Earth’s Spin to Speed Up Briefly, Causing Shorter Days This Summer

Reports indicate that for three days this summer – July 9, July 22 and August 5 – Earth’s rotation will speed up slightly, trimming 1.3 to 1.5 milliseconds off each day. Imperceptible in everyday life, this shift underscores how the Moon’s position influences our planet’s spin. For reference, the shortest day on record was July 5, 2024, lasting 1.66 milliseconds less than 24 hours. Over billions of years Earth’s rotation has slowly lengthened, but recent data show speedups. Scientists say monitoring these tiny changes is important for understanding Earth’s dynamics and timekeeping.

Causes of Faster Spin

According to timeanddate.com, the shortest-ever recorded day was on July 5, 2024, which was 1.66 milliseconds shy of 24 hours. The acceleration is largely driven by the Moon’s gravity. On those dates (July 9, July 22 and August 5), the Moon will lie far north or south of Earth’s equator, weakening its tidal braking on our planet’s spin. As a result, Earth rotates a bit faster – like spinning a top held at its ends. Seasonal shifts in mass distribution also affect rotation. Richard Holme of the University of Liverpool notes that summer growth and melting snow in the Northern Hemisphere move mass outward from Earth’s axis, slowing the spin in the same way an ice skater slows by extending her arms.

Timekeeping and Technology

Shifts in day length are handled by precise timekeeping. The International Earth Rotation and Reference Systems Service (IERS) monitors Earth’s spin and adds leap seconds to keep Coordinated Universal Time (UTC) in sync with solar time. Normally a second is added when Earth’s rotation slows, but if the spin-up trend continues, scientists have floated a “negative leap second” – removing a second – to realign clocks.

Dr. Michael Wouters of Australia’s National Measurement Institute says this fix would be unprecedented, and notes that even if a few seconds accumulated over decades, it would likely go unnoticed. Dr. David Gozzard of the University of Western Australia points out that GPS satellites, communications networks and power grids rely on atomic clocks synced to nanoseconds, and that millisecond-scale changes in Earth’s rotation are easily absorbed by these systems.