A partial eclipse occurs when a part of the Moon passes through the shadow of Earth. Partial eclipses of the Moon are not rare. A partial eclipse can be re-created using common spheres, such as a marble, a tennis ball, or a grapefruit. A larger sphere can represent the Earth and a smaller one the Moon. By placing a sphere on a table and holding a lighted flashlight near the table’s surface, you can create a long shadow that extends from the object and narrows like a cone down to a point. This shadow is called the umbra, the Latin word for ‘shadow’. The Earth casts this same type of shadow in space because the Earth is always standing in the path of some of the light from the Sun. This circular shadow is widest at the Earth’s surface, and when people on the Earth rotate into the shadow, they refer to it as ‘night’. Earth’s shadow stretches about 850,000 miles out into space, but the circle gets smaller and smaller before it narrows down to nothing. In all that immense distance, however, there is only one major object that the shadow can fall upon. Our Moon, as it orbits the Earth at 2,280 miles per hour, occasionally crosses the path of this shadow, and some or all of the sunlight falling on the Moon’s surface is blocked out until the Moon emerges from the shadow once again.
By using a flashlight and two spheres, one can demonstrate why an eclipse can occur only during the time of a full moon. The Moon has to be in line with the Earth and the Sun in order to enter the Earth’s shadow, thus anyone on the night side of Earth would see the Moon fully lighted until it touches that shadow. Why don’t we have an eclipse during every full moon? The necessary alignment is very precise. Think of trying to align a grapefruit (Earth), a ping-pong ball 12 feet away from the grapefruit (Moon), and a large house about a mile down the road (Sun)! Even more critical than this, however, is the fact that the Moon doesn’t orbit the Earth on the same plane that the Earth orbits the Sun. The Moon’s orbit is tilted about 5 degrees, and thus the Moon usually passes above or below the Earth’s shadow.
Amazingly, when some ancient Greek thinkers (Pythagoras and Aristotle) witnessed partial eclipses, they noticed that the shape of the shadow on the Moon was always curved in the shape of an arc of a circle. Through reasoning and logic alone, they concluded that the Earth could not be flat – it had to be a giant sphere.
Click here to return to the Astronomy index page