Planetary Motion

Moon, Phases and Gravity - Read the passage and watch the visual example of how the moon is "locked" in its orbit around Earth. 

So what causes the moon’s phases? Let’s begin with the basics. The Moon is a natural satellite of planet Earth, taking about a month to revolve all the way around our planet. Its orbit is very nearly circular; it stays about 380,000 kilometers away from us as it moves counterclockwise (as viewed from a northern hemisphere perspective). It also stays fairly close to the Earth's equatorial plane (an imaginary extension of Earth's equator out into space).

As you may expect, the Moon doesn’t just revolve, it also rotates about its own axis – but in quite an interesting fashion. Over the millennia, the Moon has become "locked" into a special kind of motion around the Earth. It rotates on its axis at the same pace as it revolves around the Earth. As a result, the Moon keeps the same face toward us throughout its orbit. (Watching a video is perhaps the best illustration of this). So, you may often hear astronomers talk of the “nearside” and “farside” of the moon. While Earthlings can only view the nearside, astronauts and spacecraft have successfully taken images of the farside. Note that there is not actually a “dark side” of the Moon – with this demonstration, you’ll see how the sun’s rays will strike the entire surface!

The observed phase of the Moon is determined by its position relative to Earth and the Sun. In the 29.5-day period that the Moon takes to orbit the Earth, it will appear as different shapes because of our planet’s viewpoint. In actuality, half of the Moon’s surface is being hit by sunlight. But, because of our relative positions, we’ll see the Moon swell from the new Moon, through the crescent, to the first quarter, to the swollen gibbous moon, and then the full Moon, before waning to the new Moon again. 

Bank of Experiment Examples (or you may choose your own).

Gravity

Craters and Meteorites