The 2024 summer solstice will be the earliest for 228 years. Here's why.

Today (June 20) the summer solstice — the longest day of the year in the Northern Hemisphere — will begin, and it will be the earliest solstice in 228 years.

And the solstice is set to get progressively earlier for the next 72 years. But why? And why do the days change in length throughout the year to begin with?

"Solstice" means "sun stands still" in Latin. In the Northern Hemisphere, the summer solstice is the day when the sun rises and sets at its most northeasterly and northwesterly points in the year. The phenomenon is caused by Earth's 23.5 degrees of axial tilt with respect to the sun; so as our planet orbits our star, one hemisphere leans away from the sun and the other towards it. 

This means that the solstice is the opposite in the Southern Hemisphere — the June solstice is the shortest day of the year and marks the winter solstice in the south. 

Today's solstice is the earliest since 1796, the year in which George Washington issued his farewell address. But this variation is caused by humans and boils down to the Gregorian calendar. 

Related: The 1st 'major lunar standstill' in more than 18 years is about to occur. Here's how to see it.

The calendar divides one year into 365 days. However, Earth's actual full orbital time around the sun is 365 days, 5 hours, 48 minutes and 45 seconds — which we account for by adding a leap year every four years to round the calendar up. But this creates its own problem: four times 5 hours, 48 minutes and 45 seconds is 23 hours and 15 minutes, meaning it's 45 minutes short of a full day.