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The diagram above shows what happens during an eclipse of the Sun. Diagram not to scale.
As the Moon orbits around the Earth, the Moon crosses the direction from the Earth to the Sun once every 29 days 12 hours and 44 minutes. We call this moment New Moon. The Moon actually becomes visible again a day or two after New Moon, as a thin crescent in the evening sky.
The plane of the orbit of the Moon around the Earth is tilted at 5 degrees to the plane of the Earth's orbit around the Sun. Hence at the time of New Moon, the Moon usually passes above or below the Sun, as seen from Earth, and there is no eclipse.
The picture above shows how the Earth (blue) orbits the Sun (yellow), while the orbit of the Moon (grey) around the Earth is tilted (diagram not to scale).
There are two places in the Moon's orbit where it can pass exactly between the Sun and the Earth. These are called the nodes of the orbit. If the Moon is at or near either of these nodes at New Moon, an eclipse of the Sun occurs. There is an interval of 31 days that occurs every 5.8 months, when the alignment to give us eclipses can happen.
As the sizes of the Moon and Sun as seen from the Earth are almost the same, the Moon can completely block the light of the Sun over a small part of the Earth. In the same way, you can block out a distant building by holding your hand in front of it, even though the building is much bigger than your hand.
The Moon passing in front of the Sun produces a shadow that moves from west to east over the Earth. Where the sunlight is completely blocked, in the "umbra", a total eclipse is seen. Where the sunlight is only partly blocked by the Moon, in the "penumbra", a partial eclipse is seen.
Note that the diagram above is not to scale! The Sun is really 110 times bigger than the Earth, and very much further away than as it is shown in the diagram above.
To get an idea of the sizes and distances of the Moon and the Sun, we can make a simple scale model of the Earth, Moon and Sun. Let's start by using your head as a scale model of the Sun. Let's say the diameter of your head is 14 centimetres. The Earth would then be 15 metres away, or about 20 paces. The model Earth is 1.2 millimetres in diameter - the size of the head of a pin. The Moon would be 4 centimetres, or about the width of two or three fingers, from the Earth. The Moon would be a tiny 0.4 millimetres in diameter, or about the thickness of the shaft of a pin. The real sizes and distances are 10 000 000 000 times bigger than our model.
Here are three versions of this scale model:
The picture below shows the relative sizes of the Sun, Earth and Moon.
The Moon and Earth are shown above to the same scale as the Sun. The distance between the Moon and the Earth is shown to the same scale as the sizes of all three bodies. However, the Moon and Earth should be much further from the Sun - in fact 108 Sun diameters from it. The image of the Sun shown above is a real photograph - the dark patches are less hot regions on the surface, called sunspots.
Your scale model should show you that the Sun is 400 times bigger than the Moon and that it is also 400 times further from us than the Moon. This coincidence is what allows us to experience total eclipses of the Sun - as a result the Sun and the Moon both appear to us on Earth to be the same size in the sky.