Eclipse Question

[Jim Leff]

Can anyone explain why it's not a steep coincidence that moon and sun are so very closely matched in apparent size?

[owlice]

They are currently matched in apparent size, but have not always been and will not continue to be.

We got lucky!

[Jim Leff]

Even at greatest variance, they're still pretty damn close. Is it stupendous coincidence, or reasonably expected per some angle I'd not considered?

[rstevenson]

The Moon formed about 4.5 billion years ago, not long after the Earth. It is gradually getting farther away from the Earth. (For an explanation of why this is happening, see the Tidal Evolution part of the Orbit of the Moon Wikipedia page.) Now, 4.5 billion years after the Moon formed, we happen to be at a level of scientific knowledge and technical ability such that we can both understand what an eclipse is, and can use it to study aspects of the Sun normally hidden.

So, yes, it's a coincidence that you and I are here during this particular eclipse, and more broadly speaking, that our society has been and is here during this period in the Moon's very long evolution. But it is not a coincidence that at some point in the evolution of the Moon's orbit, it happens to look close to the same size as the Sun when viewed from the surface of the Earth. That was inevitable.

Rob

[Case]

But it is not a coincidence that at some point in the evolution of the Moon's orbit, it happens to look close to the same size as the Sun when viewed from the surface of the Earth. That was inevitable.

Still, kind of special, as most moons are much smaller compared to their parent-planet. If our moon was much smaller, as is ‘normal’ in our solar system, there may have been no era in which the Moon had a same anger size as the Sun (31 arcmin).
In more than 500 million years, the Moon’s orbit will be so much extended, that even in the best of circumstances (perigee) the eclipse will be annular (not cover the whole disk of the Sun).
When the Moon had just formed, it may have been 15 times closer! That’s a huge disk to move in front of the Sun, but there was nobody to witness it for a long time.

[geckzilla]

I'd say coincidence. But hey, maybe if it was possible to look at all technological civilizations that ever existed in the Universe, we'd find that they all have to have a satellite that closely matches their parent star in size to allow for fancy eclipses. Throw that in your Drake Equation and see how rare technological civilizations are.

[rstevenson]

Quite a few discussions here at the Asterisk devolve into definition games, as each of us has a sense of how we like to use certain words. That's how languages evolve, of course. In this case coincidence has two common meanings...

1. a remarkable concurrence of events or circumstances without apparent causal connection.
2. correspondence in nature or in time of occurrence.

I tend to use the word in its first meaning when talking about a scientifically explainable event like an eclipse. Our race's pre-scientific explanation for the near perfect match between the sizes of the Moon and the Sun certainly would have included the concept of coincidence, if only because we knew of no causal connection. But now we know better. However, as I said above, the fact we few are here now is a coincidence under the second meaning.

Rob

[rstevenson]

I'd say coincidence. But hey, maybe if it was possible to look at all technological civilizations that ever existed in the Universe, we'd find that they all have to have a satellite that closely matches their parent star in size to allow for fancy eclipses. Throw that in your Drake Equation and see how rare technological civilizations are.

It may turn out that fairly large tides are vital in some way for the development of life, and therefore a large moon may be necessary. But I doubt that near total eclipses would be necessary to the devlopment of intelligence, as eclipses in general will be seen anyway -- occasionally, at least -- and will therefore prompt the curious mind to work out what they are.

Rob

[neufer]

I'd say coincidence. But hey, maybe if it was possible to look at all technological civilizations that ever existed in the Universe, we'd find that they all have to have a satellite that closely matches their parent star in size to allow for fancy eclipses. Throw that in your Drake Equation and see how rare technological civilizations are.

Surely it's a coincidence that they are nearly the same size at this time:

Sun: 31.6–32.7 arcminutes
Moon: 29.3 to 34.1 arcminutes

However, we know that the Moon's tidal force has proved us with a stable axial tilt for billions of years. And the Moon's tidal force has dominated the Sun's tidal force for those billions of years only because it's angular size has been as large or larger than that of the Sun .

