Earth's Shadow

Last updated on September 16, 2023

Earth's Shadow: A Phenomenon of Twilight

When we observe the sky during twilight, we may notice a fascinating and elusive phenomenon known as Earth's Shadow. This intriguing spectacle occurs when the Earth casts a shadow into the atmosphere, creating a distinct dark blue band near the horizon opposite the sun. The shadow is most prominent shortly after sunset or before sunrise and is particularly sharp at high altitudes. However, as twilight progresses, the shadow rises and gradually becomes less defined.

But what exactly causes Earth's Shadow? The answer lies in the interplay between sunlight, the Earth's curvature, and the atmosphere. As the sun sets or rises, its rays are refracted by the Earth's atmosphere, bending them around the planet's curvature. Consequently, the Earth casts a shadow into the atmosphere, which manifests as a dark band opposite the sun.

While Earth's Shadow is typically a transient phenomenon, it can sometimes be observed for a longer duration when there is stratospheric haze present. In such instances, the shadow may persist and even appear to "set" in the direction of the sun. This captivating sight allows us to follow the shadow as it traverses the sky, adding an extra layer of intrigue to an already mesmerizing phenomenon.

Accompanying Earth's Shadow is another captivating optical phenomenon called the "Belt of Venus" or "anti-twilight arch." This rosy pink glow appears just above the dark blue band of Earth's Shadow and extends towards the zenith. The Belt of Venus is a result of backscattered sunlight reddened by the atmosphere and the deep blue color of the sky opposite the sun. Together, Earth's Shadow and the Belt of Venus create a visually stunning display that enchants skywatchers.

Interestingly, Earth's Shadow and the Belt of Venus are best observed from high altitudes where atmospheric conditions are favorable. At these elevated locations, the shadow appears sharper and more distinct, allowing for a more immersive experience. Photographers and enthusiasts often seek vantage points at high altitudes to capture the beauty of this phenomenon.

One such example is the Omu summit in the Southern Carpathians, Romania, where Alex Tudorica captured a breathtaking panorama of Earth's Shadow and the Belt of Venus during the 2008 August Perseid meteor shower. The image showcases the dark blue band of Earth's Shadow contrasting with the vibrant rosy pink glow of the Belt of Venus. Additionally, the moon can be seen majestically positioned above the Belt of Venus, adding to the celestial spectacle.

It is important to note that Earth's Shadow and the Belt of Venus are not isolated occurrences but are part of a broader field of study known as atmospheric optics. This fascinating discipline explores various optical phenomena that arise due to interactions between sunlight, particles in the atmosphere, and other atmospheric conditions.

In conclusion, Earth's Shadow and the Belt of Venus are captivating phenomena that grace our skies during twilight. These ethereal displays provide a glimpse into the intricate interplay between sunlight, the Earth's curvature, and our atmosphere. Whether observing from high altitudes or capturing them through photography, these phenomena continue to captivate and inspire awe in those fortunate enough to witness them. So, the next time you find yourself gazing at the twilight sky, keep an eye out for Earth's Shadow and the enchanting Belt of Venus.

Earth's Shadow & Belt of Venus from Omu summit (2,507m) in the Southern Carpathians, Romania.

Alex Tudorica imaged this panorama (scroll!) during a watch for the 2008 August Perseid meteors. The dark blue band close to the horizon and opposite the sun is unlit air shadowed by the earth itself. The shadow is best seen shortly after sunset or before sunrise and is sharpest at high altitudes. The shadow rises as twilight progresses and becomes less and less distinct - But when there is stratospheric haze it can sometimes be followed across the sky to 'set' in the sunward direction. The rosy pink glow is the antitwilight arch or 'Belt of Venus', it is a combination backscattered sunlight reddened by the atmosphere and the deep blue of the anti-sunward sky. At top right the Moon rides high over the antitwilight arch. Image ©Alex Tudorica, shown with permission.

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Reference Atmospheric Optics

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