The vernal (spring) equinox on 20 March heralds the arrival of spring and it is a great time of the year for observing one of the night sky’s most elusive phenomena: zodiacal light. This light will appear as a false dusk that is created by a triangular beam of light from the Sun being reflected off a fog of tiny interplanetary dust particles when it is beneath the horizon. It’s so difficult to see and many astronomers have never witnessed it, however, I was lucky enough to spot and photograph it at Castle Tioram on a dark moonless night on the first day of March last year. It should be visible on dark moonless nights over the next few weeks, so do keep an eye out for it.
Have you ever been outside at this time of the year, looking west and noticed what you think is lingering evening twilight, or the light of a nearby town shining up from the horizon? If so, you may well have seen zodiacal light in the sky and not realised.
It is a triangular beam of light that shines along the line of the Zodiac, an 8° wide band that straddles the ecliptic, the invisible path that the Sun traces as it moves around the sky. It is the region of the sky where we can find the Sun, Moon and planets and is only 8° wide because most of the planets have orbits that are only slightly inclined to that of the Earth. The exception is Pluto, whose inclination of 17° takes it out of the Zodiac during part of its orbit.
At this time of year, the Zodiac rises steeply from the horizon at dusk meaning zodiacal light does the same. People used to think that it originated somehow from phenomena in the Earth’s upper atmosphere, but today we understand it as sunlight reflecting off dust grains that circle the sun in the inner solar system, along the line of the Zodiac.
The grains of dust were once thought to be left over from the process that created our Earth and the other planets of our solar system around 4.5 billion years ago, but in recent years, there’s been discussion about them originating from dust storms on the planet Mars. They are thought to be about a millimetre and less in size, densest around the immediate vicinity of the Sun and extending outward beyond the orbit of Mars and, when sunlight shines on these dust grains, it creates the light we see.
The light is fainter than the Milky Way, so the darker the night sky, the better chances of seeing it. It is best to go to a location with little or no night pollution, on a night when the moon is out of the sky and look for it in the west in the hour or two after sunset. If you are lucky, you may spot it as a ghostly pyramid of light rising steeply from the horizon.
The best time to look for it in the coming month will be on the days either side of the equinox (20 March) as this coincides with a New Moon, meaning that there will be no moonlight to drown it out. With Venus and Jupiter, our night sky’s two brightest planets, being low down in the west after sunset, they’ll be visible close to or in the zodiacal light. Shortly after, on 23 and 24 March there will also be a thin crescent Moon sitting in the midst of the triangular beam of light as it shines upwards from beneath the western horizon.
If you don’t see it in the next few weeks, you can try again in the autumn, but instead of looking for it in the west at dusk, you need to look for it in the east at dawn. This is because the Zodiac elliptic rises steeply from the eastern horizon at dawn in the autumn instead of rising steeply from the western horizon after dusk in the spring.