CLAIM

Lunar eclipses happen only during full moons.

TRUE

RATING

TRUE

ORIGIN

Prior to the occurrence of a penumbral lunar eclipse on 10 February 2017, a number of news outlets ran headlines suggesting that this celestial event would be even more rare because viewers would get to experience three events at the same time: A full moon, a lunar eclipse, and a comet (viewable with binoculars).

The Washington Post, for example, ran with the following headline and lede:

A full moon, lunar eclipse and comet all in one night? Here’s how to watch it Friday.

USA Today‘s reporting took a similar tack:

Triple treat: Eclipse, comet, full moon all coming Friday night

While the close approach of a comet during an eclipse is rare, the presence of a full moon during an eclipse is required. The orbital arrangement of the sun, earth, and moon necessary for an eclipse to occur only happens during a full moon, as explained by the European Space Agency:

A lunar eclipse occurs when the Earth comes between the Sun and the Moon and the Earth’s shadow passes over the Moon. This phenomenon can be seen by any observer on Earth for whom the Moon is above the horizon. Lunar eclipses occur at the time of a Full Moon, but not every Full Moon, because the Moon has to be near one of the nodes of intersection between its orbit and the ecliptic plane.

Sources:

European Space Agency.   “Lunar Eclipse Overview”
21 September 2006.