Today, February 29th 2020, is a leap day with an extra 24 hours added to the calendar. The extra day is added to keep our calendar aligned with the seasons. The need for a Leap Day is a result of the physics with the Earth rotating on its axis while simultaneously revolving around the Sun. Physics also explains why the need for leap years will eventually disappear!
Our calendar year is designed to match the seasons, which occur because of the axial tilt of the Earth. At the June solstice, the northern hemisphere is tilted towards the sun but it is tilted away from the Sun at the December solstice. Neither hemisphere is tilted towards or away from the Sun at the equinoxes.
Image credit: www.solarsystemscope.com/
But the Earth is also simultaneously spinning on its own axis, with a period of 24 hours that make up our cycle of days and nights. The 24-hour solar day is the time it takes between successive “noons” – the time when the Sun reaches its highest point in the sky as seen from Earth. The solar day is about 4 minutes longer than the time the Earth to rotate by 360 degrees (called the sidereal day) because the Earth is moving around Sun during each day.
The calendar was originally intended to start at the Spring Equinox with a calendar year measuring the time it takes for the Earth to revolve around the Sun and make it to the next Spring Equinox.
The calendar year is the period of our orbit around the Sun but it does not match up with the whole number of 365 solar days – it takes a bit of an extra day for the Earth to get back to the same position with respect to the Sun.
The calendar year is 365.24219 solar days.
The extra 0.24219 days is close to 0.25 or ¼ of a day so adding an extra day every 4 years keeps the seasons and the calendar synchronized. It is a bit more complicated because the extra 0.24219 is not exactly ¼ and the Gregorian Calendar accounts for this with its leap year rule:
A year is a leap year if it is evenly divisible by four,
but years that are divisible by 100 are not leap years,
unless they are also divisible by 400.
For example, 2020 is a leap year, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.
In the long term, changes in the Earth’s rotation rate need to be taken into account. The Earth’s rotation rate is slowing down, mostly due to the tidal forces and friction between the Earth and Moon. The day gets about 1.4 milliseconds, or 1.4 thousandths of a second, longer roughly every 100 years. A leap second is occasionally applied to accommodate for this slowdown in the Earth’s rotation and other irregularities. If you can wait around for about another four million years then the day will have lengthen by about 56 seconds. That is enough so that a calendar year will have exactly 365 solar days and we won’t need a leap year!
So enjoy the leap day while you can and a Happy Birthday any leaplings born on Feb 29th.