# Seeing a new moon?

1. Sep 15, 2006

### pivoxa15

The reason why we see the full face of the moon (when it is opposite to the sun) is the effect of diffraction of light around the spherical surface of the earth. Correct?

There can't be another explanation since without diffraction, the moon should be dark just like when it is on the same side as the sun. So this lends naturally to the confirmation of the diffraction theory of light? Hence prior to "Fresnel's spot" experiment, this simple observation should have at least convinced a lot of people to the wave theory of light.

Newton was unconvinced about the wave nature all his life? Or did he accept it after Fresnel's spot and how did he reconcile it with his particle theory of light. In any case, how did he explain seeing a full moon prior to Fresnel's spot and diffraction?

Last edited: Sep 15, 2006
2. Sep 15, 2006

### Integral

Staff Emeritus
Recall that when the earth sees a new moon, the moon sees a "full earth" the surface of the moon is lit by light reflected from the full earth.

3. Sep 15, 2006

### Staff: Mentor

This sounds like you're asking about the full moon, not the new moon (which is when the moon is on the same side of the earth as the sun). We can see it because the the moon's orbit is inclined to the earth's equator, so it usually "misses" the earth's shadow as it passes behind the earth. Sometimes it does pass through the earth's shadow, and that's when we have a lunar eclipse.

We can still see the moon (faintly) during a lunar eclipse because some light is refracted by the earth's atmosphere. If the earth didn't have an atmosphere, the moon would be almost totally dark during a total lunar eclipse. I seriously doubt that diffraction has a significant effect here.

4. Sep 15, 2006

### pivoxa15

You are correct about the title, it should have the word full instead of new.

How does 'the moon's orbit is inclined to the earth's equator, so it usually "misses" the earth's shadow as it passes behind the earth.' work? Is there a diagram?

5. Sep 15, 2006

### pivoxa15

The bit of the earth the moon sees, is also the part where the light from the sun cannot reach. The earth at that position, of opposite the sun is also near midnight. Back in pre electricity days, what kind of light from earth would be shining to the moon at that position and time?

6. Sep 15, 2006

### Staff: Mentor

They are inclined planes. Go about 5 slides deep in this java applet and you can see how Pluto's orbit is inclined to the plane of the other planets: http://liftoff.msfc.nasa.gov/academy/space/solarsystem/solarsystemjava.html

The moon's orbit works the same way. And they are inclined to the ecliptic or celestial equator - not the earth's equator.

Attached is an image showing the view from earth: the line is the ecliptic: the plane on which the sun travels during the year, from month to month. The moon in that pic is slightly below that plane due to its inclined orbit, but there are two places on that orbit when the moon crosses the ecliptic and at those times, you may get an eclipse.
You are getting full and new moon mixed up again:

-When the moon is full, the sun and moon are on opposite sides of the earth. The moon is fully illuminated by the sun and the side of earth facing it is completely dark.
-When the moon is new, the sun and moon are on the same side of the earth. The moon is partially illuminated by light reflected from earth, as the side of the earth facing the moon is completely lit.

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7. Sep 15, 2006

### Staff: Mentor

Here's my attempt to draw this. The earth's and moon's orbits are circles, but viewed or inclined at different angles. An important point is that the axis of the moon's orbit always points in the same direction with respect to the surrounding space regardless of where the earth is in its orbit.

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8. Sep 17, 2006

### pivoxa15

Nice picture. I now understand why we tend most of the time see a full moon even though its position is lined up withe earth when viewed from top down.

One question that arises is how close does the moon's orbit intersect the earth's orbit? Close enough for a collision? If so than why hasn't there been a collision for ages (if at all)?

Do you know that angle of inclination of moon's orbit as shown in your diagram?

9. Sep 17, 2006

### pivoxa15

With the last statement about the new moon, does that mean the new moon always occur in the day and never at night (for people on earth)?

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10. Sep 17, 2006

### Labguy

Do you think that might have anything to do with where you might live on Earth? We do spin around, you know. I heard that it happens every day...

