When can you see Earth’s shadow?

Just like you or me, Earth casts a shadow. Earth’s shadow extends into space, in the direction opposite the sun. There are several good times to think about and be aware of Earth’s shadow.

See Earth’s shadow any evening, ascending in the east. You can see Earth’s shadow any clear evening ascending in the eastern sky at the same rate that the sun sets below the western horizon. The shadow is a deep blue-grey, and it’s darker than the blue of the twilight sky. The pink band above the shadow is called the Belt of Venus.

Earth’s shadow, with full moon in October 2012. Photo from EarthSky Facebook friend Birgit Boden in northern Sweden.

Earth’s shadow and October 2012 full moon from EarthSky Facebook friend Cattleya Flores Viray in San Diego.

Dark blue Earth’s shadow near the horizon, with an almost full moon in the Belt of Venus above the shadow, in October 2012 as seen by EarthSky Facebook friend Lance Bullion in Baton Rouge, Louisiana.

The shadow of the Earth is big. You might have to turn your head to see the whole thing. And, just so you’ll recognize it more easily, remember that the shadow is curved, in just the same way that the whole Earth is curved.

See Earth’s shadow during an eclipse of the moon. Earth’s shadow extends into space so far that it can touch the moon. That’s what a lunar eclipse is – just the moon within Earth’s shadow.

December 10, 2011 total eclipse of the moon, as seen from Saskatoon, Saskatchewan, Canada. EarthSky Facebook friend Colin Chatfield shot this photo shortly before the total part of the eclipse began.

When the sun, the Earth and the moon are aligned in space (nearly or perfectly), with the Earth in between the sun and moon, then Earth’s shadow falls on the moon’s face. Then people on Earth see the shadow gradually turn a bright full moon dark in an eclipse of the moon. There are typically two or more lunar eclipses every year. Some are total, some are partial, some are a special kind of eclipse known as penumbral. During a lunar eclipse, a very small amount of light from the sun filters through Earth’s atmosphere onto Earth’s shadow on the moon. It’s why – at the middle part of a total lunar eclipse – the shadow on the moon looks reddish.

Night falls when the part of Earth you’re standing on enters Earth’s shadow. Click here to expand this image. Image via NASA

Night is a shadow. The fact is that night on Earth or any other world is a shadow. When night falls, you’re standing within the shadow of Earth.

Bottom line: Check out Earth’s shadow. You might see it as an ascending line of darkness in the east just after sunset. Or you might see it brushing the moon’s face during a lunar eclipse. Or think about night as a shadow, when you’re standing outside in darkness after sunset … maybe tonight.

- EarthSky