What's the difference between a new moon and a lunar eclipse?

Can you explain to my boyfriend what a lunar eclipse is and why it's different from a new moon every month?
Question posed by Charlene

What is a New Moon?

In the following section I'll ask you to 'imagine', but there's no real reason why you can't actually do the things I'm asking you to imagine if it would help to see what's going on.

Imagine standing in a darkened room with a football held out in front of you at arm's length. Now imagine a mate of yours is on the other side of the room holding a torch, and they're shining it at you. With the football still held out at arm's length, slowly turn on the spot, keeping your eye on the surface of the ball in front of you. What happens?

When you're facing in the opposite direction to your mate with the torch, you should see that the part of the ball that's facing you is lit up entirely*.

As you turn slowly on the spot, you should see a crescent of shadow creeping around the part of the ball that's facing you, so that less of the visible surface of the ball is directly lit by the light from your mate's torch. As you turn to face the torch-wielder, less and less of the ball is lit up (from your perspective) until, when you are directly facing the torchlight, the opposite side of the ball to you is lit up, and the side that's facing you is in darkness.

As you continue you turn, now to face away from the torchlight you'll see a crescent of light appear on your ball's face, growing until, as you face away from the light, the entire face of the ball is lit once more.

This is exactly what happens as the Moon orbits the Earth (or any other body orbits any other body!): The Moon is always (roughly) half-lit and half-shaded, but because of the movement of the Moon around the Earth, how much of the lit area we see changes due to our perspective. The diagram below may help to illustrate this (click to go to the originator's website).

A new moon is the time in this cycle when the Moon lies between the Sun and the Earth and so the Moon appears to us to be in complete darkness. In reality, the opposite side of the moon to us is fully lit, and the side facing us is in its own shadow.

What is a lunar eclipse?

Imagine again the situation in which you are holding your football at arms length (feel free to have a rest if you're tired...), and your friend is shining a torch at you from the other end of the room. If you move the torch, ball and your own head around so that they are in a straight line, your head will block some of the light from falling onto the face of the ball that would otherwise be fully lit.

When the Moon is on the opposite side of the Earth from the Sun, sometimes the Earth's shadow gets in the way, and what should be a full moon ends up with a curved slice 'missing' from it (a partial lunar eclipse), or being completely covered by the Earth's shadow (a full lunar eclipse). The image below may help to clarify this:

The Earth, Sun and Moon don't orbit each other in exactly the same plane**, so they don't always line up like this. That's why we don't see a full lunar eclipse every month- the Earth's shadow only relatively rarely crosses the surface of the Moon.

So what's the difference?

• New moons occur because of our changing viewpoint of the Moon; lunar eclipses occur when the Earth gets in the way of the light from the Sun and stops it hitting the Moon.
• New moons occur when the Moon is between the Sun and the Earth; lunar eclipses occur when the Earth is between the Sun and the Moon.

Other interesting bits

• The relative sizes and distances of the Earth, Sun and Moon mean that the Earth's shadow covers the Moon almost perfectly during a total lunar eclipse, and the Moon covers the Sun almost perfectly during a solar eclipse. This is not due to any special effects of the universe; it's just a remarkable coincidence.
• The above coincidence is made even more remarkable by the fact that the Moon is moving away from the Earth. This means that in the far past eclipses were less dramatic as the Moon appeared larger, and that in the far furture, eclipses will be considerably less dramatic as the Moon will appear smaller (and will not, for example, be able to cover the entire disc of the Sun): we are in a unique position, both in space and time.
• Lunar eclipses sometimes appear red, due to refraction of light by the Earth's atmosphere.
• There are many other moons in our solar system, but Earth's Moon is much bigger, relative to its parent planet, than any of the others.

Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.

Footnotes:

* As long as your head's not in the way of the light, of course.

** A 'plane' is a flat, two-dimensional (imaginary) slice through space: imagine marbles rolling around on a table top- they're all moving in the same plane. If you could imagine slotting two pieces of card together and an angle, and being able to roll marbles on the two separate sheets of card whilst at angles to each other, this would give some idea (although quite exaggerated) of what I'm talking about!