The length of an overall day varies slightly throughout the year.
We all learn when young about the seasons, and the earths axis, and how the longest ‘daytime’ occurs when our part of the world tilted slightly toward the Sun. This longest ‘daytime’ normally occurs on the summer solstice in late June in the Northern Hemisphere, or late December in the Southern hemisphere, and although we often refer to the ‘longest day’, it is in reality only the longest ‘daylight’ or perhaps longest ‘daytime. Even more celebration historically surrounds the winter solstice, which is when days finally stop getting shorter and then get longer right up until the next summer solstice.
What affects us far less, is that the length of each day from true midday to true midday also varies very slightly throughout the year. What varies is the time from when the Sun is exactly overhead one day, to exactly overhead the next day, and the variation is sufficiently small that we do not adjust our clocks, other than when there are ‘leap seconds’, official midday one day is exactly 24 hours after midday the previous day, and is not when the Sun is exactly overhead unless we are in the exactly appropriate location in our time zone. That is, provided there is anywhere in our time zone when the Sun is directly overhead at midday.
The length a day is longer than one rotation of the earth.
The Earth rotates once in around 23 hours and 56 minutes, or enough less than one day for their for one extra full rotation every year.
To understand why, first imagine that a day was one rotation of the Earth. Every day at midday a person looking up, would be looking in the same direction. The diagram to the right illustrates how the direction to the Sun in April, is no longer the direction towards the Sun in July.
The period of ‘a day’ allows for one rotation of 23 hours 56 minutes, and then another approximately 4 minutes of more rotation, to correct for the fact as the Earth has moved almost 1 degree around its orbit. Because the Earth moves each day, we need to allow an extra 1 degree of rotation every day in order for the Sun to be in the same position in the sky at the same time on the next day.
Note that 24 hours is 24 x 60 minutes, which when divided by 360 to give 1/360th of a circle, results in 4 minutes. Of course, technically it would be a little less than 1 degree, as the are 365.25 day in a year, not 360, although a 360 day calendar does exist.
So a ‘day‘ is not just one 360 degree rotation of the Earth, but an almost 361 degree rotation, which is why the stars, which are not moving relative to us, appear in a different position each night.
The length of the day varies throughout the year.
There are two main factors that vary the length of a day:
- The speed of the Earth in orbit around the Sun is fastest when closest to the Sun.
- The rate of spin of the Earth fluctuates.
The first factor is simple, and quite easy to predict. Our orbit around the Sun is not a perfect circle, but an ellipse, which gives a closest distance at ‘perihelion’ in January and currently January 3rd to 5th, when at one end of the ellipse an furthest distance from the Sun at aphelion in July, and currently July 4th to 7th, when at the other end of the ellipse. As the Earth moves fastest when closest to the Sun, and slowest when furthest from the Sun, if the speed variation was the only factor, then the shortest overall day would be in early July each year.
The changing rate of spin is a little more complex, but is again at least reasonable predictable. Imagine a spinning ice skater. They can vary their spin by bringing in their arms, and slow their spin by moving weight further from the axis of their spin. The Earth has a molten core, which cycles when weight shifts, which slightly varies the spin, in what are currently cycles around 6 years long. So the shortest day results from a combination of being near aphelion at a point when the weight is slightly more to the centre, so the Earth is spinning faster.
So why the shortest day ever recorded?
In fact, we set a new record every year lately (2020, 2022). Why? Because the spin speed is increasing.
I would expect the Earth to gradually reduce spin speed, as the tides are pulled against the direction of spin. The only way to increase spin, is the skaters trick of bringing weight to the centre.
Lately, the Earth has been speeding up, and the best explanation is due to the melting ice caps. The Earth is not a perfect sphere, but a slightly squashed oblate spheroid. This means the equator is further from the centre than the poles, and there are two reasons why:
- Centrifugal force, which can be observed by spinning a water filled balloon.
- Some squashing from the polar ice caps.
The first one is obvious, the second is not so obvious to me, but ice does make the Antarctic the highest continent on Earth, so there is logic.
Centrifugal force is a brake on changing the spin, becoming more oblate due to the speed of spin would reduce spin, and becoming less oblate as spin slows would speed spin up. So that is no going to cause a change.
However, the second factor means that melting of the polar ice would make the Earth less oblate and thus possibly spin faster.
While there could be other internal factors at work, there have been predictions made which support this theory stacks up.
Yet another measure of global warming, but at least not a scary one.
Earth has recorded its shortest day since scientists began using atomic clocks to measure its rotational speed.
On June 29, 2022, Earth completed one spin in 1.59 milliseconds less than 24 hours. This is the latest in a series of speed records for Earth since 2020.
Earth Sets New Record for Shortest Day
Note, the record is the shortest day ever recorded, not shortest day ever. We have not been around to record day lengths very long, and have had equipment to accurately record the lengths of days even longer. Billions of years ago days were shorter, and the tides and other effects have gradually made days longer.
And the record will probably not last long.
In recent years, Earth has been speeding up. In 2020, timeanddate reported that Earth had achieved its 28 shortest days since accurate daily measurements using atomic clocks began in the 1960s.
The shortest day of all in 2020 was -1.47 milliseconds on July 19.
Earth continued to spin quickly in 2021, although the shortest day of the year in 2021 was fractionally longer than in 2020.
Now, in 2022, things have speeded up again. On June 29, Earth set a new record for the shortest day of the atomic-clock era: -1.59 milliseconds.
Earth nearly beat its record again the following month, posting a length of day of -1.50 milliseconds on July 26.
Time and date.
Updates.
- 2022 August 12: Just typo fixes.
