BS: Half moon - science question - ???

Subject: BS: Half moon - science question - ???
From: Little Hawk
Date: 15 Mar 08 - 05:30 PM

Here's something that's puzzled me from time to time.

We all know that it is the shadow of the Earth on the Moon (or the lack of said shadow) that creates the phases of the moon that we see...such as half moon, quarter moon, new moon, full moon, etc...

Fine.

Now, here's the problem. I'm looking at a nice half moon today, easily visible in the bright blue afternoon sky on a brightly sunlit day. You can see the half of the moon that's lit by the sun, but the dark half is invisible. The moon has climbed about halfway to the Zenith.

The sun is still far above the western horizon, shining brightly.

Now here's my question. Given the fact that I can plainly see the entire moon, well above the Earth's horizon...and the entire sun, also well above the Earth's horizon, both with a lot of blue sky between them and the Earth's horizon....

How the hell can the Earth be interposing itself at this time between the sun and the moon in such a way as for the Earth's shadow to block out half the moon from the sunlight?

I can see plainly from where I am standing that that makes no geometrical sense at all, because the sun and the moon are both way "above" every part of the Earth that is visible from where I stand. WAY above it.

So how can any part of the Earth itself be getting in between them right now and causing its shadow to put half the moon in darkness?

This totally puzzles me. It appears to defy the normal rules of geometry. Light goes in a straight line, correct? If anything was in between the sun and the moon right now, it would have to be way up above me, I would think, since they are both way above the eastern and western horizons. Anyone got the explanation for this apparent anomaly?

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 15 Mar 08 - 05:40 PM

1000 words.

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 15 Mar 08 - 05:42 PM

10 x 10 x 10 words.

Subject: RE: BS: Half moon - science question - ???
From: Jean(eanjay)
Date: 15 Mar 08 - 05:50 PM

Little Hawk,

I wouldn't have a clue but I just love your posts.

PS:

I love Peace's posts too.

Subject: RE: BS: Half moon - science question - ???
From: Little Hawk
Date: 15 Mar 08 - 05:53 PM

Ah-hah! Got it. Thanks, Peace. I was simply getting confused between the shadow effect and the just plain illumination of the sun on the moon effect. ;-) Totally obvious when seen in your pictures. The Earth's shadow effect I was thinking of would only happen in a lunar eclipse, of course.

Subject: RE: BS: Half moon - science question - ???
From: McGrath of Harlow
Date: 15 Mar 08 - 05:55 PM

It is wise to be cautious when coming across the expression "We all know "...

Subject: RE: BS: Half moon - science question - ???
From: John O'L
Date: 15 Mar 08 - 06:02 PM

That was a brave question Little Hawk.

We don't get an eclipse every night. The sun can only shine on the part of the moon that is facing it.

The dark side of the moon is so named because the sunlight is obscured by the bright side.

Subject: RE: BS: Half moon - science question - ???
From: Little Hawk
Date: 15 Mar 08 - 06:14 PM

Yup. ;-) Got it, John. I'm not an astronomer, and I haven't given much time to that subject or I would have known immediately that it wasn't the Earth's shadow causing the phases of the moon. The moment I saw Peace's link it was like..."Oh!!! Of course! The illuminated part shows, the dark part doesn't."

I'd been wondering about it for awhile whenever I'd see a partial moon on a sunny day, but one is always afraid to ask such questions...because you think, "Well, some people out there will think I'm a dummy if I ask this...and I just couldn't go on living if that happened...!"

Being afraid to ask such seemingly basic questions which you figure many others must already know the answer to can end up making one a dummy on any particular subject for one's entire life. I've seen that happen to more than a few people. It was quite a problem for many back in school, I recall. The old "even a fool will be thought wise if he remain silent..." syndrome.

I'll have to see if I can come up with any further glaring gaps in my basic knowledge here. ;-) Hmmm. Gimme some time to think about that.

Wait. I got it. How do you get a girl pregnant?


(That's a joke. Don't bother answering it, please!)

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 15 Mar 08 - 06:24 PM

LH, you asked a question re the moon that bothered me for decades. Heck. You looked at the pictures and 'got' it. I looked at similar pics way back and a physicist friend of mine took an entire afternoon explaining what I was looking at and THEN I got it.

Subject: RE: BS: Half moon - science question - ???
From: Bert
Date: 15 Mar 08 - 06:29 PM

Good question and good answer. Now Here's a song about the moon.

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 15 Mar 08 - 06:31 PM

Regarding the pregnancy question, here's a pic that explains it.

