Say goodbye to the grand stretch because it’s all downhill from here.

Today is the longest day of the year, with the summer solstice due to occur at 9:50pm this evening.

The solstice usually occurs on June 21st; however, as you may have noticed, this year we are seeing it on June 20th.

In fact, 2024 marks the earliest summer solstice in 228 years.

'Non-leap century'

That is because 2024 is a leap year in a ‘non-leap century’ – a century that does not end with a leap year.

Under our Gregorian calendar, every normal year has 365 days.

However, it actually takes the earth 365.242189 days to travel around the sun.

This means that the solstice happens around six hours later each year.

That is, until we get to leap year, when it happens just over 18 hours earlier.

The Winter Solstice at Brú na Bóinne, Newgrange, County Meath, 14-12-2019. Image: John Lalor/RollingNews

Even with all this, however, the calendar runs ever so slightly faster than the length of time it takes the Earth to travel around the sun.

Cumulatively, this brings the solstice forward by around 45 minutes every four years.

This means that while the 2024 solstice is the earliest since 1796, the event will actually happen 45 minutes earlier each leap year for the rest of the century.

'Non-leap century'

That, however, is where things get really weird.

In order to combat the slow drift backwards, the calendar has ‘non-leap centuries’ built into it.

This mechanism will see the leap year that is due to occur in 2100 effectively cancelled.

That will allow the calendar to ‘catch-up’ by a day – pushing the solstice back 23 hours and 15 minutes compared to the previous leap year.

From there, the whole process begins again.

How do they decide when to hold a ‘non-leap century’ you ask?

Believe it or not, in order for a turn of the century year to remain a leap year, it must be divisible by 400.

That, perhaps rather arbitrary sounding, rule is the reason why 2000 was a leap year, but the years 1900, 1800 and 1700 were not.