The Sun, the Moon, and Some Family History

The apparent coincidence that our moon so perfectly covers our sun is what makes solar eclipses so spectacular. Coincidence? Maybe. Yet Oxford astrophysicist Steve Balbus has a speculative theory that, 400 million years ago, this may have provided the impetus to encourage our aquatic ancestors to investigate life on land.

Eclipse and tetrapod
An early tetrapod and a solar eclipse. Adapted from images by DiBgd and Oregon State University under Creative Commons license.

Next week I will be travelling to the US to join the huge numbers of tourists gathering to view what promises to be a spectacular natural phenomenon—a total eclipse of the sun. The sun is four hundred times further away than the moon. Its diameter is four hundred times larger. This means when viewed from our planet, the two bodies appear the same size. Depending on the time of year, sometimes one is slightly larger, sometimes the other, but they are very closely matched. This is what makes our eclipses so awe inspiring, as the moon is just wide enough to block the blinding light of the sun’s surface, but small enough to let us then view the solar corona—the stream of hot plasma which surrounds the sun, shaped by its magnetic field.

Perhaps, come cheap interstellar travel, tourists will flock to our world from across the galaxy to view such an astronomically awesome sight. Because, as far as we know, this balance is uncommon. Models of planetary system formation, and our small, but rapidly growing knowledge of extrasolar planets, suggest that it is actually very unusual to have an Earth-sized planet with such a large moon.

Are we just lucky? Maybe. But there is a theory suggested by Oxford astrophysicist Steven Balbus that this may not be entirely coincidental. This theory contains a fascinating story about the common ancestors of humans and other animals.

As a consequence of being the same apparent size, the tidal forces of the sun and moon are also about the same. The gravitational field from both bodies pulls the side of the Earth facing them a bit more than the far side, causing the ocean to bulge along this direction. As our planet spins around, sea levels across the world rise and fall with a twelve hour period. But the magnitude of the tidal force—and hence the height of high tide—can vary a lot. When the moon and sun are at the same place in the sky (during a new moon), we get the highest spring tides. Seven days later, tides are much smaller.

Now let’s go back 400 million years to the Devonian Period and imagine the life of our water-dwelling ancestors, who were then splashing around, playing hide-and-seek in the Rheic Ocean between the continents of Gondwana and Euramerica. The shallow waters were no-doubt a great place to play, with lots of food and plenty of good hiding places. But with a big risk. The narrow tapering western end of the Rheic Ocean would have had big tides. So as the sea level dropped, you could find yourself stranded in a rock pool away from your friends.

Not such a big deal in the lower pools, where you would just have to twiddle your lobe fins for six hours or so until you could escape. But if you end up stuck in one of the highest pools, it could be another month until the sea returns. Thus you face death by starvation, or if the pool evaporates.

It thus becomes imperative to flip your fishy body out of the water and attempt the epic perilous journey down the beach to the sea, or at least a lower rock pool. Millions of aquatic critters must have perished on such journeys. But those who survived—those with the stubbiest fins, best able to survive out of the water—would have lived to raise families of little amphibious pioneers.

Thus evolved the first tetrapods able to move across land. Which was an enormous advantage when seeking food, as they could then hop out of the water and visit the isolated pools. In these high-tide Devonian sushi bars, they would find plenty of stranded seafood delicacies to snack on and thus remove from the gene-pool.

Once our forefathers had a finhold on the land, there was no stopping them exploring further. A few hundred million years later their descendants are happily galloping across the prairies, climbing trees, and looking up at the sky wondering why the sun and moon should be the same size.

Thus, Prof Balbus speculates, having the sun and the moon of the same size, created the right conditions which allowed our ancestors to move on to land and flourish. Unfortunately the lack of data on life on other planets makes it difficult to test, but it’s a cute theory nonetheless.

Further reading: Dynamical, biological and anthropic consequences of equal lunar and solar angular radii, Steven A. Balbus. Proceedings of the Royal Society A.

Adapted from a post published on Fimfiction 31 March 2015.

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