Solar Eclipse - Are we just lucky?

[cramdogs]

Looking at the earth-moon-sun system, it just so happens that:

. The sun is ~400 times the size of our moon
. Our moon is ~400 times further away from the sun than from the earth

The sun and moon look ~ the same size from earth and hence during alignment, allows the almost perfect conditions for total solar eclipses with spectacular full corona viewing.

Was the earth-moon-sun system always bound to have this almost perfect distance/size ratio, or are we just extremely lucky.

[Borek]

IMHO we are just lucky.

But could there is some explanation. I see it in the same category as Titius–Bode law - it is either a coincidence, or there is some deeper reason.

[Ryan_m_b]

It's hardly perfect (whatever "perfect" means). Because the orbits are not in exactly in alignment we get partial, annular and hybrid (http://en.wikipedia.org/wiki/Solar_eclipse#Types) eclipses more often than total thanks to the variance in the Moon's orbit. Also thanks to the moon's rugged surface we get a phenomenon known as Baily's beads (http://en.wikipedia.org/wiki/Baily's_beads).

I would shy away from the term "perfect" because it implies that there is some ideal that a system should aspire to. In reality the Moon is the way it is thanks to it's formation and it's orbit. Remember that the Moon as we see it is a lot further away than it used to be. IIRC when it first formed it was three times closer and is edging further away all the time.

[Janus]

Looking at the earth-moon-sun system, it just so happens that:

. The sun is ~400 times the size of our moon
. Our moon is ~400 times further away from the sun than from the earth

The sun and moon look ~ the same size from earth and hence during alignment, allows the almost perfect conditions for total solar eclipses with spectacular full corona viewing.

Was the earth-moon-sun system always bound to have this almost perfect distance/size ratio, or are we just extremely lucky.


Due to the eccentricity of both the Moon's and Earth's orbits, there is some variation between the Sun's and Moon's apparent size. Sometimes the moon is a bit larger than the Sun and sometimes it is a bit smaller.

This last case is what leads to the annular eclipses mentioned in the previous post. The Moon, even when centered on the Sun, cannot cover the whole and we see a ring or "annulus" around the Moon caused by the Sun peaking around the edges.

[cramdogs]

It's hardly perfect (whatever "perfect" means). Because the orbits are not in exactly in alignment we get partial, annular and hybrid (http://en.wikipedia.org/wiki/Solar_eclipse#Types) eclipses more often than total thanks to the variance in the Moon's orbit. Also thanks to the moon's rugged surface we get a phenomenon known as Baily's beads (http://en.wikipedia.org/wiki/Baily's_beads).

I would shy away from the term "perfect" because it implies that there is some ideal that a system should aspire to. In reality the Moon is the way it is thanks to it's formation and it's orbit. Remember that the Moon as we see it is a lot further away than it used to be. IIRC when it first formed it was three times closer and is edging further away all the time.

well i used almost perfect quite loosely. Even with the orbital variance, I find it quite amazing that we have this system in a fine window where these total solar eclipses with full veiwable coronas are possible. Or maybe the window isn't as 'fine' as I think...

I just wondered if for some reason this distance/size ratio for the earth-moon-sun system was 'favoured' during its formation or say after millions of years, the system wanted to approach this setup. But yes, the evidence that the moon is slowly receeding away from earth possibly indicates that the conclusion to make, is that simply we are lucky to experience such an event!

[I like Serena]

Remember that the Moon as we see it is a lot further away than it used to be. IIRC when it first formed it was three times closer and is edging further away all the time.

I didn't know that!
Afaik the moon's orbit is unstable, it may edge further away, but it also may get closer. It may even impact on earth!
Do you have any basis for your statement?

[Janus]

I didn't know that!
Afaik the moon's orbit is unstable, it may edge further away, but it also may get closer. It may even impact on earth!
Do you have any basis for your statement?

It has to do with the tidal interaction between the Moon and Earth. As the Moon raises tides on the Earth it slows the Earth's rotation. In turn some of the Energy lost by the Earth is transferred to the Moon, pushing it into an higher orbit. The present of orbital increase is ~3.8cm per year.

http://eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html

[Ryan_m_b]

I didn't know that!
Afaik the moon's orbit is unstable, it may edge further away, but it also may get closer. It may even impact on earth!
Do you have any basis for your statement?

What Janus said. The moon is receding from Earth and in the process slows down the Earth's spin. I'm afraid I don't have a link from where I heard it (it was a documentary years ago) but IIRC the day length on Earth was much shorter too.

[Borek]

Not only day was shorter, as a side effect year had more days - and effects are visible in fossils.

[Drakkith]

Not only day was shorter, as a side effect year had more days - and effects are visible in fossils.

Is it? That makes sense lol. I didn't know we could discern how many days in a year from fossils.

[Borek]

Is it? That makes sense lol. I didn't know we could discern how many days in a year from fossils.

In some fossils there are tow kinds of growth rings - daily ones and yearly ones. That allows calculation of how many days per year. I think it was done using diatoms frustules.

I have read about it in one of von Ditfurth books.

[Drakkith]

Ah ok. Thanks Borek.