If you’re one of those charlatans who thinks it starts in March, I can only assume you are generally untrustworthy and probably like Barry’s Tea.

But really. The date of the first day of spring can often be a point of argument between people. Is it the start of February? Or the start of March? If you ask me, there’s only one proper answer, and it involves astronomy. But there are a couple of different reasons behind different dates.

When does Midsummer’s Day fall? I’ll give you a hint: it’s not in the middle of spring, autumn, or winter. Midsummer’s Day is based around the summer solstice, the longest day of the year. As Midsummer’s Day and the solstice fall in the middle of summer, and because each season lasts three months, we can deduce that summer lasts from 1.5 months before to 1.5 months after Midsummer’s Day/solstice.

The summer solstice in the Northern Hemisphere occurs around June 21st, and is opposed by the winter solstice, which takes place in December (again, usually the 21st, six months later). So, if the summer begins one and a half months before this, then we find that summer begins about a week into May, in astronomical terms. We know that each season lasts three months, so therefore, spring starts at the beginning of February.

The year and its seasons can be divided out along a circle, which resembles the Earth’s orbit around the Sun. Earth’s rotational axis is tilted slightly, so the point where the northern hemisphere is tilted most towards the Sun is the summer solstice, and where it’s tilted away is the winter solstice.

The Quarter days, from NASA

The above diagram shows the quarter days (two solstices and two equinoxes) as positions of the Earth along its orbit around the Sun. Four more points – each located halfway between each Earth in the diagram – are called cross-quarter days, and were particularly important in pre-Christian and Celtic Ireland. If the Quarter days above marked the middle of the season, then the cross-quarter days marked the beginning of the season.

It makes astronomical sense to use the equinoxes and solstices as markers for the middle of the seasons, as it means that spring and autumn will receive the same amount of solar radiation over three months, where summer and winter will be “opposite”, each one receiving a maximum or minimum of solar radiation. It’s nice and symmetric, and symmetry is good in science.

So why do some people say that spring starts in March? This is where meteorology comes in. Because of the way climate works, weather and temperature experiences a ‘lag’ of about six weeks. This means that the three warmest months of the year, i.e. meteorological summer, arrive about six weeks after astronomical summer.

Average temperatures, from Met Éireann

As you can see above, the warmest months from a meteorological perspective are June, July, and August, whereas we have found earlier that maximum solar radiation from the Sun occurs on May, June, and July.

There you have it. If you’re a meteorologist, spring begins on March 1st. If you’re an astronomer, it’s  February 1st (or a week-ish later if you’re particularly pedantic). I’m an astronomer, so I have no doubt that spring has well and truly sprung and the lambs are frolicking in the fields as we speak. Ahem.