All About Space

What is zodiacal light?


If you go to an area with low light pollution, you may notice that the western sky in the evening appears bright even after twilight. The dim light appears to be distribute­d from the ecliptic plane and lingers until late evening. A similar phenomenon occurs in the morning towards the east. This is zodiacal light. It is a phenomenon where dust particles near Earth are illuminate­d by sunshine and the light is reflected towards ground-based observers.

Astronomer­s have debated for over a century about the origin of the dust particles. As is well known, comets – small icy bodies from the outer Solar System – eject dust and gas when they approach the Sun. Asteroids also emit dust due to collisions or rapid rotation. Some asteroids in the outer asteroid belt – between the orbits of Mars and Jupiter – may contain ice underneath and eject dust together with gas. These small bodies of the Solar System are significan­t dust sources that become observable as zodiacal light.

The contributi­on of dust particles from beyond the Solar System used to be discussed, but was conjecture­d to be ignorable. However, motivated by the recent discoverie­s of interstell­ar comets such as ‘Oumuamua, astronomer­s may consider the contributi­on of interstell­ar dust once again.

Before modernisat­ion, it should have been possible to observe zodiacal light from almost every part of the world, with records of it being visible from the centre of Tokyo, Japan, during World War II. However, due to rapid industrial growth and the increase of artificial light around us, zodiacal light has been obscured behind severe haze and light pollution. The night sky illuminate­d by artificial light is indistingu­ishable from zodiacal light and, for this reason, it is now challengin­g to observe this phenomena in most areas of the world.

Although it does give off a rare celestial glow, zodiacal light and the Milky Way are not comparable. The Milky Way glows due to the shining of millions of stars, whereas zodiacal light is reflecting the Sun’s light via debris.

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