Around 23 September, the sun crosses the celestial equator — the imaginary line above Earth’s equator — and autumn officially begins in the northern hemisphere. On paper, this is the moment day and night stand in perfect balance: twelve hours each, a celestial scales held level before darkness slowly takes charge and carries us towards winter.
It’s an idea humans have marked for millennia. Neolithic monuments across Britain — including Stonehenge, built in stages from around 3000 BC — appear aligned to both solstices and possibly the equinoxes, suggesting our ancestors tracked these turning points with obsessive care. They farmed by them, and in a sense so do we, even now: harvest festivals, the return of schools, the slow ritual of pulling jumpers back out of wardrobes.
But the tidy story of “equal day and night” doesn’t quite survive contact with reality. In fact, here’s something you can test yourself from Monkton Heathfield: on 23 September, look up the sunrise and sunset times for Taunton and do the sums. You’ll find the day is still a few minutes longer than the night. Somewhere between seven and ten minutes of extra daylight, depending on your latitude. The precise tipping point — the true “equal night,” which meteorologists call the equilux — arrives three or four days after the equinox itself.
Why the delay? Two culprits, both wonderfully pedantic. First, we define sunrise as the moment the sun’s upper edge appears above the horizon, and sunset as the moment its upper edge disappears. Since the sun’s disc takes a couple of minutes to fully rise or set, this definition buys us bonus daylight at both ends of the day. Second — and more strangely — the atmosphere bends light. Refraction lifts the image of the sun above the horizon even when it has geometrically sunk below it, the same trick of physics that squashes the setting sun into an egg shape. In total, we see the sun when it isn’t there, at both dawn and dusk, for several minutes each day.
Add it together and the astronomical books don’t balance until several days after the official equinox. Precision matters, and the equinox rewards those who check.
There’s a second, more surprising imbalance hiding in September — one you can feel rather than calculate. By late September, our day lengths almost exactly match those of late March. Same sun, same duration, same geometry. And yet anyone who has stood at a school bus stop in a biting March wind knows the seasons feel nothing alike. Late September mornings smell of harvest and ripening blackberries; late March mornings smell of cold earth and rebirth.
The culprit is thermal lag. The ground and — crucially for us on this peninsula — the oceans have spent the whole summer banking warmth. Water is slow to heat and slow to cool: the Bristol Channel and the Celtic Sea around our coasts reach their warmest temperatures in August or even September, roughly two months after the longest day. All that stored energy keeps feeding warmth back into the air long after the sun’s arithmetic has shifted. September is the receipt for July’s spending; March is the receipt for February’s — a month when the sea around Britain is at its coldest, still punishing us for the winter we’ve endured.
So the same day length delivers wildly different outcomes: a mild, mellow equinox versus a raw, hopeful March. When people say autumn in Somerset “comes in gently,” that oceanic thermal lag is the unseen hand behind it.
And you can watch the balance tipping in our own data, right here. The shortening days mean measurably less solar energy arriving each week — visible in the gentle decline in sunshine hours and UV readings on our Environmental Data page, and in the slow retreat of afternoon warmth as the longer nights get more hours to radiate heat back into space. Look at the diurnal range in our temperature records: the gap between daytime highs and night-time lows widens through autumn, because clear September nights — especially under the high pressure spells that often grace this corner of the west — allow efficient cooling once the sun sets.
There’s a practical footnote to all this for gardeners and dog-walkers around the village: dew and fog become features of the morning landscape as night length exceeds day. Longer nights mean longer hours of cooling, more overnight condensation, and increasingly frequent mist drifting in from the Somerset Levels to the south and east of us. If you want a reminder of why, our posts on dew point and relative humidity cover the mechanics.
The equinox, then, is less a switch than a hinge — the moment the arithmetic of sunlight quietly turns against us, even as the banked warmth of summer delays the reckoning. Whether autumn then delivers gold or gloom is another matter altogether, decided by the jet stream’s wanderings and the Atlantic’s mood.
Keep watching the station.