At What Phase Is It Possible to See an Eclipse of the Moon?

Eclipse Prerequisites

• Two conditions must exist for a solar or lunar eclipse to occur. The first condition is for the correct moon phase -- full moon for lunar eclipses and new moon for solar eclipses. Moon phases simply describe the relationship between the moon, Earth and Sun during the moon's orbit. The second condition requires the moon to lie on the same plane as the Sun and Earth so that the shadow of one body passes over the other, a condition that occurs twice a month.

Moon Phases, Briefly

• As the moon orbits around the Earth, half of the Moon is lit by the Sun and the other half is shrouded in darkness. However, observers on the Earth notice a nightly change in the appearance of the moon as it progresses from the dark new moon to the completely illuminated full moon and back to new moon in a cycle that lasts almost a month. A full moon appears lit because the moon is "behind" the Earth in relation to the Sun giving observers a view of the side of the moon facing the Sun. Conversely, a new moon is "in front" of the Earth in relation to the sun giving observers a view of the darkened side of the moon.

Deviation from the Ecliptic

• It may seem that if the moon is behind the Earth during a full moon and in front of the Earth during a new moon then there would be a lunar eclipse every full moon as the moon passes through the Earth's shadow and a solar eclipse every new moon as the moon's shadow falls upon the Earth. But, this does not happen since the moon orbits the Earth at a slight (5.2 degree) angle from the ecliptic. Put another way, if the Earth and Sun were on a disc, the Moon would orbit the Earth slightly off the disc. An eclipse can then only occur when a full moon or new moon coincides with the Moon crossing the ecliptic. This narrows their frequency to only two to three times per year.

But Why a Red Moon?

• As the moon passes through the Earth's shadow during a lunar eclipse, the shadow of the Earth lowers a dark veil across a previously fully lit moon. After the moon is darkened, the moon surprisingly turns red before darkening again and then passing out of the shadow back to a brightly lit full moon. The red color during the height of the lunar eclipse results from sunlight refracted through the Earth's atmosphere. The atmosphere filters blue light leaving red-orange colors to fall upon the Moon.