Earth’s shadow: When can you see it?

Earth’s shadow is easy to see

Like all worlds orbiting a sun, Earth casts a shadow. It’s easy to see in the sky, just after sunset and before sunrise. In fact, you’ve probably already seen Earth’s shadow, many times, as day changes to night.

That’s because night itself is a shadow. When night falls, you’re standing within the shadow of Earth.

The best time to watch for Earth’s shadow is when it’s creeping up on your part of Earth … Like all shadows, the shadow of Earth is always opposite the sun. So, you’ll want to look eastward after sunset for the shadow (or westward before sunrise).

In the photos above and below, Earth’s shadow is the dark blue line above the horizon. And the pink band above the shadow is the Belt of Venus.

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What to look for to see Earth’s shadow

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 the Belt of Venus.

The shadow of the Earth is big. You might have to turn your head this way and that – along the arc of the horizon opposite the sun – to see the whole thing. And, just so you’ll recognize it more easily, remember that the shadow is curved, in exactly the same way that the whole Earth is curved.

And, once you spot it, don’t go back inside just yet. Wait awhile, and watch Earth’s shadow ascending or descending at exactly the same rate that the sun is rising or setting on the opposite horizon.

Our shadow is why we see lunar eclipses

Earth’s shadow extends so far into space that it can touch the moon. That’s what a lunar eclipse is. It’s the moon within Earth’s shadow.

When the sun, the Earth and the moon align in space (nearly or perfectly), with the Earth between the sun and moon, then Earth’s shadow falls on the moon’s face. That’s when people on Earth see the shadow gradually turn a bright full moon dark in a lunar eclipse.

As seen from Earth’s surface, there are typically two or more lunar eclipses every year. Some are total, some are partial, some are a subtle 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.

The view from space

Another way to get an awareness of Earth’s shadow is simply to think about it as seen from space.

The image below provides a beautiful global view of Earth at night. It’s a composite image, assembled from data acquired by the Suomi National Polar-orbiting Partnership (Suomi NPP) satellite over nine days in April 2012 and 13 days in October 2012.

The dark part is, of course, Earth’s shadow.

Bottom line: Look for Earth’s shadow in both the evening and morning sky. It’s a blue-gray darkness in the direction opposite the sun, darker than the twilight sky. The pink band above the shadow – in the east after sunset, or west before dawn – is the Belt of Venus.