Regarding phases of the Moon, you should have begun to wonder why you are able to see a Full Moon. Shouldn't the Moon be in the Earth's shadow at this time? You have probably witnessed a few eclipses and numerous Full Moons, so by your own observations you infer that something else is involved, and that is indeed the case. The diagram below shows the orbit of the Moon around the Earth.


At Full Moon, the Moon is in line with the Earth and Sun – so why isn't the Moon in the Earth's shadow? The deception is because the figure is only showing two of the three dimensions of the Moon's orbit. The next diagram shows a side view – the orbit of the Moon around the Earth is tilted by 5° to the Ecliptic (the angle is exaggerated in the diagram). This value does not seem like much, but for comparison, how large is the angular size of the Full Moon on the sky? It is only half of a degree. So the full extent of the change of the Moon's position above and below the Ecliptic is 20 times its own diameter!


The size of the Earth's shadow at the distance of the Moon's orbit is also a factor. It is 1.5° or about three times the size of the Moon's diameter. So, although the Moon can easily fit completely inside the Earth's shadow, it can be significantly above or below it at the time of Full Moon.


Now let's consider a Lunar Eclipse. Recall that the revolution of the Moon around the Earth is in the same direction as the motion of the Earth around the Sun, namely counterclockwise. The Moon is in orbit around the Earth, so it moves into the Earth's shadow. There are two types of lunar eclipses: partial and total. A partial eclipse occurs when the Moon's disk is not completely within the shadow of the Earth, whereas it is completely within during a total eclipse. The maximum duration for a total eclipse is 1 hour and 42 minutes. Because lunar eclipses occur during Full Moon, the middle of the eclipse event is always near midnight.


During a lunar eclipse, the shadow's curved edge is seen. The ancient Greek astronomers noticed that every eclipse has this characteristic. Being experts in geometry, they reasoned correctly that (1) the Moon was entering the Earth's shadow and (2) the Earth was spherical, for that is the only perfect geometrical solid that would always cast a curved edge. For example, a cube would not.


Today lunar eclipses provide little scientific returns other than a gross study of the Earth's atmosphere. Remember that when the Full Moon is on the horizon it has a red color since only these wavelengths pass through the thick atmosphere. During a lunar eclipse the reason we can see the faint Moon is because some sunlight passes through the Earth's atmosphere, hits the Moon, and reflects back to us. This light is highly reddened due to its long path through the Earth's atmosphere, so the eclipsed Moon usually has an orange-red tint. The extent of this tint, though, depends on your location. Different viewers sometimes see a different tint due to varying levels of atmospheric aberrations.


The Moon has entered the Earth's shadow (February 2008).

(Photograph was taken by the author.)


Totality during the Lunar Eclipse of February 2008. Notice the red color of the Moon compared to the previous two photographs. The bottom picture includes Saturn to the left and the star Regulus (in Leo) to the upper right.

(Both photographs were taken by the author.)



The Naked-Eye Sky (copyrighted) by James Sowell, 2013