# Why does the moon always have a dark side?

Discussion in 'Digital Photography' started by Richard Henry, Apr 7, 2004.

1. ### Richard HenryGuest

Because there is only one sun?

Actually the "dark" side can be lit up quite a bit from reflected Earthshine
when the angles are right.

Richard Henry, Apr 7, 2004

2. ### John PopelishGuest

The Moon is lit by the Sun. The Sun is off to one side of the Moon.
One side lit, one side dark. You would need two suns to light both
sides at the same time.

John Popelish, Apr 7, 2004

3. ### vhlGuest

Its because by some fluky chance, the moon rotates at the
same rate as the earth does so the near side is always
facing the earth, so even when its in shadow, its still
the same side. We can see more than half the sphere as
we can see it from different parts of the earth.

vhl, Apr 7, 2004
4. ### SquishuaGuest

Over the course of a lunar day (29.5 earth days) all sides of the moon
receive sunlight. If you are operating under the assumption that one
particular side of the moon *never* recieves light, you are mistaken.

The "dark side" of the moon refers to the "forbidden" side that is
never viewed directly from earth (or at least has not been viewed
directly for eons) because the same side of the moon always faces us.

Regards,

Squishua

(remove the capital letters from my e-mail address to contact me)

Squishua, Apr 7, 2004
5. ### Ida NoeGuest

Ahh..so what kind of light fill ratio should one use to bring out the

How much depth of field would you need to get the whole moon in focus?

Would you tell the moon to say "cheese"?

Ida Noe, Apr 7, 2004
6. ### Jason PawloskiGuest

No such thing as the dark side of the moon. Matter of fact, its all dark.

(Blatantly stolen from Floyd)

Jason Pawloski, Apr 7, 2004
7. ### ObakeGuest

one side of the moon is heavier then the other. the heavy side was/is
attracted by earth's gravity and over the many many years has stabilized so
only one 'side' of the moon faces the earth.

Obake, Apr 7, 2004
8. ### NachoGuest

This is completely off-topic, but it is not true. The point is tidal
forces. Tidal forces creates intense deformations in the moon when it
rotated, time ago. This deformations uses energy, obtained from its
rotation. So it slowly stopped. It could have been stopped in any other
position.

Best regards.

Nacho, Apr 7, 2004
9. ### Robert J. KolkerGuest

Any light seen from the Moon is relected light. Since the only
significan t source of the light is from the Sun and there is only one
Sun, then only one side of the Moon is lit up by the Sun.

Bob Kolker

Robert J. Kolker, Apr 7, 2004
10. ### Sam WormleyGuest

We (posters) should add that only half of the spheroid is illuminated,
a opposed to more than half, because the illumination source is far
enough away to approximate a point source.

Sam Wormley, Apr 7, 2004
11. ### MikeGuest

Good question and I see you did not get a straight answer from the
physics "gurus" hanging out in this NG.

What is called a "dark side" of the moon is actually the "far side"
which is never seen by an observer on earth. But the moon gets sun
shine on all of its sides as it turns. The reason there is a "far
side" is that over the millions of years the moon goes around the
earth, the gravitational pull of the earth has slowed moon's rotation
about its axis down so it matches the period of its revolution around
the earth. One can easily see that when this is true there is always a
"far side" not seen by the earth and called the "dark side" but it
actually gets sun shine all over the place.

Mike

Mike, Apr 7, 2004
12. ### Richard HenryGuest

It looks to me that all but 2 of the responses were "straight".

Richard Henry, Apr 7, 2004
13. ### Uncle AlGuest

Why does the Earth always have a dark side?

Uncle Al, Apr 7, 2004
14. ### Chris BrownGuest

Actually, he's quite right - although tidal forces are involved, your
explanation is trivially wrong:
The problem with this is that the Moon hasn't actually stopped rotating at
all. It rotates on its axis once every 28 days. This is the same as its
orbital period, so that we always see the same side. If it had stopped, we'd
see opposite sides at 14 day intervals.

The reason the same side faces us is because the orbital and rotational
periods coincide. The reason they coincide is indeed because tidal forces
slow the rotation down, but, and this is the important bit, the Moon's
centre of mass is offset from its centre of rotation, like a bicycle wheel
that has a weight on one side. Over the N billion years that the Moon has
been orbiting the Earth, tidal forces have slowed the rotation until it
reached the optimal rotational speed, which is the speed where the Moon's
centre of gravity is always as close as possible to the centre of the Earth,
i.e. always facing down.

Chris Brown, Apr 7, 2004
15. ### briggsGuest

Under the tidal forces associated with the Earth's gravity, the
moon naturally assumes a slightly elongated shape -- something
like an egg. Those same tidal forces naturally pull the long axis
into alignment with the earth.

When the moon was rotating at a different speed than it revolved around
the Earth, it was something like a gushy soft-boiled egg -- always a
little elongated, but with various pieces of the lunar crust continuously
rotating into and out of the bulges. Since the moon is (or was) neither
completely rigid nor completely fluid, the viscosity would cause the
bulges to lag a little. And this would, in turn, cause a tidal torque,
eventually bringing the moon's rotation rate into lockstep with its

A moon that was merely heavy on one side would continue to spin
out of synch forever (*), much like an unbalanced frictionless
bicycle wheel. It takes a gushy satellite to efficiently lock up with
its partner.

I've never learned this out of a textbook in so many words, but it
seems pretty obvious.

John Briggs

(*) the rotation of an unbalanced satellite can eventually damp due to
other effects, but the time frames are much longer than the induced
tidal drag scenario described above.

briggs, Apr 7, 2004
16. ### Jay WindleyGuest

|
| Good question and I see you did not get a straight answer
| from the physics "gurus" hanging out in this NG.

Well, be fair. It wasn't clear whether itnwas a phase angle question or a
tidal locking question. Contrary to the original poster's statement, the
title did not say it all. I think both interpretations of the question were

Jay Windley, Apr 7, 2004
17. ### Ian StirlingGuest

No, it's not.
Somewhere around an extra half a degree, or some extra 10Km of illumingation
round the edge of the illuminated area.

Anyway, the correct answer is that there has been a dark side of the
moon since 1973, as that's when the album was released.

Ian Stirling, Apr 7, 2004
18. ### ZZBunkerGuest

Because the moon does not orbit the Earth,
it is locked in Earth orbit.

And also because the dark side of the moon
is do much dark as it is colder than
Mars.

ZZBunker, Apr 7, 2004
19. ### Jim HuttonGuest

But it does NOT have a dark side.
How anthropocentric!
J

Jim Hutton, Apr 7, 2004
20. ### JeffGuest

Damn gravity.

Jeff, Apr 7, 2004