11. Sep 17, 2006

### Farsight

Sometimes I'm not sure if questioners are serious.

12. Sep 17, 2006

### Staff: Mentor

The moon is in orbit around the earth, so no, there can't be a collision.
5 degrees. http://en.wikipedia.org/wiki/Moon
The moon isn't necessarily in your half of the sky at the moment when it is "new", so it is better to say that the new moon occurs when the sun and moon are at their closest to each other in the sky. It is during the day somewhere.

13. Sep 17, 2006

### neutrino

You might get a better idea of the relative positions of the Sun and the Moon by trying to observe a New Moon. (Dark Moon is a term I prefer to use to denote the actual conjucntion )
http://skytonight.com/observing/objects/projects/3308686.html

It's a challenge to find a Moon aged within 24 hours since conjunction (my personal best is 51 minutes past the day). The location (azimuth) changes every month, and some geographical locations will be more advantageous than others. It's quite fun trying to find the sliver in a sky which is still filled with some sun light. You can also do it the other way, trying to find "oldest Moon", i.e. before conjucntion.

14. Sep 17, 2006

### DaveC426913

I have to agree, I wonder if these questions are serious.

When you are looking at a new moon, where is the Sun to cause you to see the shadow-side of the Moon? You'd have to be looking at the sun too. Pretty hard to do that ... at night.

15. Sep 17, 2006

### Labguy

The OP didn't seem to state the question well, but others have filled in most of the blanks.

But, from your quote above, explanations so far have been WHEN there is a new moon and not just when you can SEE a new moon. Wherever you may live, the odds are that there will be a 50/50 chance that new moon occurs when you can "see" it and when it is not in your particular hemisphere (night time for you).

There is an easy way to "see" a new moon. Neutrino's 51 minutes past is pretty close to closest possible, but many people here may have seen it exactly at new moon and not thought about it. I have seen that twice. When is the quizzer.

When the moon is on the "solar plane" (ecliptic?) and is exactly new, in the day, and you can see it would be: (Tah-Dah!)

16. Sep 17, 2006

### sdemjanenko

concerning diffraction, for a body to undergo diffraction it must be narrow. the earth is not narrow. And while technically there is a diffraction, it is almost unnoticable. If you have ever done a double-slit interference lab, the slots are on the order of 10^-5 m to 10^-6 m in width across, its most noticable when it is close to the size of a wavelength of light.

17. Sep 17, 2006

### DaveC426913

I know. I didn't raise that because that would just further complicate the issue.

18. Sep 18, 2006

### neutrino

Just to be clear, I meant 24h51m after the "new moon."

Do you mean that these people (and yourself) were looking at the location were the moon was supposed to be but never saw anything?

19. Sep 18, 2006

### Arancaytar

Technically, yes, depending on what you mean by "occur". If the new moon "occurs" when the moon is in the sky from your perspective, then that is during the day. The sun and moon are in the sky at the same time. You don't see the moon then, of course, because it doesn't reflect any light. You can see its shadow if it passes directly in front of the sun - making an eclipse (all solar eclipses occur during a new moon).

So if you look at the sky at night during a new moon, the moon isn't absent because it's invisible. It just plain isn't there - it's on the other side, along with the sun.

Last edited: Sep 18, 2006
20. Sep 18, 2006

### Labguy

Good, I thought that was verly early, maybe a record.
No, we were looking directly at a new moon exactly at the hour and minute it was new. This can be done only during a solar eclipse (annular or total) as in my photos posted above. This is when the moon directly covers all of the sun (corona photo) or covers just most of it leaving the ring of sun around the moon (annular photo). The black circle in both photos is the moon.

Most photos of this type are exposed to show the sun's detail(s) or the annulus, but a much longer exposure can be made where the solar corona or ring would be greatly overexposed and the moon would be more visable due to being lit by reflected Earthlight. Same as when you can see the some of the "dark" part of the moon when it is a very thin crescent.

By "detail(s) of the sun, I mean chromosphere,etc. as in this photo: