Does Earth Really Rotate Slower in March?

You might have heard that Earth rotates more slowly on its axis in March than in September. It's a fascinating claim—but it's actually completely backwards.

Earth's rotation speed does vary throughout the year, but the pattern is tied to summer and winter, not spring and fall. Our planet spins fastest during Northern Hemisphere summer (July-August) and slowest during Northern Hemisphere winter (January-February). March and September, being transition months between seasons, don't show any special difference in rotation speed.

The Real Science: Why Summer Days Are Shorter

In July and August 2025, Earth experienced some of its shortest days ever recorded, with certain days being about 1.05 milliseconds shorter than the standard 86,400 seconds. The shortest day of 2024 was July 5, when a complete rotation took 1.66 milliseconds less than usual.

This isn't some cosmic accident—it's all about atmospheric angular momentum, a fundamental principle of physics.

The Atmosphere and the Planet: A Balancing Act

Think of Earth and its atmosphere as dance partners spinning together. The total angular momentum of the system must stay constant, so when one partner slows down, the other speeds up.

During Northern Hemisphere summer, the jet stream moves and atmospheric winds slow down significantly. When the atmosphere loses rotational speed, Earth's solid surface picks up the slack and spins faster. Conversely, during winter, stronger west-to-east winds increase the atmosphere's angular momentum, forcing the planet itself to slow down slightly.

The Northern and Southern Hemispheres each have a primary polar jet stream flowing from west to east. These jet streams are fastest during each hemisphere's winter and slowest during summer. When Northern Hemisphere summer brings a decrease in westerly wind speeds, the solid Earth must spin more rapidly to maintain the system's total angular momentum.

How Much Does It Really Change?

We're talking about incredibly tiny variations—typically between 0.8 and 1.7 milliseconds per day. That's far too small for humans to notice, but it's significant enough to matter for:

• GPS satellites, which need precise timing to calculate your location
• Atomic clocks, which measure time to billionths of a second
• Deep space communications, where even millisecond errors can throw off calculations
• International timekeeping standards, which occasionally add "leap seconds" to keep our clocks synchronized with Earth's actual rotation

Scientists can measure these tiny changes using a global network of atomic clocks and extremely precise astronomical observations. Modern technology is so advanced that we can detect variations of just a few milliseconds in a 24-hour period.

Other Factors That Affect Earth's Spin

Seasonal atmospheric changes aren't the only thing affecting our planet's rotation. The Moon's gravitational pull plays a significant role—when the Moon is at its furthest distance from Earth's equator, it changes the gravitational impact on our planet's axis, causing temporary speed increases.

Over much longer timescales, tidal forces from the Moon are actually slowing Earth's rotation gradually. About 1.4 billion years ago, a day was only about 18 hours long. Eventually, billions of years from now, Earth's rotation will slow so much that one day will equal one month, with the same side of Earth permanently facing the Moon.

Even climate change is now affecting Earth's rotation, as melting ice redistributes mass from the poles toward the equator, subtly altering our planet's rotational speed.

So What About March and September?

These months fall during the spring and fall equinoxes, when neither hemisphere is tilted significantly toward or away from the Sun. They're transition periods between the faster summer rotation and slower winter rotation, but there's no particular difference between March and September rotation speeds. The myth likely arose from a misunderstanding of seasonal variations in Earth's rotation, mixing up which months see the actual changes.