Why do we have leap years and leap seconds?

For around five million ‘leapers’ February 29 is a special day, their birthday. It only happens once every four years and signifies an important reorganisation of time. But what is a leap year and are there any other ‘leaps’ that keep our clocks accurate to the microsecond?

We may think that our system of time-keeping is perfect, that at any point during the day the time is exactly the same as it was the previous day. Unfortunately, this is not the case. In fact, by the end of a normal calendar year our clocks will be out by almost six hours from where they were at the start of the year.

Why do we have leap years?

The calendar and the planet fall out of sync because of a mismatch between the length of a day and the length of the year. It doesn’t take exactly 365 days for the Earth to complete an orbit around the Sun. It takes 365 days, 5 hours, 48 minutes and 45 seconds. 

To resolve this discrepancy, our calendar (the Gregorian calendar) adds one day (24 hours) to the calendar every four years.

Again, this adjustment is not exactly correct because it effectively adds 6 hours per year rather than the exact amount of the discrepancy. To compensate for this, most century years (like 1700, 1800 and 1900) are not leap years. The exception to this is years divisible by 400 (1600, 2000, 2400 and so on), which are still leap years. 

The chart below shows how the date of the northern summer solstice will drift over the course of a 400-year leap cycle.

Why do we have leap seconds?

In addition to the leap year, our clocks and calendars also undergo finer adjustments: leap seconds, which are extra seconds inserted into certain days to adjust for irregularities in the rotation of the Earth.

Why do these irregularities occur? It’s in part thanks to the moon.

The tidal forces that cause water to rise and recede along coasts all over the world are caused bythe gravitational attraction of the Moon and the Sun. However, as the Moon rotates around the Earth, it does so much more slowly than the Earth rotates on its axis. We see this as the moon moves across the sky rather than standing still.

The affect that this has on each celestial object is that the Earth’s spin is very slightly slowed down, by around 0.002 seconds each day, while the Moon speeds up in its orbit, causing it to move away from Earth by around 4 cm each year.

Why does this 0.002 seconds matter? Well, if a clock ticks at 1 tick per second, or 86,400 ticks per day, it will slowly fall out of sync with actual days as measured by the rotation of the Earth.

To combat this, the leap second was introduced in 1972, using atomic clocks – which only lose one second every 100 million years – as a more ‘correct’ standard for time. Since its first use, a leap second has been added around once every 1.5 years.