This lunar eclipse won't occur again in the United States until 2021. A lunar eclipse is when the sun, Earth, and moon are aligned in a straight line and in that order. On all other days, the sun shines on the moon and the moon reflects the light onto the Earth. In a lunar eclipse, the Earth blocks the moon from getting any of the sun's light and makes it appear red. A solar eclipse is opposite. The order of the three planets goes sun, moon, Earth. The moon blocks the sun from reaching the Earth.