Wherever there is light and an object, there is a shadow. By blocking sunlight, Earth also casts a shadow.
But can you see the shadow of the earth? Astronomers told LiveScience that yes, you can see it cast on several objects. In fact, under the right conditions, you can even spot the Earth’s shadow every day.
Shadow on the moon
Because sun is an extended light source (meaning it is not a single point of light), the Earth’s shadow has three parts: a dark central umbra, a lighter peripheral penumbra, and an antumbra that appears at a distance where overlapping penumbral areas replace the umbra.
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Earth’s shadow is the most prominent under total lunar eclipses. During a lunar eclipse, the full moon passes through the Earth’s shadow. The Moon enters the Earth’s penumbra, and then its umbra, before passing through the penumbra again. It is difficult to distinguish the penumbra of the Earth, since the Moon dimmer just a little. In contrast, the umbra, which can be up to 2.7 times the lunar diameteris much easier to see.
However, when the Moon is completely enclosed in the Earth’s umbra, it is usually not completely black. Most times it is faint to deep red. That’s because the planet’s atmosphere scatters sunlight and then bends it red rays into the umbra.
“In a way, we see sunrises and sunsets on Earth reflected from the Moon!” Emily Ricean astronomy professor at the Macaulay Honors College at the City University of New York told LiveScience in an email.
The shadow can also tell us about the Earth’s atmosphere. “The cloudier and dustier the Earth’s atmosphere is at the time, the redder the moon will appear,” she noted.
By comparison, it is possible to see a blackened moon when moon borders above the Earth’s umbrella-penumbral boundary. This stems from a contrast effect, since the part of the moon in the penumbra is 500 times brighter than in the umbra, wrote astrophysicist Fred Espenak EclipseWise.
The Earth’s shadow is also visible in partial lunar eclipses, when only part of the Moon passes through the umbra.
Daily joy
But you don’t have to wait for an eclipse to see the Earth’s shadow. Immediately before sunrise and just after sunset, the planet’s silhouette – cast in space – becomes visible in the sky near the horizon in the direction opposite the sun.
“During the day, the clear sky is bright because direct sunlight illuminates the gas molecules and particles in our atmosphere, which can scatter some of this sunlight.” Raymond L. Leeadjunct professor in the Mathematics and Science Division at the US Naval Academy, told LiveScience in an email. “When the sun sets or rises, the (roughly) spherical Earth blocks the lowest rays of the sun from illuminating the atmosphere directly, thus casting a curved shadow on the corresponding lowest parts of the atmosphere opposite the sun.”
However, the exact properties that make up the Earth’s shadow are debated. A study from 2017 in the journal Applied optics says that the shadow consists of an upper part dark blue ribbon and a lower brown band. But Giovanni di Giovannian amateur astronomer who has a master’s degree in physics from the University of L’Aquila, in Italy, said neither is the real shadow. Instead, our planet’s shadow is “a thin, very dark band, but with little contrast compared to the other bands above it,” he told LiveScience in an email.
In any case, the shadow is present when the sun is just above the horizon until it is about 4 degrees below it. The planet’s silhouette persists for about 15 minutesbefore that disappears with the rising sun or merge with the darker night sky.
Seeing the Earth’s shadow requires a clear, dust-free sky at high altitudes. “Very few observers would notice it … only from the top of high mountains, and perhaps with less difficulty from an airplane,” di Giovanni said.
Earth’s shadow on other objects
Detecting Earth’s shadow on other objects is possible, but more difficult. If The International Space Station passing overhead on a cloudless evening at sunset, a couple star binoculars can help you see it muted when it enters the shadow of the earth.
In addition, telescopes can show how geostationary satellites “disappear” in the Earth’s shadow. Such satellites orbit the Earth just above the equator, at the same speed as the planet rotates, which makes them appear fixed in the sky. According to the satellite operator Space NorwayGeostationary satellites are mostly bathed in sunlight. But it changes for about 21 days around each equinox. In accordance Australian Space Weather Forecasting Centreduring these “eclipse seasons” the satellites slip into Earth’s shadow for several minutes each night.
Earth’s shadow even appears on passing asteroids. For example, astronomer Gianluca Massi, founder of The Virtual Telescope Project, wrote in a blog post that one of their telescopes recorded the house-sized asteroid 2016 VA dimming and disappearing as it passed through Earth’s shadow for 10 minutes, 50 seconds at a distance of 120,000 kilometers.
But there is a limit to how far the earth’s shadow extends. “(Earth’s) umbra is a very long cone shape that extends about 1.4 million km (870,000 mi) away from Earth,” Rice said. So the Earth’s shadow will not reach Mars, which at its closest is 55 million km away. Still, with many objects passing through Earth’s shadow, there are plenty of ways to keep looking at — and marveling at — the planet’s silhouette.
