Melanie Windridge
Plasma scientist
Melanie explains how Earth and Sun interact with one another to create alluring auroral displays.
The status of International Dark Sky
Park is awarded to areas where the night-sky brightness is regularly equal to or darker than 20 magnitudes per square arcsecond. The land may be public or private, but there must be public access to the relevant areas. There is, however, no required minimum land area.
An International Dark Sky Reserve must experience brightness of no more than 20 magnitudes per square arcsecond and must be public or private land of at least 700km2. It must contain a “core area” that meets the minimum criteria, as well as a “peripheral area” that supports the preservation of the core’s darkness.
Parks and Reserves are further divided into gold, silver and bronze tiers. Gold means a visual limiting magnitude – in other words, the faintest stars you can see – equal to or greater than +6.8 under clear skies, silver means +6.0 to +6.7 and bronze means +5.0 to +5.9.
Gold calls for a complete lack of lighting on towers or buildings within the area boundary. An array of phenomena should be visible like aurora, airglow, the Milky Way, zodiacal light and faint meteors. Silver means the Milky Way must be visible in summer and winter, while “minor to moderate” illumination from artificial skyglow is allowed. Bronze is awarded to areas that don’t meet the silver standard, but which still maintain visible natural sky.
Filling the void
A Dark Sky Sanctuary is typically an isolated location with few or no threats to the quality of darkness. The designation was introduced in 2015 to bridge the gap between the Park and Reserve definitions. Such areas are sometimes inaccessible to the general public due to the fragility of their natural darkness and so cannot be considered a Park, but also lack the core/ peripheral areas of Reserves. The Sanctuary designation is intended to ensure that there is awareness of their delicate nature, but also that the potential damage caused to the area through public awareness is minimised. To achieve any of these four designations, a site must apply to the IDA, which gives advice on any changes needed and then provides support to make them happen. There’s also a separate Dark Sky Friendly Development of Distinction award, which recognises small communities and developments that promote sensitive outdoor lighting. Areas must be nominated by an IDA member and receive support from a community leader and residents.
IDA guidelines say that designations “almost always begin with a small group of individuals who organise to seek formal protection of their nightscape”. So if you and your fellow amateur astronomers want to protect darkness in your local area, visit the IDA website to find out more.