Blood moon 2018: the lunar eclipse on July 27/28

A lunar eclipse occurs when the Moon passes directly behind Earth and into its shadow. This can occur only when the Sun, Earth, and the Moon are aligned (in syzygy) exactly or very closely so, with the planet in between. Hence, a lunar eclipse can occur only on the night of a full moon. The type and length of an eclipse depend on the Moon's proximity to either node of its orbit.
During a total lunar eclipse, Earth completely blocks direct sunlight from reaching the Moon. The only light reflected from the lunar surface has been refracted by Earth's atmosphere. This light appears reddish for the same reason that a sunset or sunrise does: the Rayleigh scattering of bluer light. Due to this reddish color, a totally eclipsed Moon is sometimes called a blood moon.