The Belt of Venus

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.

Belt of Venus
Wikimedia Commons / flagstaffotos.com.au / GFDL
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.

Tony Flanders

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.

Tony Flanders

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.

Tony Flanders

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.

13 thoughts on “The Belt of Venus

  1. Jeremy Perez

    This post is a nice treat, Tony. That’s a great set of photos showing its progression from intense to subtle as it rises in the sky. I drive eastward on the way home from work, and at the time of year when sunset coincides with that drive, the Belt of Venus is always a welcome treat. It tempts me with what’s waiting behind that beautiful, rising curtain if only I take the time to get the telescope out the door when I get home.

  2. Michael Harrison

    This is an interesting subject but the vertical bands in the pictures get in the way of seeing the belt. What are those artifacts?

  3. Derek

    A nice article about a little-understood and under-appreciated phenomenon we have the chance to see frequently – if we only open our minds to it.

    Crepuscular rays are another lovely phenomenon also under-appreciated and little-understood. Simply, they are shadows of clouds or geology like mountains cast across the terminator at either dawn or dusk. Because their scale can be hundreds of miles long, they often show convergence effects on the set sun and its anti-point.

    They show as bands arcing or arching upwards into and sometimes across the sky.

    However, the bands in Greg’s photos are most likely artifacts from post-processing of image-stitching (merging of more than one image panorama from a camera).

  4. Alison

    What a cool article :) I have often admired the pretty pink in the sky at sunset while walking on the beach or photographing moonrises, but never thought about the explanation. I’ll pay more attention next time.
    Thanks for an interesting article on something that, as you say, people often pay little attention to.

  5. Barbara

    What a wonderful article. As a child we often looked east during sunset to see the “skyblue pink” as my Mother called it. She would have been thrilled to know the science behind it. Thank you, you have made this phenomenon more special than it already was.

  6. Mike

    I had always interpreted this ‘belt’ as part of what Minnaert had called the ‘purple light’. Althouh he describes it viewing west at sunset, the figure 169 (Light and color in the Outdoors, 1993 Springer-Verlag) illustrates that it is seen to the east as well. This is the first time I’ve heard the term Belt of Venus applied.

  7. Jim

    Great article. I climbed Mt Hood in Oregon a few years ago, arriving at the peak at sunrise. Looking west I clearly saw the Belt of Venus but didn’t realize it was a named phenomenon. As the sun rose, the belt moved lower to the horizon. In addition to the Belt, I could clearly see the shadow of Mt Hood projected westward. I guess the shadow is the same effect as the crepuscular rays mentioned by Derek, except that the shadow was below me rather than above. The shadow forms a perfect pyramid that I think many climbers misinterpret as mirroring the shape of the mountain, but it is almost certainly just a distance/convergence effect. I have some good photos of this.

  8. Theresia Taylor

    I love to watch the sunrise and sunset. The beauty of them are simply fascinating to me. I had never heard of the Belt of Venus but thoroughly enjoy the colors in the eastern sky at sunset. They are beautiful and your photos are awesome. Your explanations will make me focus in on more of the details of this amazing wonder in the sky.

  9. John Whitby

    Here in the Colorado Rockies at 8000 to 10000 feet we see this every clear evening – about 300 times per year! We call it Alpenglow – the pink lighting of the mountains in the east after sunset.

  10. Patrick

    12/1/11
    Years ago, when I was taking Astronomy 101, my professor told us that if we turned our back on the Belt, bent over, and looked at it through our legs, it would appear much brighter. We thought he was just trying to make fools of us, but when we tried it, it worked! It actually does appear significantly brighter that way.

    And you don’t actually have to look between your legs. If you stand facing the Belt and just lean over sideways, you get the same effect.
    Someone want to try to explain this?

    P.S.: I’ve had great fun over the years teaching people how to do this.

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