Stephanie Thompson, of Corran School, asks :-

Why is there an extra day every four years?

Tim Armstrong, the physicist in charge of New Zealand's national time standards at Industrial Research Ltd, responded.

To understand the answer to your question it is necessary to first understand what makes a year. A year is the period of time required for the earth to make one complete revolution around the sun. This a commonly called a solar year. The period of time required for a solar year is close to 365 and a quarter days.

The calender that we use, called the Gregorian calender, has exactly 365 days in a normal year so each calender year is slightly shorter than the time required for a complete revolution of the earth around the sun. We therefore add an extra day every four years to keep the calender year in line with the sun.

The loss of a quarter of a day every four years may not seem too important. However, if we didn't make this correction, then after a number of years all of the seasons would get out of line with the calender. After 800 years had passed December on the calender would correspond to the middle of the solar year and New Zealanders might find themselves having a White Christmas!

It might seem a nuisance that the days and the years do not agree exactly but a day is the period of time for the earth to make one revolution about its own axis. As this is completely unrelated to the earth's rotation around the sun, there is no reason why they should agree.