https://www.reddit.com/r/askscience/comments/wfo51b/is_the_earth_rotating_faster/
created by YakDaddy96 on 04/08/2022 at 01:29 UTC
0 upvotes, 1 top-level comments (showing 1)
I've recently seen a lot of people and even some magazines talk about the Earth spinning faster. They claim we have just had the shortest recorded day.
I'm always skeptical of things like this so I've turned to the one place I can trust, the internet.
Comment by CrustalTrudger at 04/08/2022 at 11:29 UTC*
34 upvotes, 4 direct replies
There have been a few news tidbits about changes in the length of day (e.g., this one[1] or this one[2]). With these, the first thing to clear up is that we're talking about *extremely* small changes, typically fractions of a millisecond. The other thing to clear up is that there is *a lot* of variability within this very small range from day-to-day. For example, consider this record[3] giving average, min, and max day lengths for years going back to 1973 (relative to 24 hours, e.g., is the day longer or shorter by fractions of milliseconds with respect to 24 hours). From this you can see that there is a lot of oscillation, e.g., if you pick almost any year, regardless of the average the minimum will be less than 24 hours and the maximum greater than 24 hours, with a few exceptions here and there, and the average is generally never exactly 24 hours.
2: https://phys.org/news/2022-08-earth-faster-usual-experts-shortest.html
3: https://www.timeanddate.com/time/earth-rotation.html
What's causing these oscillations? Lots of things. Ignoring forces outside of Earth for the moment (i.e., tides), anything that results in a movement of mass toward or away from the center of the Earth will result in a change in the rotation speed because of the conservation of angular momentum. The simple analogue often used is a figure skater spinning with their arms out, where they can increase their rotation rate by pulling their arms inward and slow down by putting their arms out again. So what's moving mass around in the Earth? We return to "lots of things", earthquakes (e.g., Anderson, 1974[4]), movements of water and air at the surface of the Earth (e.g., Barnes et al., 1983[5], Holme & de Viron, 2013[6]), or oscillations of the inner core (e.g., Wang & Vidale, 2022[7]). Then there are tidal forces, which in a variety of ways and on a range of temporal scales, are modifying the length of day (e.g., Luo et al., 1987[8], McCarthy & Luzum, 1993[9], Ray & Erofeeva, 2014[10]).
4: https://www.science.org/doi/abs/10.1126/science.186.4158.49
5: https://royalsocietypublishing.org/doi/abs/10.1098/rspa.1983.0050
6: https://www.nature.com/articles/nature12282
7: https://www.science.org/doi/10.1126/sciadv.abm9916
8: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JB092iB11p11657
9: https://academic.oup.com/gji/article/114/2/341/564256
10: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013JB010830
All of the above are focused on largely oscillations around a 24 hour length of day with varying periods and magnitudes, but is there a long-term trend? Yes, and it's unquestionably a lengthening of the day (i.e., Earth's rotation is slowing down). Here the question is the timescale of the measurement. The "shortest day ever recorded" news from above is considering the timeframe over which we have extremely sensitive measures of the length of day, back to about ~1960. However a range of less precise (but much longer records) highlight that on average, the length of the day is increasing, mostly driven by changes in the dynamics of the Earth-Moon system, i.e., the Moon is moving away (e.g., Lambeck, 1975[11]). I'll leave a discussion of the dynamics of this to someone who is more well versed on tides and tidal forces, e.g., /u/dukesdj. Dynamics aside and looking toward what is preserved in the geologic record, at the longest time-scale, much of our reconstruction of the length-of-day variations over geologic time comes from tidal rhythmites, which are deposits in the geologic record that record the periodicity of ocean tides (e.g., Mazumder & Armina, 2005[12], Coughenor et al., 2009[13]). From these, we've been able to get estimates of the approximate length of day at a few points in geologic history, e.g., at 2.5 billion years ago - length-of-day was 17.1-18.9 hours, at ~~9000~~ 900 million years ago - length-of-day was 18.2-20.9 hours, and at 620 million years ago - length-of-day was 21.9 hours (e.g., Williams, 1989[14], Sonnett et al., 1996[15], Sonnett et al., 1996b[16], Sonnett & Chan, 1998[17], Williams, 1997[18], etc). As discussed in many of these (but especially Williams, 1997), there is a decent amount of uncertainty in many of these estimates and reasons to be skeptical of the exact numbers, but even within that uncertainty, these paint a pretty clear picture of a long-term trend of the lengthening of the day by *hours* (compared to what we're talking about above measured in *milliseconds*), i.e., a slowing of the rotation rate of the Earth.
11: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JB080i020p02917
12: https://www.sciencedirect.com/science/article/pii/S0012825204000923
13: https://www.sciencedirect.com/science/article/pii/S0012825209001445
14: https://www.lyellcollection.org/doi/abs/10.1144/gsjgs.146.1.0097
15: https://www.science.org/doi/abs/10.1126/science.273.5271.100
16: https://www.science.org/doi/pdf/10.1126/science.274.5290.1068
17: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/98GL00048
18: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/97GL00234