Lunar eclipse
During a lunar eclipse, also called a blood moon, Earth passes directly between the Sun and the Moon. The resulting shadow on the Moon turns it orange to dark red. Lunar eclipses only happen around a full moon.
The shadow behind Earth can be divided into two regions. When the Moon lies entirely within the ‘umbra’ region of full shadow, we witness a total lunar eclipse. When the Moon lies only partially within the umbra we see a partial lunar eclipse, where part of the Moon is still directly lit by the Sun. When the Moon lies fully within the partially shaded ‘penumbra’, it will dim slightly, but this is often difficult to detect with the naked eye.
The reason that the Moon turns red is that sunlight passes through Earth’s atmosphere and gets redirected into the umbra. Thanks to an effect called Rayleigh scattering, only light on the red end of the visible spectrum makes it through the atmosphere, while shorter-wavelength colours like blue and violet are filtered out.
The white dashed lines indicate how rays of sunlight travel from the Sun to Earth. Because these rays travel in straight lines, tracing these rays from the Sun’s edges also traces out the umbra and penumbra.
Lunar eclipses can be seen from anywhere on the night side of Earth and often last for an hour or longer. They are safe to observe without eye protection.
[Image description: This infographic explains how lunar eclipses work. A central diagram shows the formation Sun-Earth-Earth, with Earth casting a shadow on the Moon as it blocks the Sun's light. The fully shadowed region is labelled 'umbra'. A surrounding partially shadowed region is labelled 'penumbra'. The Sun, Earth and Moon themselves are also labelled. Dashed lines mark out the path of sunlight. A heading at the top says 'Lunar eclipse' and the European Space Agency logo sits at the top right of the infographic.]
