[HN Gopher] Ocean Tides and the Earth's Rotation (2001)
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       Ocean Tides and the Earth's Rotation (2001)
        
       Author : susam
       Score  : 32 points
       Date   : 2025-04-15 19:19 UTC (3 days ago)
        
 (HTM) web link (core2.gsfc.nasa.gov)
 (TXT) w3m dump (core2.gsfc.nasa.gov)
        
       | PopAlongKid wrote:
       | >Currently the secular change in the rotation rate increases the
       | length of day by some 2.3 milliseconds per day _per century_.
       | [emphasis added]
       | 
       | >suppose the rotating earth is our clock and it's been 100 years
       | [...] Then after 1000 days our earth clock loses about 2.3
       | seconds,
       | 
       | I think the math here is very wrong, or else I haven't had enough
       | coffee yet.
        
         | doph wrote:
         | >it's been about 100 years so now each day is 2.3 milliseconds
         | longer
         | 
         | >after 1000 days 1000 * 2.3 milliseconds = 2.3 seconds
         | 
         | I don't think the example helps at all to explain the concept,
         | but I think the math is right
        
           | lifeisstillgood wrote:
           | So, if my maths is right that's about 1 second extra slower
           | every 500 days, which means a day will take 48 hours in about
           | 43 million years
           | 
           | That seems very very fast on galactic time scales - we would
           | get tidally locked (rotate at speed of orbit?) in about
           | 150x43 million ... oh about 3 billion years ... yeah never
           | mind
           | 
           | Edit A slightly different way of looking at it is we add a
           | leap second every 2 or so years on average (27 in past 53
           | years). This seems about right with the above maths - it just
           | amazes me
           | 
           | What amazes me more is they are going to stop using leap
           | seconds https://en.m.wikipedia.org/wiki/Leap_second by 2035
        
             | FredPret wrote:
             | An interesting similar thought is to work out the length of
             | a day during the Jurassic period
        
             | ta1243 wrote:
             | No, it's 2.3ms per day slower every 100 years. After 100
             | years the cumulative "loss" would be 100 _365.25_ (2.3/2),
             | or about 11 hours, but each day would be about the same
             | length as it is today.
             | 
             | If rotation was 86400 seconds per day today, then in 100
             | years time it will be 86,400.0023 seconds.
             | 
             | In 1,000 years time it will be 23ms slower
             | 
             | In 10,000 years time 230ms slower
             | 
             | In 100,000 years time 2.3 seconds slower.
             | 
             | In 1 million years 23 seconds, a 86400s day will take 86423
             | seconds
             | 
             | In 43 million years at that speed of 2.3ms per 100 years, a
             | day will take 86,989 seconds, or 24 hours 16 minutes.
             | 
             | (there are other factors effecting earth's rotation)
        
         | ta1243 wrote:
         | Not 2.3 seconds per day, 2.3 seconds _in total_ over that 1000
         | days, not 2.3 seconds per day which would be crazy.
         | 
         | On average each day over the last 100 years is only about 1.2ms
         | longer, so total extra length after 100 years is 100 _365.25_
         | 1.15. A cumulative 42 seconds has been added in 100 years.
        
       | morninglight wrote:
       | If you are interested in math, computers and tides, check out:
       | 
       | Charles Petzold - "Computer of the Tides" (2014)
       | 
       | https://www.youtube.com/watch?v=O-N5DuEUap8
        
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       (page generated 2025-04-18 23:01 UTC)