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 15 Mar 08 - 06:33 PM

Bert, that's beautiful.

Subject: RE: BS: Half moon - science question - ???
From: Slag
Date: 15 Mar 08 - 06:34 PM

Wrong! Earth's shadow has nothing to do with Moon phases, only lunar eclipses. And your half-moon is really a quarter-moon! Take a large and a small ball and a single light source from a distance. Set them side by side and imagine your point of view as some point on the big ball and as you do this remember that the moon's orbit about the Earth is highly incline, i.e. it does not follow around the equator but it is like a hat brim tilted back on your head. Very seldom does the Moon ever cast its shadow on the earth or vice versa.

As you move the moon ( the small ball ) in a circle about the big ball (Earth) you will see that half the ball is always in light but that lit portion is not always fully visible from Earth. When it is on the farther point away from the light source ( the Sun) we see a full Moon. In seven days the Moon is at a right angle from the Earth/Sun line. That's when we see the "half-moon" which is the third quarter: three quarters of the way around the Earth. When the Moon is more or less between the Earth and Sun (7 days later) it disappears and this is the "New Moon". Seven more days it is at a right angle to the left again we see a "half-moon" and this is the first quarter which is on the PM side of things. The Moon transverses about 13 degrees of sky a day and makes an orbit in about 29 days. Approximations because there are other considerations which show up as variables.

If the Earth/Moon/Sun system actually did lie in the same plane as on a floor then the would be a lunar eclipse at every full moon and a solar eclipse at every new moon. But such is not the case. Again, the orbit is inclined so such encounters are relatively rare events. I think I remember reading that in 360 years nearly every spot on Earth will have experienced a solar eclipse, but I'm not waiting! 2012 I hope to be in Australia for such an event!

In interesting phenomenon can be seen clearly around the crescent phases and that is Earthshine falling on the otherwise darkened portion of the Moon. Sometimes a faint reddish glow is seen caused by the crud in Earth's atmosphere. I hope this has helped.

As the Earth and the Moon plow around the Sun in their annual journey they weave an interesting dance. The Earth is little moved by the Moon but the Moon traces a wavy line, in and out, back and forth across the Earth's track. It's a fairly easy thing to plot this out, either in whole or in portion, on a piece of paper and then connect the dots.

Tom

Subject: RE: BS: Half moon - science question - ???
From: Georgiansilver
Date: 15 Mar 08 - 06:34 PM

Peace...using ropes for sex never did appeal to me!

Subject: RE: BS: Half moon - science question - ???
From: Slag
Date: 15 Mar 08 - 06:36 PM

Sorry I took so long typing the above! Seems as though hyou got the answer before I got it posted! Too many interuptions!

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 15 Mar 08 - 06:39 PM

That's not ropes you twisted soul. That's the guy thingy following the arrow going to the girl thingy. Then there's stuff that happens and the stork comes and delivers the cabbage leaf. It's elementary.

Subject: RE: BS: Half moon - science question - ???
From: Slag
Date: 15 Mar 08 - 06:41 PM

Enough of the Cowboy sex!

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 15 Mar 08 - 06:45 PM

Huh. Laugh all ya want. HERE'S the science on it.

Subject: RE: BS: Half moon - science question - ???
From: Little Hawk
Date: 15 Mar 08 - 06:54 PM

Wow. Complicated. This explains why I don't have any children, I guess...I just couldn't manage to follow that whole sequence without losing track somewhere along the way.

Hey, Bert, nice song!

Subject: RE: BS: Half moon - science question - ???
From: JohnInKansas
Date: 15 Mar 08 - 10:14 PM

Dang it! You guys got Little Hawk all straightened out before I got a chance to confuse him with the "ephemeris jump" effect.

Explainin' to him that the moon isn't where it looks like it is any time it's not straight overhead could have diverted him from figurin' out his question and we might have kept him from gettin' educated for weeks (or at least for a few more good posts).

John

Subject: RE: BS: Half moon - science question - ???
From: folk1e
Date: 15 Mar 08 - 10:35 PM

Would it be pedantic to mention that there is no darkside of the moon?
The moon rotates at the same rate it orbits, ie it always shows the same face to earth (tidal effect). But all points on the moon are lit at one time or another. The only ecception to this was thought to be Mercury, but we now know it does rotate at a different (slightly) rate to its orbit!

