Twilight comes like clockwork, twice a day, morning and evening. Literally that time between day and night, twilight is rigorously defined by the Sun's angle below the horizon. Backyard gazers use twilight for setting up their mounts, adjusting their finders, and preparing their charts and lists. Morning twilight is a sleepy time for the few dedicated observers pulling all-nighters. Most are too busy, or too tired, to enjoy twilight.

When the Sun sets, we are cast into the Earth's shadow. The Earth's shadow then appears to rise slowly upward, projected against the eastern sky directly opposite the sunset. Although few people recognize this "twilight arch" for what it is, it is a surprisingly common phenomenon and quite easy to see in cloud-free skies with a low horizon. It can be recognized by the pinkish band (known as the Belt of Venus) above the darkened gray - blue band of the shadow itself.

The zodiacal light represents the last vestige of the Sun's glow in the evening sky. It appears as a tilted wedge of light from the horizon tapering upward along the zodiac. It is highest and best seen in the evening in the Spring and in the morning in the Fall. Brighter than the Winter Milky Way, it plainly visible from a truly dark site. The zodiacal light is believed to be sunlight reflected off interplanetary dust particles on either side of the Earth in its orbit.

Twilight is more than a time to anticipate the clear evening to follow. Finish your alignment, set aside the chart and take in the sights of the half-light between day and night.

Top: Red Mill Pond, looking northeast into the Earth's shadow, the Belt of Venus visible as a pinkish band through the middle of the photo, November '98. Right: The zodiacal light towers above the west ridge at Star Hill Inn, New Mexico, February '97. The brightest trailed "star" embedded in the glow is Saturn. Color photos are available at the web page: http://udel.edu/~dmiller/moondark/march99.html.

For more information on the Earth's shadow - Belt of Venus, the zodiacal light, other twilight phenomena and elusive glows, see Fred Schaaf's Seeing the Sky: 100 Projects, Activities & Explorations in Astronomy (1990), John Wiley & Sons. Moondark is written by Doug Miller and published on the web, in the Delmarva Star Gazers' Star Gazer News and in the Delaware Astronomical Society's FOCUS. Please address comments and suggestions to dmiller@udel.edu. This document was last revised on 21 February 1999. All text and images copyright © 1999 Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.