Why is the summer solstice so early this year and what has it to do with leap years?

We tend to refer to the solstice as one whole day, but in fact it marks an exact moment in time.

In the northern hemisphere, the solstice marks a point where the Earth's tilt towards the sun is at a maximum. This means that the sun appears at its highest elevation, making the day of the solstice the longest day of the year where we get the maximum amount of daylight.

The solstice occurs every year between the 20 and 22 June. The length of day varies depending on your location, but on average here in the UK there is between 16 and 17 hours of daylight.

What is special about this solstice?

The solstice occurs on the same moment in time for everyone, so depending on where you live in the northern hemisphere it could occur during the day or the night.

In the UK, this will occur at 9:50pm (BST).

But there is something a little unusual about the solstice of 2024. This June solstice is the earliest in 228 years when it was at 18:45 on June 20 in 1796.

Part of the reason is 2024 is a leap year.

So how is a leap year calculated?

To be a leap year, the year number must be divisible by four – except for end-of-century years, which must be divisible by 400.

This means that the year 2000 was a leap year but 1700, 1800 and 1900 were not leap years.

Over the course of 400 years, it means the planet loses three leap years, shifting time slightly and making things too fast by about 45 minutes.

How are the numbers worked out?

An astronomical year is 365.25 days. An extra day is added every four years to keep calendars and season from gradually drifting out of sync, known as a leap year.

If we didn't have a leap years, calendars would be out of sync by 24 days every 100 years.

A year is more precisely 365 days, five hours, 48 minutes and 46 seconds.

So every four years there are about 11 minutes 15 seconds missing to make a full 24 hours. To balance this out, the calendar skips three leap days every four centuries to prevent any drifting of seasons.

The last time there was no leap day was 1700, 1800 and 1900.

There was however a leap year in 2000 and in the future there will be no leap day in 2100, 2200, 2300 or 2500.

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