Why does the moon sometimes appear close and at other times far away?

Beacause asit goes around the Earth it becomes closer and further away during its cycle. The moon is in an oval orbit.
Because it is closer sometimes than others. Der McFly!

It is not on a perfect radial distance from the earth.
Its also a trick of the light, kind of like the way light bends through water.
Because your vision changes from long to short distance.. nah not really, Ive got no idea, just thought Id answer anyway
It's an optical illusion that has never been explained satisfactorily. When the moon is close to the horizon you can look at it in relation to the buildings or the landscape - and your brain thinks it quite big. When it's high in the sky, there's nothing for your brain to compare it too, and it appears smaller.

But, trust me - it doesn't get any closer or further away.
the moon sometimes looks closer because of light reflecting from were you are standing , e.g if you are in a dark place then the moon will look bigger but if you are in a lighted place then the moon will not look as big to your eye , it,s all about refraction look that up in your dictionary it will help you understand better
It is an optical illusion - the moon is never closer or further away from the earth at different times, its just the angle that we see it at that makes it look bigger and smaller.
Objects in the rear view mirror are closer than the seem !
When the moon is high up and surrounded by nothing but sky, it looks small. When it is low on the horizon it seems bigger, as now it has other things around it. It is just an illusion.
its because of the gravitational pull of the Earth and moon, the greater the gravitational pull, the closer the moon and Earth is.
beacuse chang er don wan to see lu yi.! hahax.
The moon is sometimes closer to the earth than at other times. The difference between its furthest distance and its closest is about 40,000 km. This gives a variation in its angular size, varying between 29 minutes and 34 minutes of arc. The apparent change in 'size' and thus appearance of being closer or further away is much greater than accounted for by the approximately one tenth of a degree variation in angular size.

So the people who have answered that it is due to a psychological effect are right. Although there is a very small effect due to actual distance, most of the effect is due to use perceiving the moon as much larger when we look at it horiztonally (ie near the horizon) than when it is further above the horizon.

Apparently if you stand on top of a 100ft building and look down at objects, they look much smaller than if you look at them 100ft away along a road - though I've never tried it out myself with measured distances.
Its all relevant to the horizon and how your eyes view of the world distorts its view of the moon.
If you really want to learn how your eyes decieve you on this try this little experiment.
Go outside, bend over and look at the moon upside down through your legs.

Now try to figure that out?
solar system made of a few planets all going around the sun the only time the moon looks really big is when earth is in direct line to the moon which means it closer
it's the position in the sky
depends if it is a new moon or a full moon.

a new moon is not as bright because it is using reflected light so it appears smaller.

a full moon is using light direct from the sun so it appears brighter and bigger.

for all your full and new moon dates follow this link.

Cause the moon is sometimes closer to the Earth than other times. It is all because of hte Moon's revolution.
It's on a piece of elastic, and the Gods are playing swing tennis wiv it
The moon illusion is an optical illusion in which the moon appears larger near the horizon than it does while higher up in the sky. This optical illusion also occurs with the sun and star constellations.

One explanation is the apparent distance theory which may now be rejected by modern vision researchers who have investigated the moon illusion. Humans may tend to perceive the sky as more or less a surface, but unlike a hemispherical surface, it does not seem to be equally distant from us at all points. When we see clouds, birds and airplanes in the sky, those near the horizon are typically farther away from us than those overhead. If we see an airplane overhead, its image gets smaller and smaller as it nears the horizon. This results in the perception of the sky as a comparatively flat surface. In other words, we perceive the sky near the horizon to be farther away than the sky overhead. If we perceive the moon to be in the general vicinity of those other things we see in the sky, we would expect it to get equally farther away as it approaches the horizon as well, which should result in a smaller retinal image. But since its retinal image is approximately the same size whether it is near the horizon or not, our brains, attempting to compensate for perspective, assume a low moon must be physically larger. This effect is known as the Ponzo illusion. I.e, regardless of eye elevation, the horizon moon will be perceived as being much larger than the zenith moon because the terrain is viewed as a plane extended outward from the observer. Thus, if two objects have the same projected size but appear to lie at different distances from the viewer, the object which seems farther away will appear larger. However, very few people (only about 5%) see the horizon moon as being both larger and farther away, and vision researchers have found that most people see the horizon moon as both larger and closer than the zenith moon. This common disagreement between the apparent distance theory and the available data is referred to as the "size distance paradox", and new theories are being developed to replace the apparent distance theory.

Relative size hypothesis
This hypothesis states that the perceived size of an object depends not only on its retinal size, but also on the size of its immediate visual environment. But, relative size can be either relative angular size or relative linear size, or both, and the moon illusion begins as an angular size illusion.
perhaps the effect is simply an illusion: could it be that the image is merely perceived as bigger when the moon is near the horizon?
This is a well recognised astronomical phenomina, the reason the moon looks closer or larger at sometimes is because when the moon is lower down and closer to the horizon the light is bent more by the earths atmosphere (abit like a lens or a mirror can bend light). Hence when the moon is high in the sky the light is bent less by the atmosphere and so it seems further away. In other words this is caused by an optical effect.
oh my god, i looked in the sky last night and the moon was soooooooooo big and yellowy looking! i aint gota clue why but it DOES change size in the sky its not just how you are looking at it like some people are saying.. it was massive last night!
the moon appears closer near the earths stratosphere as it gets magnified by the water vapour droplets in the air and on cold crisp nights with less vapourized atmosphere it is smaller. there you go.
It really does depend on how pissed you are.
Errrr, What twonk says the Moon DOESNT GET CLOSER?

The Moon DOES get closer / futher away because its not in a 'Circular' orbit around the earth!

The Earth's Moon is the fifth largest in the whole solar system, and is bigger than the planet Pluto. The Moon has a nearly circular orbit (e=0.05) which is tilted about 5° to the plane of the Earth's orbit. Its average distance from the Earth is 384,400 km. The combination of the Moon's size and its distance from the Earth causes the Moon to appear the same size in the sky as the Sun, which is one reason we can have total solar eclipses.
It takes the Moon 27.322 days to go around the Earth once. Because of this motion, the Moon appears to move about 13° against the stars each day, or about one-half degree per hour. If you watch the Moon over the course of several hours one night, you will notice that its position among the stars will change by a few degrees. The changing position of the Moon with respect to the Sun leads to lunar phases.

Have you ever heard the term the 'far-side' of the Moon? Because of the effect on the Moon of tidal forces due to the Earth, the same side of the moon always faces the Earth. In other words, it takes the Moon the same amount of time to rotate around once as it does for the Moon to go around the Earth once. Therefore, Earth-bound observers can never see the 'far-side' of the Moon. Tidal forces cause many of the moons of our solar system to have this type of orbit.