The Sun and Moon coincidence, examining a cosmic conundrum

When you look at a solar eclipse, you probably haven't considered a staggering coincidence. A coincidence without which we wouldn’t see this beautiful effect.

Consider, if the Moon’s apparent size from the Earth was small compared to the Sun it would just be a transitory dot as it passed in front of it. Conversely, if the Moon were larger it would just obscure it completely as it passes between the Earth and the Sun. Of course, what we’re hinting at here is that the Moon and Sun have approximately the same apparent size when viewed from the Earth - despite being vastly different in terms of size and distance from us.

So what is going on?

In simple terms the sun is about 400 times the diameter of the Moon - that’s quite a lot. Remember the sun is big - really big - it’s about 865,000 miles across which is over 100 times the size of the Earth. Equally, although in astronomical terms it’s very close to us, compared to the moon it is not near. The sun is about 93 million miles from us, so far in fact that light takes about 8 minutes to get from the sun to the Earth. This means that if the sun were to go out, we wouldn’t find out for 8 minutes!

So what about the Moon? As we have already said it’s around 400 times smaller than the Sun, but you guessed it, it’s also approximately 400 times nearer than the Sun at around 239,000 miles. Now I know that 239,000 multiplied by 400 is not 93 Million, but we’ll come to that later. Orbits aren’t circular so these are averages and approximations.

But here’s the point - despite the Sun being a star that is massive in human terms (not astronomical) and the Moon crudely being a lump of rock that orbits the Earth relatively closely, the result is that the Sun is around 400 times the diameter of the Moon and around 400 times further away. They look the same size from here. This is a mind-blowing coincidence! One of these bodies is a burning ball of gas, the other a lump of inert rock.

So, is this a coincidence? Or is it proof that God only has one template? Or maybe there is some evolutionary benefit to this that means we are here to see it because it allowed us to be here to see it (an anthropic principle).

Let’s delve into this in a little more detail.

---

The Numbers

The Sun

The Sun is 1,392,000 km in diameter. The orbit of the Earth means that its distance varies from 147.1 million km at its closest to 152.1 million km at the furthest.

All of this means that from Earth the Sun has an apparent size of between .52 and .54 degrees.

The Moon

The Moon is 3475 km in diameter. Its orbit of the Earth means that its distance varies from 362,000 km to 405,000km.

Doing the same calculation as before means that the Moon has an apparent size of between 0.49 and 0.55 degrees.

So, from a technical point of view although the Sun and Moon do not have identical apparent sizes from Earth - given the vagaries of orbits there is a significant overlap, and it’s perfectly sensible to suggest that they are the same!

Has this always been true?

Interestingly, modern measurements show that the Moon is moving away from the Earth a few centimetres (3.8) each year. Now this isn’t much but will add up over the millennia.

So 200 million years ago, the Moon would have been approximately 7,600 km closer to the Earth assuming this rate was constant. Plugging this into the angular size calculation means an apparent size of .56 at its closest. This doesn’t feel like a large change and there is plenty of evidence to suggest that the rate hasn’t been constant over large periods of time anyway.

Equally, if we project far enough into the future the Moon will have moved far enough from the Earth that it would not cover the Sun in an eclipse and it would begin to look like a smaller body. However, these timescales are large and the trends debatable. The receding Moon also opens up other questions with regard to its orbit and the speed of rotation of the Earth - but these are not the focus of this article.

Let’s do one final calculation to see how far in the future we would need to go for a truly visible difference to the layman assuming a constant rate of change.

For the sake of argument let’s say that to be truly obvious the angular size of the Moon would need to be .4 as a minimum, so 20% smaller than it appears today. Presumably, most people would spot that!

That means the Moon would need to be at a minimum of approximately 500,000 km from Earth for this to be the case. This would take around 2.5 billion years to happen. I think that’s far enough in the future to ignore for now!

What does it mean?

So where does all of this leave us? It certainly looks like a freakish coincidence - and humans hate coincidences - so let’s see if we can come up with another explanation.

Before we do this, let's discard the elephant in the room. ‘God’ I hear you cry, has a lack of imagination or only the one template, and so on. Well as the Hitchhiker's Guide to the Galaxy explained long ago, God requires faith not proof, so sadly there is no way of proving this so further discussion is effectively pointless.

Now more interesting is, as mentioned earlier, an anthropic principle - put simply, if without 'something' we would not exist then we only observe that thing because it makes us possible. Without it, we would not exist and therefore could not observe it. So not really a coincidence at all, because all the cases when this isn’t the case mean that there is no one to see it. This principle can be weakened to 'likely' and 'unlikely' so that it's just more 'likely' that we exist due to 'something'.

For this to be a possibility with regard to the Moon there needs to be a benefit to the Earth and to us of the Moon being where it is and the size it is. Below are some ideas that we can explore:

• Evolutionary

• Psychological

• Physical protection

• Many-Worlds

Before we do this, it is of course true that the relationship involves the Sun as well, both its size and distance from the Earth. Now we could talk about the output of the sun and ‘goldilocks’ zones but that feels like a distraction. For us to have evolved as we are, dependent on liquid water and the mix of our atmosphere, the habitable zone for the Earth is relatively small. The Moon seems a much more interesting subject until we consider the impact of all this on the search for life around other stars - but that is for another day!

