Because the orbit of the month inclines 5 degrees against a revolution side (ecliptic surface) of the earth. Even if it becomes the full moon, it is located up or down on some level of the shadow of the earth.
When the moon just crosses the ecliptic surface at the grade of a full moon, an eclipse of the moon takes place.
By the way, a gloomy part may be seen thinly with a bright place when we look at the moon well. This is because the light of the sun reflects the earth and shines light on the surface of the moon. It is called Earthshine.
Filed under: Astronomy and Space
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