Subject: RE: BS: Half moon - science question - ???
From: Beer
Date: 15 Mar 08 - 10:48 PM

Nice Bert. Very enjoyable.
Beer (adrien)

Subject: RE: BS: Half moon - science question - ???
From: Slag
Date: 16 Mar 08 - 12:30 AM

I wished you hadn't , uh, truncated that diagram! I was really getting into it. I also learned that 9 sub 27 was not the one for tying on Royal Caochmen to your leader. Cost me an 8 pound or better trout!

Subject: RE: BS: Half moon - science question - ???
From: Little Hawk
Date: 16 Mar 08 - 12:53 PM

Does it seem odd to anyone else but me that the moon rotates at exactly the same rate as it orbits the Earth, thus always presenting the same side of the moon to our vision? Quite a coincidence!

Is that not strange??? It's more than strange. It's almost unbelievable.

Just sit down for a minute and try and figure out what the odds are against it occurring that way...

I think it's got to be one of the oddest things I've ever heard of. No one on this planet ever got to see the far side of the moon until the space missions of the late 60's.

I wouldn't think this would be the case with many planetary satellites (which also rotate on their own axis while orbiting their planet). I would assume that some of them rotate (on their own axis) and that others don't, but I can't say I know for sure. What's the case with the moons of Mars, Jupiter or Saturn, for instance? Rotation? No rotation? Different rates of rotation?

Subject: RE: BS: Half moon - science question - ???
From: Bill D
Date: 16 Mar 08 - 02:29 PM

Moon spin

Subject: RE: BS: Half moon - science question - ???
From: sapper82
Date: 16 Mar 08 - 02:37 PM

You got that a bit wrong Little Hawk.
The light on the moon is direct from the sun and the phase of the moon depends on the viewing angle.
To demonstrate:-
1: Get yourself a torch with a good strong beam and a ball.
2: Set up torch and ball so that the torch shines DIRECTLY onto the ball from a distance of 4 or 5 feet.
3: Draw the curtains and turn the lights off.
4: Walk around the ball and look at the way that, from different angles, the light from the torch only lights up a portion of the ball as seen from each angle.
If you view from behind the torch you will see the full ball;
If you view from 90 degrees to the light beam you will see a half ball.
At different angles you will see a different "phase of the ball."
The phases of the moon are caused by the same effect.

Subject: RE: BS: Half moon - science question - ???
From: GUEST
Date: 16 Mar 08 - 08:00 PM

"Does it seem odd to anyone else but me that the moon rotates at exactly the same rate as it orbits the Earth, thus always presenting the same side of the moon to our vision? Quite a coincidence!"
Hey LH, Bill D's reference may not be the whole story. It also seems that there is uneven mass distribution with the globe of the moon; this was demonstrated by analysis and the difference between the expected and actual velocities of the various Apollos and other spacecraft as they orbited the moon. These Mass Concentrations, or Mascons, made the capsule speed faster, of sometimes slow down. This was a separate effect from speed changes caused by Kelperian motion.
The far side of the moon (not dark side, that's a great stoner recording) has substantially more craters that then "our" side. Our side has huge "seas"(the dark spots in the surface of lovely Luna) all across the surface. These were at one time great lakes of molten rock that welled up from the core as the moon was being subjected to insane levels of meteor bombardment during the period after it was ripped from the Earth when our Terra Infirma was hit by a mass about the size of Mars.
The lava lakes cooled to form great swathes of the closer side with greater density than the average surface material. The Earth was also hit with about the same violence, but the evidence has been mostly eroded by erosion and geologic subduction.*
This mass tends to keep itself pointed in our direction because of the greater effect that the earth's pull exerts on the mass of the close side.
The Moon certainly had a different rotational period a VERY long time ago. The coupling effect forces eventually put the moon in sync with the Earth. It also changed the length of the day to 24 hours. At one time the cooling molten Earth spun at a very different rate (about 30 hours, as I recall. Don't quote me).
If you are VERY unlucky, some other smart-ass will complicate the matter even further by discussing how interconnected gravitation fields of orbiting bodies result in various LaGrange Points.
* for a view of one very large impact crater in your own Canada,eh, just have a look at the notch cut out of the South-East shore of Hudson's Bay. There are many others on the general area, but this is easy to spot on any map.

Subject: RE: BS: Half moon - science question - ???
From: GUEST
Date: 16 Mar 08 - 08:01 PM

That Guest is me, Skivee. Looks like my cookie spoiled.

Subject: RE: BS: Half moon - science question - ???
From: Amos
Date: 16 Mar 08 - 08:05 PM

SKivee:

What? Where was I when all this was going on?


A

Subject: RE: BS: Half moon - science question - ???
From: Skivee
Date: 16 Mar 08 - 08:07 PM

You weren't even a gleem in the cosmo's eye yet.

