Fred Schaaf's Northern Hemisphere's Sky column for June 2010 discusses some phenomena that are visible in twilight, one of them being the Belt of Venus.Fred simply mentioned the Belt of Venus, but I thought it needed more explanation.So I picked up a phrase from the internet, and described the Belt of Venus as "an arch of pinkish light above the shadow that Earth casts on the atmosphere opposite the sunset." Fred demurred, describing it as "pinkish border to Earth's shadow."
Unfortunately, I had no first-hand experience to fall back on. As an astronomy writer, I knew the term, of course. And reading about it in various sources, it was obvious that I must have seen it dozens or hundreds of times, but I had never recognized it.
Fortunately, it was clear the next few nights, so I was able to observe the Belt of Venus several times in a row, both in the evening and morning. The bottom line is that as usual, both Fred and I are right. It is a border to the shadow, but it also forms a striking arch — though an exceedingly low and broad one.
All you need to see for yourself is a clear evening and a site with an unobstructed eastern horizon. A hilltop, lakefront, or beach is ideal.
Right after the Sun sets, tear your eyes away from the arresting scene in the west, and look east to see a bright pink band opposite the Sun. This is the light of the sunset where you're standing being reflected off the atmosphere some 50 or 100 miles east of you, as shown above.
Three minutes later, the pinkish band has become fainter, but with a richer hue. Surprisingly, it has started to lift off the horizon. Now there's a thin band of bluish sky below the pink.
Six minutes after sunset, the dark blue band below the pink ribbon is beginning to take on shape and substance. It is now clearly rounded, taller in the middle than on the sides.
You are, in fact, seeing Earth's shadow. The Sun is now setting about 100 miles west of you. That light is still reflecting off the atmosphere to your east, but now some of it is blocked by Earth itself — even, just a little, by you standing on that hilltop. So the lowest part of the atmosphere opposite the sunset is no longer lit up.
Fifteen minutes after sunset, the pink has dissipated. But you can still make out Earth's shadow in the east as a huge, low hump of darker sky along the horizon — exactly the opposite of the normal situation, where the sky is brightest along the horizon and darker toward the zenith.
You are watching the onset of night. Soon, Earth's shadow will grow and darken dramatically, and eventually it will cover the entire sky, allowing you to look out at the greater universe that's usually hidden by the Sun.
Why is the Belt of Venus so little known? Partly it's because it's a fairly subtle effect. But even more, though people go out to watch sunsets all the time, they rarely think to turn away from the spectacular vista in the west and see what's happening on the opposite side of the sky.