moon = sun in size on retina

[gib65]

The size of the moon and the size of the sun on the human retina are equal. They are both about 0.5 degrees. I was wondering if there was more to this than mere coincidence - like is it somehow essential that they are the same perceptual size for life to evolve on Earth. Is it important, for some obscure reason, to have perfect solar eclipses now and again?

[AzurePhoenix]

Well as far as I know, the moon is getting further and further away a little bit every years. Many millions of years ago it looked larger, and in many, many years it will look smaller, eventually making eclipses less than total.

[Sisyphus]

And it's not exactly equal. It's just close. By "perfect" do you mean complete? Because that would still happen if the moon was bigger or the sun was smaller. Anyway, I don't think a few minutes of unscheduled darkness every hundred years (or however long the time is between eclipses at a given location) would make any difference in anything.

[Martin]

The size of the moon and the size of the sun on the human retina are equal. They are both about 0.5 degrees. I was wondering if there was more to this than mere coincidence - like is it somehow essential that they are the same perceptual size for life to evolve on Earth. Is it important, for some obscure reason, to have perfect solar eclipses now and again?

 

I never heard anyone give a reason why having them about the same angular size (half a degree like you say) is especially favorable to life.

 

If there are other sentient species in our galaxy, or in other galaxies, they probably see all kinds of moon including several (like Mars has two) and mostly smaller. We have a pretty big moon. I would guess most people's is smaller.

 

having a massive moon like ours helps to stabilize the rotation plane of the earth----stabilize the axis from too much wobbling. have to get someone else here to confirm this---I dont have a link.

 

not to speak authoritatively, my opinion is that it really has benefitted life that the earth moon is around 1/80 of the earth mass----but only because that is SUBSTANTIAL

 

It is not the exact mass ratio that matters, or the exact angular size. it is just that our moon is hefty enough and near enough that it raises TIDES which I think are helpful to evolving life and it also IMO helps to stabilize the axis which stabilizes the SEASONS and helps evolution by making things kind of reliably moderate.

 

evolution can get screwed up if a planet is always going to extremes of heat and cold in its hemispheres---as with an axis tilted over 90 degrees.

 

I just dont think the ECLIPSE thing you mentioned is so important.

 

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if you are talking about the evolution of a mathematical CIVILIZATION like we got from Babylonians and Greeks then what you say is very important. Eclipses were very helpful to the first greeks who estimated the relative distances of sun and moon (Aristarchos) and reckoned the ratio of earth diameter to moon distance (Hipparchos).

 

having such good eclipses was a great piece of luck for humans in getting them to apply geometry to the heavens---and the rest is history.

[swansont]

Just to note that even now not all total eclipses are "total." The moon's orbit isn't a perfect circle. Sometimes it's a little further away, enough to get an annular eclipse.

 

picture 1

 

picture 2

[Severian]

This is an interesting question because it if a 'fine-tuning' problem. The angular size of the sun or moon is approximately given by

[math]\theta = \frac{2R}{d}[/math]

where [math]\theta[/math] is the angular size in radians, [math]d[/math] is the distance from the Earth to the sun or moon, and [math]R[/math] is the sun or moon's radius.

 

The fact that they are nearly equal tells us (in an obvious notation)

[math]\frac{d_s}{R_s} - \frac{d_m}{R_m}\approx 0[/math]

 

The actual values are:

[math]R_s = 6.961 \times 10^8 m[/math]

[math]d_s = 1.496 \times 10^11 m[/math]

[math]R_m=1.738 \times 10^6 m[/math]

[math]d_m = 3.844 \times 10^8 m[/math]

 

So the previous equation becomes:

[math]\frac{d_s}{R_s} - \frac{d_m}{R_m}

=214.9 -221.2 = -6.3 [/math]

 

Now, in principle, a theory is fine tuned if changing the fundamental parameters (in this case, the sun/moon distance and radii) by x% changes a physical observable (in this case the difference in the inverse angular sizes) by y% where y>>x.

 

Increasing [math]d_s[/math] by 10% gives [math]\frac{d_s}{R_s}=236.4[/math] so the difference becomes 15.2 which is a change of roughly 300%.

 

So the theory is fine-tuned and many modern theoretical physicists seeing the model of the solar system for the first time would have discarded it on the grounds that it is fine-tuned. I think this illustrates the dangers of discarding theories on aesthetic reasons....

[Douglas]

larger, and in many, many years it will look smaller, eventually making eclipses less than total.

The annular eclipse is an eclipse that is less than total. These are more likely to happen in July, when the sun looks a little smaller, in conjunction with the moon been closest to earth where it looks a little bigger.

[swansont]

The annular eclipse is an eclipse that is less than total. These are more likely to happen in July, when the sun looks a little smaller, in conjunction with the moon been closest to earth where it looks a little bigger.

 

You have it backwards. You want big sun + small moon. The sun is closest, and thus appears largest, in early January.

 

 

 

If I were the sarcastic type I'd make a comment about wishing I'd said something about annular eclipses.

[Douglas]

You have it backwards. You want big sun + small moon. The sun is closest' date=' and thus appears largest, in early January.

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Oops, yer right.