Earth's orbit around the Sun, and the
orbit of the Earth around the sun is an ellipse and not a circle.
The National Maritime Museum website shows two separate graphs for
these causes, and a third graph where they are combined to give the
full correction. If you have a sundial available nearby, check your
clock time against the sundial time at different times of the year
and plot the variation on a graph. For older pupils this is a good
practical example of how two graphs can be 'added' together.