Subject: RE: BS: Half moon - science question - ???
From: John Hardly
Date: 16 Mar 08 - 08:15 PM

It's funny, but I once asked (just to satisfy my curiousity) more than 20 adults (including two doctors) before I got the correct answer as to what created the phases of the moon. EVERY ONE of them thought, as LH, that it was the same phenomenon as an eclipse. I find that fascinating that SO MANY people could be so similarly confused.

Subject: RE: BS: Half moon - science question - ???
From: Skivee
Date: 16 Mar 08 - 08:20 PM

To clear up my bit about the impact crater on Hudson's Bay. It is the Southeastern feature with the essentially circular coastline, not the part that reaches even further south.

Subject: RE: BS: Half moon - science question - ???
From: Slag
Date: 16 Mar 08 - 09:36 PM

LaGrange points aside, the moon also wobbles a bit and if you were very adept at viewing you woul notice that through a year of the lunar cycle you can see about 53% of the lunar surface from Earth.

Another odd thing LH! Isn't it strange that the analogous structure of the lunar surface ( mountains and such ) is just beyond the range of the unaided human eye. If the moon rotated where we could see the "darkside" mankind might have figured out the true(r) nature of the cosmos much sooner than we did.

I haven't done the search on Google but there is something called the Goldylocks effect about all these strange little coincidences. No time right now but I will run them down at some point and let you know, that is, if someone doesn't beat me to it!

Subject: RE: BS: Half moon - science question - ???
From: Little Hawk
Date: 16 Mar 08 - 09:36 PM

Cool! Thanks for the info on rate of rotation and all that, guys. (BillD and Skivee). Very interesting stuff.

sapper - Yeah, I already had that neatly explained by some previous posters on this thread.

You know what, John, I began to realize, after thinking about it for a bit, that there was a time in my younger years when I did understand full well what causes the visible phases of the moon (from our point of view here on Earth)....but at some point I forgot all about it! The discussion here has reminded me that I did understand it at one time, I'm not sure exactly when it was....could have been in my teens or 20s, I guess.

Then at some point I completely forgot what I had known about it, and I later got the idea that it was the Earth's shadow that was doing it....which is kind of odd, really, because what would then distinguish ordinary phases of the moon from lunar eclipses? Nothing. And why would that not have occurred to me right away?

This may be what happens to the human brain over a lifetime as it slowly fills up with way too much information...it starts to forget half of it, specially the older stuff!

Either that or I'm sliding slowly into senile dementia... ;-)

Quite seriously, though, I do think it gets a lot harder to keep track of all the past backlog of information as the decades slide by. I probably knew a ton of stuff at 25 that I have since forgotten. (and, God knows, at 25 I figured I knew everything!)

Subject: RE: BS: Half moon - science question - ???
From: John Hardly
Date: 16 Mar 08 - 09:50 PM

I had the whole thing just reoccur to me once when I was in my thirties. I was outside tending my kiln and I looked up at the moon. I could tell what was causing the phase by the angle of the just set sun.

But more than that, few things can give you a sense of the immense distances quite like realizing the angle and distance the sun has to be from the moon in order to create the phases. Observing it that chilly winter evening, it took my breath away.

Oh, and it's not blue cheese. It's a potato.

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 16 Mar 08 - 09:54 PM

The eyes have it.

Subject: RE: BS: Half moon - science question - ???
From: number 6
Date: 16 Mar 08 - 10:14 PM

If you take your time and study the (medieval)picture in the link ... it will answer a lot of questions.

the earth in perspective with the universe

biLL

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 16 Mar 08 - 10:44 PM

The person in the pic seems to be wearing PF Flyers. Like, how cool is that?

Subject: RE: BS: Half moon - science question - ???
From: Peace
Date: 16 Mar 08 - 10:46 PM

Full moon.

It was just a matter of time.

Subject: RE: BS: Half moon - science question - ???
From: Little Hawk
Date: 16 Mar 08 - 11:07 PM

Shane has been known to do that on occasion.

Subject: RE: BS: Half moon - science question - ???
From: Slag
Date: 16 Mar 08 - 11:53 PM

Once in a Blue Moon?

Subject: RE: BS: Half moon - science question - ???
From: Rowan
Date: 16 Mar 08 - 11:54 PM

I'm looking at a nice half moon today, easily visible in the bright blue afternoon sky on a brightly sunlit day. You can see the half of the moon that's lit by the sun, but the dark half is invisible.

