All articles tagged with #tidal friction
Laser pulses bounced off Moon retroreflectors left by Apollo and Lunokhod missions measure the Moon’s recession from Earth at about 3.8 cm per year with millimetre accuracy, a result that tests general relativity and informs Earth–Moon dynamics; the rate depends on ocean tides and continental layout, and future reflectors from Artemis missions will enable sharper measurements.
Earth’s rotation has been slowing for billions of years due to tidal friction with the Moon, lengthening a day from roughly 22 hours in the Devonian (about 385–410 million years ago) to today’s 24 hours. Fossil corals show ~400 daily growth lines per year in the Middle Devonian, giving a day of ~21.9 hours, while Pennsylvanian corals imply ~22.4-hour days; the Moon’s recession (~3.8 cm/year) and internal mass redistribution also affect daily length in the short term. Proterozoic proxies suggest days as short as ~19 hours, highlighting how Earth’s spin has gradually slowed over deep time.
Earth will experience one of its shortest days on July 22, with a rotation 1.34 milliseconds faster than usual, part of a recent trend of Earth's rotation speeding up, possibly due to internal Earth dynamics, though the exact cause remains uncertain.