Daniel Pope

If you think the evening twilight above the western horizon is lasting a little longer (more than 90 minutes after sunset) than usual, perhaps you are seeing the Zodiacal Light. This is what the very first observers recorded experiencing long ago on moonless nights near the time of the Vernal Equinox. They also noticed that dawn seemed to come earlier than usual around the Autumnal or Fall Equinox. This is the phenomenon that is being referenced when you encounter the words, the False Dawn. Perhaps the term “False Twilight” appears somewhere in ancient literature but I have not found it yet. Later, Aristotle (384- 322 BC), noted that this faint light around the time of both equinoxes always appeared along the arc of the constellations of the Zodiac. He is credited with naming this very subtle feature of our Solar System, Zodiacal Light.

We now know that the twelve constellations of the Zodiac roughly define the plane of our Solar System. Connecting our naked eye planets and Moon gives us a much better visualization or image of the narrow plane of our Solar System, the Ecliptic. Perhaps we should change the name False Dawn or Zodiacal Light to the Ecliptic Light. The Zodiacal Light is always present but this is the best time of the year to look for it because now the Ecliptic is tilted at its steepest angle to our western horizon just after sunset.

We need a good view of our western horizon and a clear, dark, and moonless sky in order to detect the Zodiacal Light and perhaps using averted vision might help.

Look above the western horizon about an hour and a half after sunset. Look for a very faint glow in the sky that is shaped like a triangle or wedge. The bottom of the triangle will be just above the horizon centered on where the sun went down. The width is about from your little finger to your out-stretched thumb at arm’s length. The other two sides of the triangle will rise and meet at a point about half way up the sky. The Pleiades will serve as a guide for judging halfway up the western sky.

It is as awe inspiring and exquisite today as it was when our ancient ancestors observed it. Experiencing this very subtle feature of our Solar System first hand invariably produces one of those “ah ha” moments or reactions.

For a long time it was thought that this phenomenon was caused by something in the Earth’s atmosphere. It was not until 1690 that the correct explanation for the Zodiacal Light was given by Gian Domenico Cassini. He explained that the Zodiacal Light is the Sun’s light being reflected and scattered by dust and small particles in the inner Solar System. Periodically comets pass through this region leaving a trail of dust and small particles. Asteroids have collided leaving small particles of debris as well. When we see the Zodiacal Light, what we are seeing is way beyond Earth’s atmosphere. It is out in our Solar System. This is yet another way of seeing the Ecliptic.