Length of Earth's Day Affected by Planet's Wobbly Core

The length of an Earth's day is about 24 hours as our planet rotates on its axis, spinning around the Sun. At least, that's usually the length of an Earth's day. New research has shown that variations in the length of day over periods of between one and 10 years are caused by processes in our planet's core.

While Earth rotates once per day, the length of this time can vary over hundreds and millions of years. For example, about 300 million years ago a year lasted about 450 days and a day lasted about 21 hours. That's a stark contrast to the 365-day year of today.

Over time, Earth's rotation has slowed and the length of a day has increased. Yet this rotation is affected by a number of factors, including the force of the wind against mountain ranges which can change the length of the day by plus or minus a millisecond over a period of a year. While that may not seem like much, it's enough to cause a big difference in models over time.

In order to examine these variations and fluctuations, researchers created a model that showed the length of a day over a one to 10 year period between 1962 and 2012. It took into account the effects of atmospheric and oceanic processes and the effects on Earth's rotation.

"The model shows well-known variations on decadal time scales, but importantly resolves changes over periods between one and 10 years," said Richard Holme from the School of Environmental Sciences in a news release. "Previously these changes were poorly characterized; the study shows they can be explained by just two key signals, a steady 5.9 year oscillation and episodic jumps which occur at the same time as abrupt changes in the Earth's magnetic field, generated in the Earth's core."

The findings show a little bit more about the short-period dynamics of Earth's fluid core. In addition, they reveal that our planet's lower mantle, which sits above the outer core, is a poor conductor of electricity. This gives researchers new insight into the chemistry of Earth's deep interior.

The findings are published in the journal Nature.