A dark sky park allows us to enjoy the beauty of the night sky, such as is already the case in the Mackenzie Basin where the Aoraki Mackenzie International Dark Sky Reserve protects the region from light pollution.

Imagine being able to see the whole vault of the heavens and count shooting stars from an inner-city Dark Sky Park, says astronomy professor John Hearnshaw.

OPINION: There are now many proposals for what daytime use to make of the Christchurch red zone. They range from green parks, native reserves, walking and cycling tracks to a city farm, a rowing lake and white water rafting. But here is just one proposal for what should be done in the red zone at night, and that is to establish a city Dark Sky Park.

A Dark Sky Park is a place where all lighting is carefully controlled. The key features in a Dark Sky Park are that the lights must only shine down onto the ground, and not into the sky where the light is not needed. Secondly, the lights should be a warm yellow colour and devoid of the harmful rays of blue light. Thirdly, floodlights for sporting facilities or illuminating buildings or other features should be completely banned. Fourthly, those lights which do not need to be on all night should be on motion sensors, so they are off when no-one is nearby.

John Hearnshaw Downward-casting light bollards in Tekapo.

Why should we aspire to have a Dark Sky Park in the city? One huge benefit is to encourage nocturnal species, especially native birds and insects, back into the city. How magic would it be to hear ruru in central Christchurch? Or to experience a dawn chorus of bell-birds and tui in the early hours of day-break.

Like humans, many animals also have a circadian rhythm of alternating light and darkness, and they rely on this to lead a normal life. Artificial light at night can severely disrupt their lives, whether they are nocturnal or otherwise. Even trees and plants thrive better if the nights are dark and the days are bright and sunny.

Modern research has shown that humans are adversely affected by too much light at night, especially blue light. Many LED lights, often favoured by today's lighting engineers because of their luminous efficiency and longevity, emit large amounts of blue light.

Although our vision is relatively insensitive to the blue end of the spectrum, there are retinal ganglion cells in our eyes which are especially sensitive to the blue. The signals to the brain from these cells in turn control hormone production in a number of glands. Thus blue light at night suppresses melatonin production, which causes our day-night cycle to be disrupted. The result is stress and tiredness and difficulty in sleeping.

Worse still, the lack of melatonin can contribute to serious ailments, including cancer, diabetes, asthma, obesity and stress disorders. Although it is not suggested that anyone will be sleeping in the red zone at night, almost certainly people will be sleeping near the red zone and any light spill into houses bordering the zone could have harmful effects.

There is a growing worldwide movement to promote dark sky parks or reserves to allow people to see a dark starry sky in the absence of light pollution. New Zealand has one of the world's first International Dark Sky Reserves, that in the Mackenzie Basin and including Aoraki/Mt Cook National Park. The Aoraki Mackenzie International Dark Sky Reserve is now one of New Zealand's top tourist attractions, bringing more than 100,000 tourists annually to Tekapo, Twizel and Mt Cook explicitly to view the beauty of a pristine and dark night sky.

It is unlikely the red zone will ever be the same draw-card for astro-tourists. Yet a Dark Sky Park will still be a much better place to see the stars than anywhere else within the city boundary. How wonderful to lie down on a bank or in a forest clearing on a summer evening under a starlit sky, and see the whole vault of the heavens, the Magellanic Clouds and count shooting stars. Or in winter to admire the Milky Way and the numerous gaseous nebulae and star clusters. The universe can be very inspiring, beautiful and educational.

Finally a Dark Sky Park has cleverly designed lighting, where light is used only where needed, when needed and in the amounts needed. Good lighting saves electricity, and the savings over a year can be quite significant. Not only is the environment protected, but with a bonus of a cost saving, why would we not do it?

Establishing a Dark Sky Park in the middle of a city in most circumstances would be a very hard task to undertake. But here in Christchurch the Red Zone presents a golden opportunity that most cities never have and Christchurch should seize the opportunity and do this now while we can.

John Hearnshaw is Emeritus Professor of Astronomy at the University of Canterbury and chair of the Aoraki Mackenzie International Dark Sky Reserve Board. People who wish to join the Dark Sky Park movement or who would like more information can contact him at john.hearnshaw@canterbury.ac.nz.