Google Answers Logo
View Question
Q: The face of the moon ( Answered,   5 Comments )
Subject: The face of the moon
Category: Science > Astronomy
Asked by: mikehenrynyc-ga
List Price: $3.00
Posted: 19 May 2003 06:29 PDT
Expires: 18 Jun 2003 06:29 PDT
Question ID: 205793
Do we in fact always see just one "face" of the moon?  If so, does
that mean that the moon isn't spinning on it's own axis?  Do you see a
different phase of the moon in other parts of the world?
Subject: Re: The face of the moon
Answered By: willie-ga on 19 May 2003 06:55 PDT
Hi there, and thanks for the question, one that disturbed me at one

Here's a layman's explanation from "Why do We See only One Side of the
Moon?" (

"Relative to the Earth, the Moon makes one rotation every 29.5 days.
That happens to also be the time it takes for the Moon to complete one
revolution around the Earth.


Here's a more technical version, from the Scientific American, Ask the
Expert: Astronomy site (

"The moon keeps the same face pointing towards the Earth because its
rate of spin is tidally locked so that it is synchronized with its
rate of revolution (the time needed to complete one orbit). In other
words, the moon rotates exactly once every time it circles the Earth.
"It is very unlikely that the moon started out synchronized; that
would indeed be a surprising "coincidence." As Boss explains, "The
moon's synchronous spin state is thought to have arisen billions of
years ago, when the moon was much closer to the Earth, and so tidal
forces were much stronger than at present. The Earth's gravity
maintained this spin state even as other gravitational interactions
caused the moon to move outward to its present orbital radius.


Here's a good tip to aid understanding, taken from "Tides, the Earth,
the Moon, and why our days are getting longer" ( )

...grab two oranges; one for the Earth and one for the Moon. Let one
go around the other, first without any rotation and then letting the
"Moon" rotate just once on its axis for every time it goes around the
"Earth". See how if the Moon does NOT rotate, then eventually we see
all sides? Therefore the Moon does rotate, but it does very slowly:
once a month!


For all practical purposes, phases of the Moon and the percent of the
Moon illuminated are independent of the location on the Earth from
where the Moon is observed. That is, all the phases occur at the same
time regardless of the observer's position. This is because the main
factor is the position of the sun relative to the moon. Again, you can
see this demonstrated using your oranges as above. The further apart
you move your oranges (i.e. modelling the real distances involved),
the more you'll see that the position you have on the "Earth" orange
doesn't have any perceivable effect on which phase of the moon you are

There's a really nice little site explaining phases of the moon, with
a small animated movie, over at the US Naval Observatory site ( )

Hope that helps.


Google search terms used:
moon phases earth
moon face earth
Subject: Re: The face of the moon
From: f3etoiles-ga on 19 May 2003 08:34 PDT
Actually, it is not absolutely true that we always see the same face:
the Moon has a small wobbling motion (called "libration"), allowing
the possibility to see (from Earth) 59% of the total area. See for
instance (don't miss the
movie there !)
Subject: Re: The face of the moon
From: hfshaw-ga on 19 May 2003 08:43 PDT
Just a comment to point out that the moon does not always keep
*exactly* the same face pointed toward the Earth -- there is an effect
called "libration" that causes the moon to appear to "rock" back and
forth a little relative to an observer on Earth.  The net effect is
that we can actually see 59% of the moon's surface.  The other 41% is
always hidden from an earthbound observer.

There are actually two mechanisms at work here.  

1) The moon's orbit around the Earth is not exactly circular.  That
means (according to Kepler's 2nd Law) that the moons orbital speed
changes with time as it orbits the earth.  Its rotational speed does
not change, though, and as a result, the moon appears to rock back and
forth in an east-west direction.

2)  The moon's rotational axis is is not exactly perpendicular to the
plane of its orbit -- it's tilted about 6.5 degrees relative to the
orbital plane.  This causes the moon to appear to rock a little in the
north-south direction as viewed from the Earth.

There's a beautiful animated gif file of the actual moon showing these
effects at:
(look about midway down the page)
Subject: Re: The face of the moon
From: stressedmum-ga on 20 May 2003 05:46 PDT
When I travelled to the UK and saw the face of the moon, I was quite
spooked because he was a different guy from the one we see here in
Australia! Have a look at this site: and that might give you a
better idea of what I mean.
Subject: Re: The face of the moon
From: neilzero-ga on 21 May 2003 01:53 PDT
All correct information except perhaps stressedmum. In trigonometry,
the hypotenus is about 240,000 miles long so moving a few thousand
miles on Earth's surface, produces only a tiny change in the portion
of the moon that is visable from Earth.  Neil
Subject: Re: The face of the moon
From: stressedmum-ga on 23 May 2003 18:50 PDT
Hi Neilzero, just want to extend a warm and fuzzy invitation to you to
drop into Melbourne next full moon and have a look with me at the
'face' of the moon. (I'm talking about the face of the Man in the
Moon, specifically.) You can explain trajectories to me and I can
explain that even though it doesn't seem to add up, down here, the man
in the moon indeed has a different face. Truly ruly, he does!

Important Disclaimer: Answers and comments provided on Google Answers are general information, and are not intended to substitute for informed professional medical, psychiatric, psychological, tax, legal, investment, accounting, or other professional advice. Google does not endorse, and expressly disclaims liability for any product, manufacturer, distributor, service or service provider mentioned or any opinion expressed in answers or comments. Please read carefully the Google Answers Terms of Service.

If you feel that you have found inappropriate content, please let us know by emailing us at with the question ID listed above. Thank you.
Search Google Answers for
Google Answers  

Google Home - Answers FAQ - Terms of Service - Privacy Policy