The sky at night looks different depending on where you are, how old you are and what you can see. It can appear washed out in cities or become a breathtaking canopy of stars under dark rural skies. The same constellations that are barely visible from a brightly lit suburb can look completely different beneath a truly dark sky.
People also experience the night sky differently. Some can see fainter stars than others, and as we age our pupils become smaller, reducing the amount of light that reaches our eyes. There is a surprising amount of variation in what different observers can see.
There are three major ways currently to measure the darkness of the sky – taking photos from satellites, measuring from the ground up (SQM) and using your own eyes.
Satellite maps for “dark skies”, also known as light pollution maps, use orbital sensor data to measure how much artificial light radiates from the ground into the night sky.
Most light-pollution maps are not actually measuring what an observer sees at a specific location. They’re modelling sky brightness from satellite data, often at half-km (500 m–1 km) resolution, using assumptions about atmospheric scattering, terrain, and lighting types. In a place like the Wairarapa, local topography can make a huge difference that global models simply don’t capture.
Eastern Wairarapa benefits from something that most global maps handle poorly: terrain shielding. Light from Wellington, Lower Hutt, Porirua and Palmerston North is substantially blocked by mountain ranges. A satellite looking downward sees light emissions, but an observer looking upward may experience significantly darker skies because those light sources are hidden below the horizon. Modern research has shown that incorporating topography can materially alter predicted sky brightness relative to global atlas models (Linares et al. 2024).
Globe at Night research has shown that changes in sky brightness experienced by observers on the ground can differ significantly from trends inferred from satellite measurements. Satellite data and dark-sky maps remain valuable planning tools, but they do not always capture the full observing experience. (Kyba et al. 2023)
Sky Quality Meters measure the brightness of the night sky, not its darkness. They measure all light reaching the sensor, including natural sources such as the Milky Way. At certain times of the year, the bright core of the Milky Way can affect readings even when artificial light pollution is extremely low.
Sky Quality Meters do not measure how many stars you can actually see either. Furthermore, they are largely colour-blind, so a sky that appears brighter or dimmer to people because of changes in blue-rich lighting may only show a small difference on an SQM. Cloud can also fool the instrument by reflecting artificial light back towards the ground, making the sky appear brighter than it would on a clear night. And, like any measuring device left outdoors, SQMs age over time and occasionally need calibration checks to ensure their readings remain accurate.
While they are among the most widely used tools for measuring night sky brightness and provide an objective reading of the sky background in magnitudes per square arcsecond, they do not tell the whole story. Other factors, like atmospheric transparency, humidity, cloud cover and the colour of artificial lighting can all influence what we actually see overhead. For this reason, professional dark-sky assessments combine SQM measurements with visual observations, all-sky imaging and long-term monitoring.
Just as almost two thousand years ago, Greek astronomer Hipparchus came up with a scale of star brightness, American amateur astronomer John E. Bortle developed a way for observers to assess the darkness of the night sky. Rather than relying on instruments, his scale uses visible features of the sky itself. If you can see certain stars, galaxies or natural sky phenomena, you can estimate how dark your observing location is.
No dark sky meters required, just your eyes – that’s the Bortle Scale!
The scale was published in Sky & Telescope magazine in 2001 to help amateur astronomers compare observing conditions. Based on his nearly 50 years of observation, John Bortle created this scale to help other amateur astronomers appreciate how dark the sky was at their location. Read his original article here.
There is one important catch. The Bortle Scale was developed in the Northern Hemisphere, and some of its best-known indicators, such as the visibility of the galaxy M33, are less useful from New Zealand where the object remains relatively low above the horizon. Southern Hemisphere observers often rely on other indicators, including the visibility of the Magellanic Clouds, Omega Centauri, zodiacal light and airglow.
The Bortle Scale is about observability. The darker the sky, the more detail becomes visible. More stars appear, the Milky Way reveals its dust lanes and structure, and faint celestial objects that are invisible from cities begin to emerge.
But it is ultimately about giving observers a common understanding of how dark the sky is, no matter where they are in the world. Here is a summary of each level:
• The Milky Way casts visible shadows. Zodiacal light, gegenschein, and airglow are prominent. Constellations and deep-sky objects are easily visible to the naked eye.
• Sky Quality: Truly pristine and dark, with no light pollution.
• The Milky Way is highly detailed. Zodiacal light and airglow are visible. Many deep-sky objects are observable without optical aid.
• Sky Quality: Minimal light pollution, excellent for astronomy.
• The Milky Way is still prominent but not as detailed. Light pollution starts to become noticeable on the horizon.
• Sky Quality: Mild light pollution, good for astronomy.
• Description: The Milky Way is visible but lacks detail. Light pollution is more noticeable and begins to affect the visibility of fainter stars and celestial objects.
• Sky Quality: Moderate light pollution, with some impact on stargazing.
• Description: The Milky Way is faint and appears washed out. Light pollution is evident, limiting the visibility of many stars and deep-sky objects.
• Sky Quality: Significant light pollution, noticeable effect on stargazing.
• Description: The Milky Way is difficult to see and may not be visible at all. The sky has a noticeable glow from light pollution.
• Sky Quality: Strong light pollution which has a considerable impact on visibility.
• Description: The sky is bright with light pollution, making it hard to see most celestial objects without optical aid. Only the brightest stars are visible.
• Sky Quality: Heavy light pollution, poor conditions for stargazing.
• Description: The sky is very bright due to urban light pollution. Only a few of the brightest stars and planets are visible, and the Milky Way is invisible.
• Sky Quality: Severe light pollution and very poor conditions for stargazing.
• Description: The sky is extremely bright, often appearing grey or orange from streetlights. Only the Moon, planets, and a few of the brightest stars are visible.
• Sky Quality: Extreme light pollution, nearly impossible to stargaze.
As the sky becomes darker, increasingly faint celestial objects become visible. Some are distant galaxies, others are natural atmospheric phenomena, while some are structures within our own Milky Way. The following examples illustrate what experienced observers may see under genuinely dark skies.
Visible to the naked eye from dark locations, these neighbouring dwarf galaxies appear as detached clouds of light in the southern sky. Definitely easy to see in the Wairarapa Dark Sky Reserve.
This quasar, 2,4 billion light-years away, is visible from the Wairarapa Dark Sky Reserve through a 16″ telescope. How to find 3C273 →
A faint natural glow produced high in Earth’s atmosphere, often mistaken for distant cloud.
Sunlight reflected off dust particles within the Solar System appears as a faint cone of light before dawn or after dusk – in the Wairarapa Dark Sky Reserve, it is visible when the ecliptic does not intersect the Milky Way.
Under truly dark skies, even the dark dust lanes inside the Milky Way Kiwi are visible.
The Bortle Scale is not really about numbers but about what becomes visible as the sky grows darker. As artificial light fades, increasingly faint celestial objects emerge, revealing details completely hidden from most urban observers.
The most accurate understanding of a location comes from combining all three approaches: dark-sky maps, instrumental measurements and direct observation of the night sky.
Discover the story, landscapes and communities that make up one of only two International Dark Sky Reserves in New Zealand.
New to stargazing? Learn how to prepare for a night with the stars, what to bring and what you can expect to see.
