Thread by @ProfTimOB

Happy solstice! (In just a few minutes, at about 3.57pm BST, 14.57 UTC). Heard some discussion about use of the phrase 'longest day' when people really mean most daylight hours - apparently would make people think the day gets longer and shorter. Well, it does...

...the Earth spins faster (leading to slightly shorter days) and slower (longer days) all the time. Here's a graph I made of the difference between the actual length of the day and 24 hours (=86400 seconds) for each day since 1962 (data from hpiers.obspm.fr/eop-pc/index.p…)

...you see that it really does spin up and slow down all the time - due to things like variations in the flow of material in and around the Earth’s core, the effects of weather (winds and precipitation) and even earthquakes, changing the mass distribution ( oment of inertia)...

[moment of inertia]...on very long timescales, the Earth's spin is slowing, meaning the days are slowly getting longer, due to the tidal interaction with the Moon (and causing the Moon to move farther away from Earth by about 4cm each year)...

...but in recent times, the Earth has been, on average, spinning faster, reducing the length of the day. So, if you're interested, the longest day since 1962 was 12th April 1972 when it was 4.355 milliseconds longer than 24 hours :) ...

...one might ask, how teh heck do we know how long it takes the Earth to spin round and how do we measure that on a daily basis. Well , thanks to radio astronomers, we have VLBI - Very Long Baseline Interferometry - networks of radio telescopes across the planet...

...specialist geodetic (Earth-measuring) telescopes are used to measure their own positions relative to a set of quasars - supermassive black holes in distant galaxies. Effectively a fixed grid against which to measure the telescope positions (and hence rotation of the Earth)...

.... if I didn't bore you too much on this 'not really' longest day, you can read a bit more here proftimobrien.com/2016/12/make-t…