Today’s Question: Thank you for letting us know about the total lunar eclipse. I was able to capture some nice photos thanks to that. I was reading that a total lunar eclipse only occurs during a full moon. Why is that?
Tim’s Quick Answer: A total lunar eclipse can only occur during a full moon because an eclipse requires an alignment of the sun, earth, and moon.
More Detail: A total lunar eclipse occurs when the moon passes through the shadow of the earth. That requires that the moon is directly opposite the earth relative to the sun. In other words, the sun, earth, and moon line up, with the earth in between the sun and moon. This causes the shadow of the earth to fall on the moon.
By definition, if the earth is in between the sun and the moon, the moon will appear as a full moon from the perspective of the earth because the sun will be illuminating the entirety of the moon that is visible from earth.
If, on the other hand, the moon is not aligned with the sun and earth, it will not appear full from the perspective of earth. For example, if the moon is in a position that extends from a 90-degree angle relative to the alignment of the sun and earth, the moon will appear half full because in terms of the illumination of the sun we are viewing the moon from the side.
Similarly, when the moon is full it will rise or set around the same time as sunset or sunrise, because of the alignment of the sun, earth, and moon at the time of a full moon.
So, the alignment that enables the moon to pass through the shadow of the earth for a lunar eclipse also results in a full moon from the perspective of earth.