Sky Matters: Shortest day of the year happens this month — or maybe August

Since 2020, the Earth has been spinning faster – hence the uncertainty in the exact date of the shortest day in 2025.

Since we don’t know what’s causing the Earth to spin faster, we can’t exactly predict what happens next. Pictrure: iStock

If asked to name the shortest day of the year, most of us would plump for December 21. It’s the sort of slam-dunk pub-quiz question designed to lull us into a false sense of “knowing stuff” – or perhaps that’s just me and pub quizzes. But the answer, surprisingly, is not December 21, but rather July 9 or possibly July 22 or maybe even August 5, at least in 2025.

To resolve this apparent conundrum, we first have to define what is meant by a “day”. If it was the time between sunrise and sunset, then December 21 would be the correct answer. If it is the time taken for the Earth to spin once on its axis then it's more complicated, leading to the three possible dates in July and August.

Since 2020, the Earth has been spinning faster. Not by much, about 1.66 thousandths of a second faster on July 5 last year (compared to the total length of an average day of 24 hours or 86,400 seconds). In fact, that day holds the record for being the shortest day since records began in 1973 when satellites could measure the precision of the Earth’s rotation with sufficient accuracy – after all, measuring a change of 1.66 thousandths of a second in the rotation of an entire planet is no mean technological feat!

We don’t yet know what causes this increased rate of spin and associated shortening of the day. It could be due to melting ice causing a change in the way water is distributed around the Earth, or to complex interactions between the Earth and the Moon or to something going on deep in Earth’s core. Since we don’t know what’s causing it, we can’t exactly predict what happens next – hence the uncertainty in the exact date of the shortest day in 2025.

Not knowing the spin rate of the Earth, even by that mere 1.66 thousandths of second, influences the accuracy of satellite navigation or large-scale financial systems which rely on knowing the time extremely accurately. For most of us the financial effect is too small to be noticeable or most likely zero, but if the winner of the recent €250 million Euromillions Lottery jackpot is investing in stocks and shares the uncertainty might shave (or add) a fraction of a cent here or there to the value of their stocks.

There’s no suggestion that this increased rate of spin is here to stay. Indeed, over long periods of time the Earth’s rate of spin has been gradually slowing and will continue to do so, largely thanks to the influence of the Moon and the tides it raises in our oceans. For example, around 1.5 billion years ago the average length of a day was about 19 hours, while some 300 million years ago our ancient ancestors experienced days that were 22 hours long.

Measuring the movement of the stars has been a crucial way to establishing time. Early clocks were not capable of keeping time accurately without the need for frequent corrections. For hundreds of years the most accurate way to keep clocks in line was to watch for the moment when particular stars would pass their highest point in the sky – their transit - and clocks could be reset to agree. Telescopes designed to measure the exact moment (to within minutes or sometimes seconds, not thousandths of a second) were called Transit Telescopes. Today, our timing is kept in good order by atomic clocks, which are capable of accuracies of one second in one hundred million years.

But it’s an interesting thought that because the Earth’s rate of spin is slightly uncertain this July, the exact positions of the stars will also be slightly uncertain, if only by the tiniest amount. In these somewhat uncertain times it’s an irony that might not be lost on us.