Why is our moon exactly half when half?

[cjl]

I do not think that this diagram is drawn to scale! Which makes it not very convincing !

It isn't to scale, no, but the geometric effect is still correct. To scale, you'd have a very hard time making out much of anything, since the sun would be so far away.

 

[cjl]

actually not really equal

there usually 2 total lunar eclipses a year and only one total solar eclipse per year
and when you look at the size comparisons of the sun, moon and Earth its kinda easy to see why its a lot easier for the moon to go into the Earth's huge shadow, being so close to the earth, than it is for the small disc of the moon to cover the small disc of the sun

Dave

Not true at all - there are anywhere from two to five solar eclipses per year. Actually, from what I can find with some quick googling, it looks like solar eclipses may in fact be more frequent than lunar eclipses, though I would have to stare at the geometry for a while to figure out why that may be (I don't know off the top of my head).

 

[sophiecentaur]

In short, it should be easier to cover the moon than to be covered by the moon, right? :-)

But I do think the equal frequency is interesting (but not so strange if I think about it).

Roger
PS
By the way, I liked your explanation of the Sun-issue. I am not on-line all the time so I didn't have the time to comment while the topic now is closed. Very interesting debate indeed!

"And some fell on fertile ground"
I don't think PF got what I was on about - but at least one of you did!

 

[sophiecentaur]

It isn't to scale, no, but the geometric effect is still correct. To scale, you'd have a very hard time making out much of anything, since the sun would be so far away.

The Sun doesn't actually need to be in the picture for the effect to be clearly shown. The Penumbra is a very secondary issue but even that could be included without showing the actual source of the illumination.
It could also be a good idea to show pictures of what the two Eclipses would look like to someone on the Moon - for completeness.

 

[technician]

To show the scale of things it is worth making a model of the Sun, Earth, Moon system
The Sun is about 100x the diameter of the Earth so if you have a cardboard disc 100cm dia the Earth would be a disc about 1cm dia (a typical coin?). The Moon is about 1/3 the dia of the Earth so a 3mm disc (a small pea?)
Take this lot outside, place the Sun disc 100m away..place the moon disc about 25cm away...this set up is a fair scale representation of the Earth, Moon, Sun.
If you are involved in education this is a great demonstration for kids to see and to get a feeling for the distances involved.

 

[rogerk8]

To show the scale of things it is worth making a model of the Sun, Earth, Moon system
The Sun is about 100x the diameter of the Earth so if you have a cardboard disc 100cm dia the Earth would be a disc about 1cm dia (a typical coin?). The Moon is about 1/3 the dia of the Earth so a 3mm disc (a small pea?)
Take this lot outside, place the Sun disc 100m away..place the moon disc about 25cm away...this set up is a fair scale representation of the Earth, Moon, Sun.
If you are involved in education this is a great demonstration for kids to see and to get a feeling for the distances involved.

Thank you technician, this was very fun to know!

I am however right now primarily interested in the Solar Eclipse phenomenon.

I have studied the nice picture supplied by DrGreg and did not get it for quite some time.

But now I think I finally get it!

A question does however come to mind.

Has the shadow on Earth the same area everytime a Solar Eclipse occur?

Or is the Lunar Orbit also elliptic?

Because the picture shows everything between a total eclipse to an anular eclipse.

These phenomenon clearly depends, among other things, on Earth to Moon distance (LU).

And I'm still waiting for critics regarding my calculated fact that the LU is too large for generating a total eclipse/moving shadow on the Earth.

There could only be an anular eclipse in my book.

But perhaps there never is "total darkness" while there is a Solar Eclipse?

Perhaps our Solar Eclipses are "just" anular?

Roger

 

[davenn]

Not true at all - there are anywhere from two to five solar eclipses per year. Actually, from what I can find with some quick googling, it looks like solar eclipses may in fact be more frequent than lunar eclipses, though I would have to stare at the geometry for a while to figure out why that may be (I don't know off the top of my head).

you DIDNT read what I said !

TOTAL ECLIPSES

I stand by what I wrote

Dave

 

[davenn]

Roger this diagram of total and annular eclipses over a 20 yr period
will give you a good idea of the area of the Earth's surface covered in each

cheers
Dave

 

[davenn]

But perhaps there never is "total darkness" while there is a Solar Eclipse?

it gets pretty dark, lots of stars become visible, the birds are all going to roost.
You could equate it to a deep twilight. Being in the shadow of totality is awesome, something I never tire of

You will see a total eclipse passing over the USA from NW to SE in August 2017 ... that's my next planned
shadow seeking trip

Perhaps our Solar Eclipses are "just" annular?

no, an annular eclipse has a very specific meaning... the sun isn't fully covered by the moon
and you get an annulus ( ring) of sun visible around the edge of the moonDave