For most of us in the Northern Hemisphere, summer starts on the 1st of June—at least, that is what the meteorologist on the news says. However, for astronomers, and for a lot of human history, that wasn’t the case. Traditional time was marked by the passage of the Sun over the course of the year, defined by four important dates.
There is the date when the day is at its shortest (the winter solstice), the date when it’s at its longest (the summer solstice), and the two times when day and night are of equal length (the spring and autumn equinoxes). This is how humans originally broke the year into seasons, and they built huge monuments, such as Stonehenge, to track when these dates happened. This was important because these astronomical events actually move.
So, why do the solstices and equinoxes shift? It happens for two main reasons. The first is leap years. It takes roughly 365 and a quarter days for the Earth to orbit the Sun, but our calendar rounds it down to 365. To account for the drift, we add an extra day every four years.
The second reason is that the Earth’s path around the Sun is elliptical rather than perfectly circular, meaning the Earth doesn’t travel at a constant speed. These factors mean that the spring equinox happens sometime between 19 and 21, March the autumn equinox falls between 21 and 24, September the summer solstice occurs between 20 and 22, June and the winter solstice lands between 20 and 23. December.
Why, then, do meteorologists say summer starts on 1st June? It actually comes down to keeping simple records. Meteorologists break the seasons into neat, three-month blocks based on the civil calendar and annual temperature cycles, which makes climate statistics much easier to track. However, the weather itself lags behind the Sun like a toddler with a parent in a supermarket.
During the spring and summer, the oceans and land absorb a massive amount of the extra solar energy hitting the Earth. Because water and Earth take time to heat up, this delays when the atmosphere reaches its peak warmth.
Likewise, in the winter, the land and sea act as a thermal sink, holding onto that warmth and releasing it slowly as the days shorten. This “seasonal lag” is exactly why the coldest weather hits well after the shortest day, and why the hottest days usually happen weeks after the summer solstice.
Fun fact: the Romans fixed the winter solstice on 25th December. It stayed that way for centuries, eventually causing absolute chaos for the calendar down the line.
