Jessica Davidson, Balclutha Primary School, asks :-
Why does the Moon look red sometimes? What does it mean?
Daniel Schumayer, a physicist at the University of Otago, responded.
The ever-changing colours of the sky have always fascinated and inspired humans. However, before we discuss the apparent colour of the Moon, we have to understand what happens to a light ray when it enters the Earth's atmosphere.
Light is a wave, similar –in some sense– to a water wave. As a wave travels and hits objects, its course is disturbed. Imagine raindrops falling into a pond. You see circular ripples forming where a droplet hits the pond. Throwing a pebble into the pond changes the ripples profoundly, while sprinkling only a few grains of sand into the pond has practically no effect on the ripples. This is because the size of a pebble is approximately the same as the distance between two consecutive crests of a ripple, called the wavelength, while the size of a grain of sand is much smaller than that. Since light is also a wave, it is subject to similar disturbances by the particles hovering in the atmosphere, such as dust, volcanic ash, smoke from large Australian bush-fires, water vapour, etc.
Moreover, the light entering the atmosphere contains all kinds of waves, e.g. violet and blue with short wavelengths, and orange and red with longer wavelengths. Coincidentally, most of the airborne particles are of a size capable of disturbing, or scattering, the blue waves more effectively than the red ones.
When the Moon is close to the horizon, its light has to travel a lot through the atmosphere until it reaches our eyes compared to when it is right above us. The greater the distance the light has to travel, the more blue waves will be scattered out of it by the particles. Thus most of the blue waves do not reach our eyes, leaving the red colour the dominant colour and making the Moon appear reddish to us. However, the phenomenon of reddish Moon is less obvious if the air does not contain enough airborne particles which could scatter the blue waves. As the air in New Zealandis typically very clean, with few particles, we usually do not see the Moon in a reddish colour.
But keep watching the sky, because it has many other wonders to explore.