Several minutes later, Zubenel watched that straight boundary drop lower until it "set" on the distant tree line. "Somehow, I intuitively knew what I had just witnessed," he says, "even though I had never seen such a phenomenon."
Zubenel shared his observations and photographs with Les Cowley at Atmospheric Optics (www.atoptics.co.uk).
After looking at the geometry of the situation, Cowley confirmed Zubenel had seen Earth's shadow in the west after sunset! We commonly see Earth's shadow rising as a dark band in the east after sunset. As Earth rotates, that band rises until it reaches an altitude of about 6°, when it blends into the darkening sky background and fades from view.
But when volcanic ash fogs up the stratosphere, we can see Earth's shadow against the glowing aerosol layer in the west after sunset, as shown in the diagrams on page 18. As Earth rotates, more and more of the thin aersol layer falls into Earth's shadow, which seems to "push" the sunlit portion lower toward the western horizon until the shadow "snuffs" it out. How long?
How long will the Kasatochi twilight displays last? According to the July 2008 Bulletin
of the Global Volcanism Network, Kasatochi's stratospheric aerosol cloud was one of the largest since the 1991 Mount Pinatubo eruption in the Philippines. So abnormal twilights may be with us for some time. Note, however, that while the intensity of the displays starts out strong, it generally softens as stratospheric winds disperse the gases.