The+Moon

The Moon is Earth's only natural satellite and has an important impact on our development and culture as humans, as well as affecting the Earth's oceans and perhaps the very composition of the planet itself. It orbits the Earth every 27.3 days (in reference to the fixed stars - a sidereal month). The time it takes for a complete lunar phase cycle to complete is slightly longer (29.5 days), as the Earth-moon system is moving round the Sun at the same time, as shown in the diagram below:

//**Difference between Lunar month (time between new Moons) and time for Moon to orbit the Earth (relative to fixed stars):**//



Importance of the Moon

 * 1) Impact on our culture - the moon has been an object of worship or study throughout the evolution of humanity. For instance, the Chinese imagined that the Moon was being eaten by a dragon as it waned.
 * 2) The race between the US and the USSR in the 1960s to be the first to land humans on the Moon resulted in many technological developments - not least the development of the integrated circuit (an early form of micro chip) which was driven by the Apollo missions, and Jack Kilby won the Nobel prize in Physics for this invention.
 * 3) The Moon causes the tides in the oceans - this is due to the gravitational pull of the Moon on the Earth being greatest nearest the Moon and least on the opposite side of the Earth. This causes more water to flow to each of these locations (think: nearest the Moon the water is being pulled the most; on the opposite side the Earth is being pulled more than the water). As there are two bulges (high tides) and two low points (low tides) and the Earth rotates once a day, there are 2 high tides and 2 low tides each day.
 * 4) One of the most widely accepted theories of how the Moon came into being suggests that a large object (about the size of Mars) collided with the young Earth. The vast amount of molten material that this created (from both the upper layers of Earth and from the impacting object) was spashed into orbit around the Earth. This slowly clumped together to form the Moon. The impactor will have contributed part of its material to the make up of the planet (and Earth will have provided part of its material to the Moon).

The Phases of the Moon
The diagram below shows how the position of the Moon and Sun result in the different shapes of the Moon that we see. Positions 2, 4, 6 and 8 are explained below.

8 - Here the Moon is on the same side of the Earth as the Sun. So the light from the Sun is shining on the side of the Moon that is away from an observer on the Earth. So no part of the "light side" of the Moon can be seen from Earth, and so the Moon is dark to an observer on the Earth. This is called the "new Moon". Note that the new Moon cannot be seen easily as any "Earth shine" that might light it up will be abscured by the residual light in the sky from the setting Sun (the new Moon and the Sun will set at about the same time).

2 - Starting from the full Moon, as the Moon moves around the Earth on its 27.3 day cycle (or 29.5 days once the movement of the Earth around the Sun is taken into account), more of the Moon become visible. At position 2 on the diagram below half of the lit side of the Moon is visible for an observer on the Earth. When the amount of light is increasing each night, is is said to be waxing, and when half the Moon is lit up, this is called the first quarter.

4 - At this point the side of the Moon facing an observer on the Earth is fully lit by the Sun. This is called the full Moon.

6 - After the full Moon the amount of the Moon's surface facing the Earth which is lit is decreasing, or waning. When half the moon is visible and the moon is waning this is called the last quarter.



Eclipses
Looking at the diagram above you may wonder why there is a full moon at position 4, as it looks a like the Earth would get in the way of the light from the Sun. Well the Earth does get in the way sometimes - that is called a lunar eclipse - but only rarely. To understand why it only happens rarely consider the diagram below. This shows that the plane of the moons orbit is inclined about 5 o to the plane of the Earth's orbit. Therefore there will only be a lunar eclipse when the moon is crossing the plane of the Earth's orbit at the same time as it happens to be on the far side of the Earth relative to the Sun. The word eclipse comes from the word 'ecliptic', which is another word for the plane of Earth's orbit around the Sun, so there is only a lunar eclipse when the Moon crosses the ecliptic during the right phase of the Moon. This will happen at least twice a year.




 * Demo** - place a stool in the middle of the room to represent the Sun, with the flat surface of the stool being the plane of Earth's orbit. Take a frisby, representing the Earth (at the centre of the frisby) with the moon orbiting in the plane of the frisby (at the edge of the frisby - some blu-tac may help). Then walk round the stood with the frisby angled up from the plane of the stool, and keeping the angle of the frisby constant with respect to the room. This should show that there can be two "eclipse seasons" when the Moon is crossing the Earth's ecliptic, and that these 'nodes' are 180 o apart.

Similarly a solar eclipse only happens when the moon is crossing the Earth's ecliptic at the same time as a new Moon. Again there will be at least two a year, however, in the case of a solar eclipses, it is rare to see one without travelling to the right place, as the shadow of the Moon on the Earth as it crosses the Sun during a solar eclipse is relatively small. For any given place on Earth, there will only be a solar eclipse once every 360 years!

In addition, as the Moon's orbit is not perfectly circular, sometimes the Moon will be closer to the Earth during a solar eclipse that at other times. When the Moon is closer, observers in the umbra will only see the central region of the Sun hidden by the moon. When the Moon is further away, observers in the Umbra will see the whole of the Sun hidden. Observers in the penumbra only see part of the Sun obscured as the Moon does not cross the middle of the Sun from their position.


 * Why is the moon important to us?**

media type="youtube" key="ug1Nq7HGoEE?rel=0" height="504" width="894" Summary: - The tides (particularly in the past when the day way 12 hours, and the moon much closer, so that the tides were bigger and more frequent) may have helped early life evolve through the regular wetting and drying of large areas of land. - The moon may help keep the Earth tilted like it is, providing regular seasons which are not too extreme. - The moon spurred a massive investment in new technology when it inspired the "space race" to put a man on the moon.