Tyrel L. answered • 12/17/19
11 year teacher with a passion for learning and teaching!
A lunar eclipse can be viewed by anyone positioned on the Earth with the moon above the horizon as the moon enters the Earth's shadow and the eclipse progresses over a few hours. This will change as the Earth turns and more areas face towards the moon and others face away. You could see an eclipse in progress as the moon sets and then you wouldn't see the rest of the eclipse occur. The eclipse could also begin before the moon rises for a given location but once it does rise the observers at that location could see the rest of the eclipse.
Lunar eclipses will occur roughly every 6 months at one or more full moons that occur within an window of time known as an eclipse season that lasts about 30 days. This is the time when the obit of the moon is oriented in relation to the Earth and the sun so that full moon occurs at one of the 2 nodes when the moon's orbit intersects with the plane in which the Earth orbits the sun. At all other times the full moon happens when the moon is above or below this plane and so the shadow of the Earth misses it. The distance from the moon from the earth changes since it's orbit isn't perfectly circular so this also affects how it interacts with Earth's shadow which is like a tapering cone that diminishes with distance.
There are 2 parts to the shadow, with one area where all points on the moon are completely blocked from direct sunlight by the earth, and one area when a given point on the moon is still receiving part of the sun's light because the sun is still partially visible. This is the same for an observer on earth that might have the full light of the sun shining on them if they were in the open, part of it if half the sun were behind the edge of a building, or none of the light if standing all the way in the full shadow of the building with none of the sun visible.
When the moon enters only the outer shadow of the E (called the Penubra) we have a Penumbral eclipse with just a faint darkening of the moon. When the moon enters the dark part of the shadow (the Umbra) we can have a total lunar eclipse if it enters completely, or a partial if it doesn't completely enter this part of the shadow.
