# How light of sun reach at moon?

1. Dec 23, 2013

### Hepic

When earth is between sun and moon,we have full moon.
But I can not really understand that. How light of sun reach at moon? Does not hit in earth?
Or maybe light follow a curve as general theory says?

Thank you!!!

2. Dec 23, 2013

### SteamKing

Staff Emeritus
You realize that the sun is many times larger than the earth, don't you?

3. Dec 23, 2013

### Hepic

I realize that,but moon is close to earth. If light follow a straight line,I do not think that is possible this fenomeno.

4. Dec 23, 2013

### phinds

So I take it you think the moon is in the plane of the ecliptic. That's your problem.

5. Dec 23, 2013

### tfr000

I think you better search for "eclipse", and learn about why they don't happen at every new and full Moon.

6. Dec 23, 2013

### Staff: Mentor

No, neither of those is a good description of the problem. The problem is simply a lack of precision of the alignment.

7. Dec 23, 2013

### Staff: Mentor

To extend this answer: The distance earth-moon is ~60 times the radius of earth. At this distance, the area where no sun is visible is a circle with a radius of 4500km (smaller than earth as the sun is larger than earth). The moon usually passes "above" or "below" that circle as the orbits of earth (around sun) and moon (around earth) are not in the same plane. Two times per year (=two times per orbit of earth), it switches between those options, and this can give an eclipse.

While the theory of general relativity (not "general theory") gives a tiny deflection of the sunlight, this is completely negligible here.
Something you can see is the scattering and deflection of sunlight in the atmosphere of earth - it makes the moon dark red during an eclipse.

8. Dec 23, 2013

### HallsofIvy

Of course, it does sometimes happen that the moon is directly "behind" the earth. That's when we get a "lunar eclipse". Even at those times the moon does not become completely dark (more of a dark red) because of sunlight bent by the earth's atmosphere that reflects off the moon.

9. Dec 23, 2013

### SteamKing

Staff Emeritus
The word you are looking for is 'phenomenon'.

Yes, light rays do travel in straight lines, discounting the effects of gravitational lensing. However, all of the rays emitted by the sun do not originate from the same point. Making a sketch would go a long way to showing this point.

10. Dec 23, 2013

### Staff: Mentor

A few more notes of expansion, as I was typing from a phone at the time:

1. The moon's orbit is only about 5 degrees inclined to the ecliptic.
2. The moon crosses the ecliptic twice a month, but the sun is only at one of those spots twice a year - and their times being at that spot may not be at the same time.
3. Because the inclination isn't large compared to the Earth's apparent diameter (2 degrees) to the moon, you can have several at least partial eclipses a year; not just one or two. Extra ones are a month apart.
4. #3 is also why you have many more lunar eclipses than solar eclipses (the sun's and moon's apparent diameters to Earth are only 0.5 degrees).

11. Dec 23, 2013

### davenn

going from your 2 responses it would appear you also dont understand how eclipses happen

the larger size of the sun in this case isnt the important thing, as from the distance of the earth and moon is from the sun, the sun subtends the same angular size as the moon
it has nothing to do with rays of the sun shining around the earth, gravitational lensing etc

it has ALL to do with what others have posted before this last post of yours

regards
Dave

12. Dec 23, 2013

### SteamKing

Staff Emeritus
I didn't say that gravitational lensing was responsible for the moon being able to shine when the earth is between it and the sun. The OP did not discuss eclipses at all, which have no bearing on how sunlight is able to strike the moon when it is on the far side of the earth.

When the moon is between the earth and the sun, the moon is able to block some of the sun's light from striking the surface of the earth, but the width of the moon's shadow on the surface of the earth during a total eclipse is only a few miles wide. There is plenty of sunlight which illuminates the earth on either side of the eclipse, outside the band of total or partial darkness.

13. Dec 24, 2013

Is it true that Solar and lunar eclipses occur every full moon and new moon?(For some place on Earth)
(Source:Stellarium)

14. Dec 24, 2013

### Bob

Is this possible?

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15. Dec 24, 2013

### SteamKing

Staff Emeritus
It's hard to evaluate exactly what your source is saying, since you provide no direct link to these statements.

However, solar eclipses are remarkable because they occur somewhat rarely.

http://en.wikipedia.org/wiki/Solar_eclipse

According to this article, 2-5 solar eclipses occur each year. The last year to have 5 solar eclipses was 1935; the next year with 5 eclipses won't be until 2206.

A lunar eclipse can occur only on a night with a full moon, but it does not necessarily follow that every night with a full moon produces a lunar eclipse. Every year produces at least two lunar eclipses.

http://en.wikipedia.org/wiki/Lunar_eclipse

16. Dec 24, 2013

Ah,(http://www.stellarium.org/) Its a sky simulating software
(I don't have it right now.This laptop is slow.I will recheck once I get to my other home(Next year)
Since the moon always come between the Earth and the sun during new moon,Why not?
At least the shadow should get to somewhere on the Earth.

17. Dec 24, 2013

### davenn

not unless there is an alignment and there isnt one every new moon

reread the earlier posts by mfb and russ_watter and tfr000
and do a google on eclipses and what is needed to produce one, even a partial one

Dave

18. Dec 24, 2013

Oh.I see.
Thank you

19. Dec 24, 2013

### Hepic

I am wonder that too.

20. Dec 24, 2013

### davenn

his drawing is out of proportion