Hence, it doesn't seem unreasonable to suggest that most of the technological civilizations that have ever existed in the Universe have had a satellite that either closely matches or exceeds their parent star in angular size (and thus allows for total eclipses).

[KayBur]

I am not quite confident, but this is the result of an amazing coincidence. Indeed, the size of the Sun exceeds the size of the Moon by about 400 times - and it is about the same distance from us. As a result, the apparent dimensions of both are almost exactly the same. This situation is unique for 8 planets of the solar system and over 160 known satellites. But every year the Moon moves away from the Earth by 4 centimeters and our descendants will probably see another view.

This is a very interesting theory. But this is probably possible in a few hundred or thousand years. It is a pity that we will not catch this phenomenon. By the way, have you ever thought that by that time humanity might no longer exist? We do not know which asteroid can, for example, collide with the Earth or something else. Inspired by the movie "Armageddon".

[Chris Peterson]

I am not quite confident, but this is the result of an amazing coincidence. Indeed, the size of the Sun exceeds the size of the Moon by about 400 times - and it is about the same distance from us. As a result, the apparent dimensions of both are almost exactly the same. This situation is unique for 8 planets of the solar system and over 160 known satellites. But every year the Moon moves away from the Earth by 4 centimeters and our descendants will probably see another view.

This is a very interesting theory. But this is probably possible in a few hundred or thousand years. It is a pity that we will not catch this phenomenon. By the way, have you ever thought that by that time humanity might no longer exist? We do not know which asteroid can, for example, collide with the Earth or something else. Inspired by the movie "Armageddon".

It isn't a "theory" in any scientific sense of the word. It's an observation. But it's an observation that we can explain using well supported theory.

[neufer]

Can anyone explain why it's not a steep coincidence that moon and sun are so very closely matched in apparent size?

• Liquid water on Earth requires our (long lived & stable) G type star to cover about half a degree in the sky.
  
  The evolution of tidal basin life has clearly benefited from the semi-regular tides of a Moon that covers about half a degree in the sky.
  
  (The influence of such tides on the human female menstrual cycle and other sorts of human development may also be significant.)
  
  And such a large apparent moon has had a stabilizing effect on the Earth's obliquity over millions of years:

Click to play embedded YouTube video.

<<Earth's axial tilt is known to astronomers as the obliquity of the ecliptic, being the angle between the ecliptic and the celestial equator on the celestial sphere. For the past 5 million years, Earth's obliquity has varied between 22° 2′ 33″ and 24° 30′ 16″, with a mean period of 41,040 years. This cycle is a combination of precession and the largest term in the motion of the ecliptic. For the next 1 million years, the cycle will carry the obliquity between 22° 13′ 44″ and 24° 20′ 50″. The Moon has a stabilizing effect on Earth's obliquity. Recent numerical simulations made in 2011 indicated that in the absence of the Moon, Earth's obliquity might vary by about 20–25°. The Moon's stabilizing effect will continue for less than 2 billion years. As the Moon continues to recede from Earth due to tidal acceleration, resonances may occur which will cause large oscillations of the obliquity.>>

[Chris Peterson]

Can anyone explain why it's not a steep coincidence that moon and sun are so very closely matched in apparent size?

• Liquid water on Earth requires our (long lived & stable) G type star to cover about half a degree in the sky.
  
  The evolution of tidal basin life has clearly benefited from the semi-regular tides of a Moon that covers about half a degree in the sky.
  
  (The influence of such tides on the human female menstrual cycle and other sorts of human development may also be significant.)
  
  And such a large apparent moon has had a stabilizing effect on the Earth's obliquity over millions of years:

While those are all true, it doesn't answer the question. There is a much, much greater range of possibilities where the two differ significantly in apparent size. And that great range doesn't alter the role of the Moon in stabilizing the Earth's axial tilt or in providing tides.

It is a coincidence, pure and simple. They happen.

[neufer]

Click to play embedded YouTube video.

It is a coincidence, pure and simple. They happen.

I'll bet you think it's a coincidence that
Boone, Moon, June & Spoon all rhyme.