Uncovering Secrets of Earth's Shadow

Sometimes we overlook common but amazing phenomena that happen right under our noses. Take sunset for example. Most of us look forward to seeing sunsets (and sunrises) and have our cameras at the ready to record the splendid light. But if we turn our backs on the sinking Sun and instead face east, we'll witness the rising of Earth's shadow, cast by the globe on the atmosphere above us. While the science is simple — Sun goes down, shadow rises — the transition from daylight to twilight is a fast-paced, multi-phase event not unlike watching a total solar eclipse, without the anxiety of weather and travel. Any clear evening will do.

You've probably seen the phenomenon before even if you didn't recognize it. At sunset, a low, gray-blue band appears along the rim of the eastern sky and slowly climbs as the Sun sinks deeper beneath the opposite horizon. The colorful anti-twilight arch, better known as the Belt of Venus, fringes the shadow's upper edge of in a rich shade of orange-pink. Reddened light from the Sun, which has set for an observer on the ground but remains "visible" from higher up in the atmosphere, is the source of the arch's light. Air molecules scatter the ruddy rays back to our eyes.

At and shortly after sunset, the solar rays scatter off the thickest, lowest part of the atmosphere, so the the Belt of Venus appears brightest. But the lower the Sun dips below the horizon, the steeper and higher its rays slice across the upper atmosphere. Thinner air at higher altitude scatters less red light, causing the Belt of Venus to fade until it disappears altogether about 15 minutes after sunset.

Since the Belt is extremely helpful in delineating Earth's blue-gray shadow, its absence — along with the darkening sky — eventually causes the rising gloom to blend into the coming nightfall.

I enjoy watching the shadow and Belt of Venus anytime I'm outside around sunset or sunrise, whether that's driving down the freeway or standing at the edge of the forest. Here are a few things to look for whether you plan your session or it happens by chance. For the best views at sunset, find a place with a wide-open view to the east. Face west if you're watching at sunrise. The closer you can see down to the horizon, the more aspects are visible. Not all apparitions are identical. Changes in air transparency as well as clouds near and below the local sunset horizon can alter the color depth, contrast and even the shape of the shadow and Belt of Venus. At times, I've seen stray rays of color appear off to one side or other of the Belt minutes after the central portion has faded away. Expect surprises.

Elevation also affects how dark the shadow appears. From an airplane window, it's dramatically sharper and darker (more purplish), presumably due to more transparent air in combination with an exceptionally long line of sight through the atmosphere. One thing that surprised me was seeing what I thought was the shadow before the Sun had actually set. How is that possible?

If you can see nearly to the horizon, the Belt of the Venus becomes visible within about 10 minutes of sundown. Then in the minute or two before sunset, its bottom edge will appear distinctly dusky. Along its bottom you'll see a dusky-gray edge, not the steel-blue of the true shadow but shading nonetheless. This "false shadow" is caused by the atmosphere casting a pall of its own through our line of sight.

Recently, I took a closer look at a shadow-rise from the mesa country of far western Oklahoma during a dark-sky retreat. What I saw convinced me that there was even more to the phenomenon than I'd realized. The following is a photographic timeline of the phenomenon recorded on February 27, 2022. While lacking an ideal horizon, I could see low enough to capture all aspects of the rising. Speaking of which, I've only discussed the shadow's appearance at sunset. You can also view the entire sequence — in reverse — at sunrise by facing the western sky.

The entire sequence lasts about 30 minutes, making watching a shadow-rise something anyone can enjoy. I suspect there's still much to be learned about Earth's atmosphere through extended observation of the phenomenon. By all means, share your observations with us here at Sky & Telescope.

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