If you'd waited until a while after sunset you'd also have seen the "invisible" half. That's because there is, in most of the orientations of the Sun-Earth-Moon system, sufficient light reflected from the earth's surface to reach the moon, illuminate even the part of its surface shaded from the sun and be reflected back to the earth. When there is very little haze you can even see the moon when it appears as only a fingernail and thus near "new".

I suspect it is an incompletely remembered understanding of this type of visibility that is at the bottom of Little Hawk's (and others') misapprehension that inspired his original question. And, while we're at it, the "perfect" full moon is an eclipsed one (a full lunar eclipse) and the light reflected from the moon's surface in that moment is sunlight that has been refracted through the earth's atmosphere, which is why it is copper-coloured. A full solar eclipse, conversely, is the "perfect" new moon, but it's not a good idea to look directly at it.

Cheers, Rowan

Subject: RE: BS: Half moon - science question - ???
From: Little Hawk
Date: 17 Mar 08 - 12:51 AM

Right you are, Rowan. It's a very interesting sight to see when the "invisible" half also becomes visible alongside the brighter section.

Subject: RE: BS: Half moon - science question - ???
From: Skivee
Date: 17 Mar 08 - 01:21 AM

Regarding the relative safety of Solar eclipses,
" it's like I told my young son, keep doing that and you'll go blind. He said , Dah, I'm over here."
*Du-dumchick***.
Try the meatloaf. I'll be here all the week. Come back with your friends.
But seriously, you have to take a few reasonable precautions. I've seen at least 5 without dammage. I also don't know any cautious observer who has damaged their eyes. Real inccidents and urban legends aside. There are folks who should be protected from viewing them; but most will have an experience that their will cherish.

Subject: RE: BS: Half moon - science question - ???
From: JohnInKansas
Date: 17 Mar 08 - 04:21 AM

On the subject of "moon spinning," I seem to recall that there is one moon in the solar system with "retrograde rotation," but don't recall which planet might be its owner.

In the commonly assumed formation of a moon by ejection from its planet by some outside impact, the moon would be expected to have the same angular velocity of rotation as the surface from which it was spawned, and under most impact/ejection conditions a lateral velocity close to the tangential surface speed from which it came.

The tendency then would be for a moon always to orbit in the same direction as the planet's rotation, and also to rotate in the same direction as the planet, as it does for nearly all the moons known. The speed of rotation could be slowed by tidal forces, as has been explained above, but probably would ever increase once the "lock" condition in which one face always points to the planet, as for our moon, has been reached - except by an extraordiary event.

I recall that someone (I think it was Foolestroupe) once suggested the General Purpose Answer for such questions, but I'm a bit too lazy at the moment to pursue the subject to something more satisfactory. Does anyone recall a "reverse rotating" moon in the solar system - or elsewhere?

John

Subject: RE: BS: Half moon - science question - ???
From: GUEST,Keinstein
Date: 17 Mar 08 - 04:36 AM

Triton is retrograde with regard to Neptune. Jupiter and Uranus have moons some of which are retrograde and some direct. It's difficult to imagine retrograde bodies in the asteroid or De Kuyper belts having lasted until now.

I think the Russians missed an opportunity when they reverted the name of their second city from Leningrad to St. Petersburg. They should have called it Retrograd.

Subject: RE: BS: Half moon - science question - ???
From: Keith A of Hertford
Date: 17 Mar 08 - 09:51 AM

When sun and moon are both in the sky, it still looks wrong.
You expect to see the sun side of the moon all lit up.
You have to remember that the moon is local and the sun is not.
The moon sees the sun in the same direction that you see the sun.
The sun does not shine across our sky to the moon.

And it is not chance that keeps the same face to us.
The effect of tidal friction is to match both.
Earth is slowing down and would one day keep the same face turned to the moon (except that the moon will have escaped by then for a similar reason)

Subject: RE: BS: Half moon - science question - ???
From: Liz the Squeak
Date: 17 Mar 08 - 10:12 AM

If you're in the UK, on TV at the moment they are advertising one of those weekly 'build it week by week if we ever get around to publishing the last installment' magazines where you can acquire and build yourself a brass orrery - only they call it a "model of the solar system". That might help you explain your 'dark side of the moon' problems.

It'll only cost you £319 (about $650) over a year.

Re: those diagrams. I think 9.27 is how I put my back out in '84.

7.22 is impossible without lubrication and a penguin.

LTS

Subject: RE: BS: Half moon - science question - ???
From: Mrrzy
Date: 17 Mar 08 - 10:41 AM

I am reminded of something but I can't remember what...