Zodiacal Light

Spring in the northern hemisphere is the best time to view the elusive, faint astronomical phenomenon known as Zodiacal Light.

Daniel López, IAC Zodiacal Light on the left and (false colour) Milky Way on the right

This light – literally “light from the zodiac” – appears only just at the end of evening twilight or just before morning twilight, and is seen as a cone of very faint light stretching up from the horizon, narrowing as it does so, following an imaginary line in the sky known as the zodiac, or to give it its more astronomically correct name, the ecliptic.

The angle which this line makes with the horizon varies throughout the year, and the steeper the angle the more evident the zodiacal light will be. The steepest angle for observers in the northern hemisphere occurs in the evening sky in March and April, or the morning sky in October and November.

How best to see Zodiacal Light

You will need to be as far as possible from any sources of light pollution. In fact the Zodiacal Light is one of the benchmarks of the Bortle Dark Sky Scale, which says that it is only visible in skies with Bortle Class 5 or better, and even in suburban/rural transition sites it is not striking. Under rural skies it is “striking”, while in a truly dark sky site it is bright enough to cast a shadow. Under exceptionally dark skies it might appear as a band stretching from horizon to horizon.

Once you’ve found your dark sky site, you need to find somewhere with as clear a western horizon as possible, and wait until the end of evening astronomical twilight (assuming you’re viewing it in the spring – if it’s the autumn then you need to start observing before the start of morning astronomical twilight). You can find your twilight times using timeanddate.com or the excellent Velaclock app for smartphones. As a general rule, for observers in the UK, you need to wait until two hours after sunset before you skies get dark enough to see this elusive light. But wait too long and the bulk of the cone of light may have set, so sunset+2hours is really the perfect time.

What Zodiacal Light looks like

As mentioned above, Zodiacal Light is a faint grey cone of light stretching up from the horizon. The darker your observing site the larger this cone will appear, and the higher into the sky it will stretch. From the very darkest sites on Earth it can stretch overhead and down to the far horizon.

What’s Zodiacal Light made of?

Zodiacal Light is sunlight reflecting off particles of dust and rock orbiting the Sun. This dust is in a lens-shaped cloud with the Sun at the centre, and the cloud lies in the same plane as the planets in the solar system (which is why it’s visible along the ecliptic). The particles in the Zodiacal Light are around 0.15mm in diameter (some smaller, some a little bigger) and probably come from shattered comets and asteroids.

Photographing the Zodiacal Light

As tricky as it is to see with your naked eyes, it’s even harder to catch on camera. Harald Edens has a great page about how best to photograph it.

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The Bortle Scale: A Flow Chart

The Bortle Scale is a useful way of estimating your sky brightness, i.e. to what extent light pollution affects your view of the night sky. By going outside on a clear moonless night and recording what astronomical objects you can see you can assign a Bortle Class rating to your observing site.

I have used the Bortle Scale to assess night sky quality many times, and always felt the lack of a handy flow chart to lead me through it. So I made one. Enjoy. (You can also download the pdf version.)

PS The content of this chart assumes some prior knowledge of astronomy, but any of the terms used are easily google-able.