Day length used to be 18 hours: Moon slows Earth
A day lasted only 18 hours 1.4 billion years ago and 23.5 hours 70 million years ago as tides add 1.7 milliseconds per century.
Moon tides brake Earth spin
Moon gravity pulls tidal bulges that lag ahead and drag rotation slower by friction. Days now lengthen 1.7 milliseconds per century, confirmed by 2700 years of eclipse records and atomic clocks plus laser ranging showing Moon recedes 3.8 centimeters yearly.
Coral daily bands and shell growth show 70 million years ago days held 23.5 hours with 372 days yearly. Older tidal rhythmites prove 1.4 billion years ago days lasted only 18 hours with 490 days yearly in a faster spinning world.
Days will keep getting longer
If braking continues days will reach 25 hours in 200 million years, adding more leap seconds. Friction heats oceans slightly while Moon drifts farther, weakening tides so braking slows and Earth never stops quickly like Venus retrograde spin.
Full tidal lock needs tens of billions of years, longer than Sun remaining 5 billion years. Before that Sun will swell and engulf inner planets, so Earth will never show one face to Moon like Moon already shows one face to us.
- Now: plus 1.7 milliseconds per century
- 70 million years ago: 23.5 hour day, 372 days
- 1.4 billion years ago: 18 hour day, 490 days
- Moon recedes 3.8 centimeters per year
- Lock needs tens of billions of years, Sun has 5