The Sun, the Moon, and a bit of Family History · 11:25pm Mar 30th, 2015
The sun is four hundred times further away than the moon. It's 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 equality has had an obvious impact on our culture from ancient myths to Friendship is Magic. It is only natural that the sun and moon should be sisters of equal standing. It is also what makes our eclipses so spectacular. 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 recently suggest by physicist Steven Balbus, that this may not be entirely coincidental. And this theory contains a fascinating story about the common ancestors of humans, ponies 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 (i.e. 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, watching cartoons and penning silly stories about the adventures of my little fellow vertebrates. 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.
You're right.
That is a cute theory.I unfortunately have nothing of value to add to the conversation.
If that is true, then we are super lucky everything worked out the way it did!
I mean the odds are astronomical! (Pun intended...)
Not... exactly. This is actually mostly because they have about the same apparent size and about the same density, with the Moon being a bit more than twice as dense as the Sun is. A red giant giant, for instance, is enormous but very diffuse.
In any case, I've heard variations on this theory previously, and the idea of tidal pools being important is a somewhat interesting one, but I think that waves, storms, and inland rivers, streams, and lakes were other likely culprits. Waves - particularly waves driven by storms - could easily strand creatures in much the same way as spring tides. However, I think another thing to consider is whether or not the creatures might have had other things than mere survival in mind - the ocean can wash a lot of stuff up on the shoreline, after all, and being able to crawl out and nibble on it safely, away from all the predators who might want to nibble on you while you were eating, seems handy. Land plants colonized land long before animals did, and anthropods scampered out onto land long before fish did, either to scavenge or to avoid predators.
Another obvious source of critters would be things which colonized freshwater wanting to go pond-hopping in marshy areas, which lack significant tides.
Not to say that the moon might not have helped, but it seems like there are a lot of other plausible routes for such things to evolve along, such suggests to me that many critters would not need gigantic moons.
What I take from this is that we all owe our existence to Princess Luna.
[As a side note, there's another theory in which circularly polarized moonlight induced the chirality of DNA, or something like that.]
The more I hear about the origins of life on Earth, the more I marvel at our sheer improbability, and the more it seems like we may be alone in the universe. Or, at the very least, the only life as we know it.
Of course, that's assuming that that theory holds any water. Though I do still wonder how many planets out there could have crashed into their siblings and created enormous axial stabilizers.
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Even with astronomically low odds, in an effectively infinite universe, I'd say it's pretty likely it managed to happen somewhere else...
This is a nice theory, but the only fish I know of that travel across land do so to escape small evaporating pond-puddles that have nothing to do with tides.
I wonder if intelligent life, intelligent being that each generation on average the knowledge of the population, as in new hunting, agricultural, and entertainment techniques can be effectively passed down(there was a term for this, I forget it though) could happen underwater. The turning point for humans was agriculture because it created a surplus of food, allowing some people to become researchers and teachers. I'm pretty sure some animals, such as crows and dolphins, are just as smart as us but couldn't accomplish agriculture.
You mean 7 days later. The tides are greatest when Luna, Sol, and Tellus are alined. They are least when a viewer on Tellus sees Luna appears τ/4 or 3τ/4 radians from Sol.
Or maybe not; via the anthropic principle, any other life in the universe would be more likely than normal to have a similar arrangement, and thus wouldn't need to come see ours.
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Correct indeed. Summarily speaking, tidal forces stretch the body in question along a line pointing towards the object which is generating said forces. As such, the default location of the high tides on Earth is on both its nearest to and farthest from sides in relation to the Moon (for lunar tides) and the Sun (for solar tides). When these happen to line up around new or full moon, you get spring tides; when they're perpendicular to each other, you get neap tides.
Of course, in practice it's rather more complex. The constraining shapes of ocean basins and inertial (Coriolis) effects prevent the water from assuming the default bulge shape a simplified analysis suggests. Instead, the tides tend to create vast basin-wide waterflow patterns, with similarities to standing waves. In particular, there exist amphidromic points which behave much like the nodes in a standing wave: locations where some or all constituents of the tide have zero amplitude.
And of course, then you get to the really interesting stuff. However, that'd probably be better dealt with in its own blog post... *cough, cough*
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The moon and sun appearing to be the same size is not in itself important here. The vital point is that they create tides whose sizes are roughly the same order of magnitude (tides from the moon are a bit over twice the height of tides from the sun, as opposed to 1/100th or less like might be expected). As such, the total tide height when they add together (spring tide - it's not named after the season) is much higher than when they cancel out (neap tide).
The idea here is that if the tides were all the same height, tidepools would only form in areas where the tide can always reach and any creatures stuck in one would only have to wait a few hours to get out. However, since the tides aren't all the same height, creatures stuck in a tidepool formed during spring tide (the highest tide) would have to wait up to two weeks to get out, since only another spring tide could reach that point. As such, the hypothesis is that creatures which could move over land would be strongly favored, since they wouldn't be left high and dry for weeks at a time, susceptible to all sorts of dangers like the tidepool drying out, starvation, etc.
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Oops - thanks for correcting that.