Why earth wobbles

Recent research by NASA found that the wobble of Earth as it spins is broken up into three primary factors: glacial rebound, melting of ice, and mantle convection. Previously, scientists believed glacial rebound to be the primary factor in causing Earth to wobble. Let's cover the three factors and what they mean.

But first, you may find it interesting to visit NASA's interactive polar motion simulation. The first factor is glacial rebound or isostatic rebound, what scientists previously thought was the primary contributor to Earth's wobble. Imagine Earth as a very large balance ball similar to the one in the photo below. As you can see, the lady on the balance ball is compressing the ball where she is in contact with the ball, causing a dent.

To compensate for this, the sides of the ball just outside of the lady's outline bulges outward. This is an example of when large glaciers cover land masses such as North America. During the last ice age, about This depressed land underneath the glaciers and causes land to bulge upward around the perimeter of the glaciers. However, as the glaciers melted, the land, just like the ball would, regain its original shape.

This process is called glacial rebound, as Earth regains its original shape. The process is quite slow, meaning Earth is still rebounding from the last ice age. This is what scientists previously thought accounted for the entire wobble of Earth. Earth's Tilt 13, years from Now. In the upper part of the diagram above labeled "Now," Earth's axis is tilted so that summer occurs in the northern hemisphere on the left side of the Sun and winter on the right.

In the lower part labeled "In 13, years," the axis has moved to where summer in the northern hemisphere occurs on the right side of the Sun and winter on the left. So what! You say. If Earth's orbit is nearly circular, then it makes no real difference.

But if Earth's orbit is slightly elongate, and perihelion occurs on the right side of the Sun as shown , then Earth is slightly farther from the Sun on the left side. In this case, northern summer are slightly cooler and northern winters slightly warmer on the "Now" part of the diagram. NASA says Greenland is the primary culprit here. This causes about one-third of the wobble.

The observed direction of polar motion, shown as a light blue line, compared with the sum pink line of the influence of Greenland ice loss blue , postglacial rebound yellow and deep mantle convection red. The contribution of mantle convection is highly uncertain. That ground slowly rises back up to its natural level, and that changes the distribution of mass and affects the wobble.

NASA says this is also responsible for about one-third of the wobble. The last third comes courtesy of mantle convection. So, this part of the wobble is decidedly not the doing of humanity.

With the causes identified, scientists can track polar motion and understand which changes are due to human activity and which are a consequence of the nature of Earth.