Evolutionary Advantage

So is there an evolutionary advantage to the Moon being the size and position it is to life on Earth?

The most obvious effect of the Moon on Earth - the tides - could have had an impact on the formation of early life in the seas. Particularly as the Moon would have been nearer to the Earth at that point and the effect more pronounced. The tides would have facilitated the movement of primitive life from the seas to the land and perhaps the thermal movements caused by the tides in the seas could even have helped with the development of DNA. This is all theoretical of course, and only relates to the very early development of life on Earth. Most experts suggest that in the end life would have evolved anyway.

The Moon also has other effects on life on Earth. Its reflected light in the dark of night certainly gave rise to creatures that use it and could well have affected the evolutionary path. It also has an effect on some animals who effectively have a ‘lunar clock’, circalunar rhythms, similar to circadian rhythms.

This is all very interesting but doesn’t appear to be critical to the development of Homo Sapiens on Earth. Quoting Jurassic Park - ‘life will find a way’.

Psychological

To early Man, the Moon in the night (and of course day!) sky must have looked like a dark sister to the Sun. Its similar size and nocturnal light would presumably have seemed like an inverse to its brighter partner: Yin to Yang, dark to light, death to life and so forth. The waxing and waning would have added to the symbolism of death and rebirth. The Sun-Moon relationship provides a sense of duality that was key in the beliefs of numerous early cultures.

The other impact on humankind is harder to quantify. The aspirational impact of such a bright body so close to the Earth could have drawn us to the heavens. The motions and patterns of the stars and the seemingly erratic movements of the more obvious planets would seem tiny compared to the sheer size and luminosity of our companion. The Moon is so large compared to the Earth that many have suggested that we are more a double planet than a planet and moon.

More recently the Moon has acted as a close object that we could reach relatively easily. Just 66 years after the first powered flight on Earth, man landed on the Moon. This is an incredibly short period in the sweep of human history. Perhaps without the Moon being so close to draw us out of our cradle, manned spaceflight would not have happened when it did. In the future, the Moon could have its most vital role - providing water and resources as a staging post to the planets and then the stars. The Moon’s low escape velocity makes it a tempting base for large non-aerodynamic ships to use rather than constructing a bespoke space station.

Physical Protection

A theory proposes that the Moon's gravitational influence on the Earth smooths wobbles in the Earth’s motion adding stability to our tilt and axis. Without the Moon, it is suggested, the Earth would be more prone to climatic changes and make survival for the inhabitants more difficult. Further studies have shown however that the Moon isn’t necessary for this extra stability and that the Earth’s tilt is actually very stable anyway. Interestingly this theory is still posted on the NASA website so this is another controversial area.

So does the Moon act like a gravitational ‘force field’ protecting us from debris, asteroids and cometary impacts? The answer to this appears to be a resounding ‘no’, the size and sweep of the Moon are too small to make any significant difference as our ‘cosmic’ protector from physical debris.

But what about cosmic and solar radiation? The early Moon had a much stronger magnetic field than today and it is suggested that this had an important role in shielding the Earth from solar winds which would have been much more significant then. This would have protected our atmosphere at a critical early time that would have then allowed for the evolution of the Earth.

Many - Worlds

The ‘Many Worlds’ theory has become popular in the media with the plot devices of parallel universes, multi-verses, and so on. This is a subject of much debate which verges on faith rather than something that can currently be proven experimentally - let’s look at it briefly!

The Many-Worlds interpretation comes from quantum theory and postulates that for every quantum outcome, all possibilities exist until observed and our current reality collapses to just one, however, all the other possibilities also exist but just not in our universe! So each quantum event creates numerous universes. Effectively this means that there are myriads of parallel universes created every second, some of which would only be slightly different to our own. In a simplistic sense, there would be a spectrum of universes which vary by degree getting larger as the differences add up. There would be no ‘prime’ universe, they would all be equally valid.

So where does this leave us with regard to the Moon? Essentially it's a rehash of the anthropic principle - we are in the universe with the Earth-Moon-Sun relationship as it is, because this is the one with the conditions which allow us to evolve to a point where we can rationalise it.

Conclusion

So after this winding road of ideas and theories have we reached any conclusions? Difficult, as some are still up for debate and the spread of life throughout the universe is not currently known. This would make it easier to define the impact of such factors upon us if we had other examples to compare.

We can conclude a few things. The Moon - Sun relationship has been fundamental in our development. Without the Moon we may have evolved both physically and psychologically as we are - but we may not have. The Moon certainly afforded protection to the early Earth and has provided a stimulus for Mankind to leave our planet. Whether these effects are strong enough to define our existence we will perhaps find out as we explore the Galaxy.

It is probably safe to say that a combination of all of the ideas considered has contributed to our sentience. Maybe not enough to explain why the Earth and Sun look the same size from here, but perhaps to make it much more likely.

During this article, we have been hinting that there is a wider implication for life in the Universe. Should we be looking for Exo-planets like the Earth, in a similar Goldilocks zone, which has a large moon in a similar orbit? I suspect this is overstating it but it can’t hurt to consider it . . .