Thursday, October 21, 2010

Our beautiful moon and everything about it

I was looking out the window today and noticed the Moon looking almost full, hanging fairly close to the horizon. It wasn't full yet (that won't happen until this Saturday October 23rd in Denmark, you can check you own moon schedule here).
I took a couple of pictures of it through our east facing window and thought it would be a good idea to talk a bit more about our satellite, the Moon.

I'm going to talk about the Moons orbit, the phases of the Moon and the reason that the Moon has a far side and a near side (there is no such thing as the "dark side" of the Moon).
The topic will be divided into three separate posts and in this first post I will talk about the Moons orbit.

As usual, you can click the images/diagrams for larger versions.
Edit: I've removed the trivia section to shorten the post a bit. I'll repost it in a trivia post at the end of this series.

The Moons orbit around Earth
The Moon orbits the Earth roughly every 27.3 days and each full cycle of the Moons phases (from full moon to full moon) takes about 29.5 days. The reason for this difference is that the Earth orbits the Sun at the same time as the Moon orbits earth. After one full Moon orbit, the Earth will have moved enough that our vantage point in respect to Moon has changed, and it takes roughly 2.2 days for the Moon to position itself similarly in the sky.

Let me explain:
The first diagram shows position of the Moon during a (non-specified) full moon.
The Earth's orbit is the black circle and the Moons orbital direction is indicated with a red arrow. The dotted line shows the Moons position relative to the Earth, and you can see that the Moon is opposite the Sun and therefore it's full.

The second diagram shows the positions one month later. You can see by the red dotted line that the Moon has completed one entire orbit of the Earth (it is in the same relative position), but it is no longer opposite of the Sun and therefore not completely full. A few days later (2.2 days) the Moon will have "caught up" with the Earth's new position (the red arrow indicates the Moons direction) and another full moon will be visible.

The Moon and the tides
Besides the visibly changing Moon and the astronomical implications (we have put astronauts on the freaking Moon, that's what!), the Moon has a profound impact on life on Earth by being the cause of the tides in our ocean (although where and how you live might mean that you'll never experience it).
Why it that?

Planets, stars and moons all affect each other through gravity as they orbit each other. Tidal forces occur because the gravitational force is different on the near and far side of an object. The part closest to the other object is attracted more than the part further away. Image yourself standing with your left side towards the moon. The Moons gravity will affect you left hand slightly more than your right, because the right hand is slightly further away. That's how gravity works, it fades with distance.

This means that the part of the Earth closest to the Moon feels the greatest attractive force, the centre slightly less and the far side the least.

The total gravity measured at any point in Earth is relative to the centre of the Earth or "down" (where the greatest mass is) and therefore it is mathematically consistent to subtract the Moons attractive force at the centre from the ones felt on either side of the Earth (or really at any point).

The resulting forces are two forces which act outwards from the centre of the Earth and causes the Earth to "bulge" at these two areas. The bulge is best seen in water (as it is most flexible) and is visible as high and low tide.

There is high tide on the side nearest to the moon and the one farthest away at the same time. That is also why we have two high tides (separated by roughly 12 hours) every day.

Part 2 is here