12 June 2018
One of the most delicately beautiful things you can see in the sky is Earth’s shadow cast up into the sky. Next time you’re watching a sunset on a cloudless evening, turn around to see this little-known but utterly gorgeous atmospheric phenomenon. Almost all of us love to watch beautiful sunsets but we rarely think to turn away from the sunset in the west and see what is called the Belt of Venus on the opposite side of the sky.
Right after the sun sets beneath the western horizon, look directly east and you’ll see a beautiful bright pink band lying just above the horizon. It’s the light of the setting sun in the west being tilted eastwards and reflecting off the atmosphere.
As the sun sinks lower behind the western horizon, the Belt of Venus lifts off the horizon and you’ll see a band of blue underneath the rising band of pink. This blue stripe, which is darker than the surrounding twilight, is Earth’s shadow being cast up on the atmosphere!
These bands of colour keep moving upwards in the minutes following sunset and, because the Earth is round, eventually forming an arch of Earth’s shadow across the sky before fading as night sets in.
The shadow of the Earth is big. It stretches across the entire eastern horizon from north to south which makes it an especially gorgeous sight when you see it stretching across the vast horizon of the Kalahari.
The Belt of Venus also occurs before sunrise when its glow lights up the western horizon, but it is far more prominent at sunset.
The name is somewhat misleading because the Belt of Venus has nothing to do with the planet Venus, but is instead gets its name from Venus, the ancient Roman goddess of love and beauty, who was incomparably beautiful and was usually depicted wearing a beautiful flowing belt.
It’s interesting to note that it is impossible for the planet Venus to appear in the Belt of Venus. This is because Venus orbits the Sun closer than the Earth, so due to geometry, it cannot appear more than about 45° away from the Sun. Thus, we always see it for a short time after sunset (when it’s known as the “evening star”) or a short time before sunrise, (“morning star”). Since the Belt is opposite the Sun, 180° away, it is impossible to see the planet Venus in the Belt of Venus.
On the other hand, for the Moon to be full, it has to be opposite the Sun in the sky. Being opposite the Sun, it then rises through the Belt of Venus at sunset and sets through it at sunrise. It is a gorgeous sight is to see the Full Moon rising through the shadow of the Earth!
Copyright © Susan Young 2018