Understanding Light Pollution Maps: A Beginner's Guide

Light pollution maps are a useful tool for astronomers, photographers, and stargazers to locate dark sites with clear skies unaffected by light pollution. These maps use NASA VIIRS NTL data from 2012-2023, VIIRS trends, World Atlas 2015 overlays, observatories, and SQM overlays. They allow users to find the darkest places near them for the best viewing of the night sky and can be adjusted for opacity and map type. Light pollution maps also provide additional information such as cloud cover, temperature, and the time total darkness begins and ends. The Bortle scale, with nine levels, is a common way to measure and understand the brightness of the night sky at a particular location. These maps are accessible through websites and smartphone applications, with some offering free basic features and others requiring subscriptions for additional tools.

Using the Light Pollution Map app

The Light Pollution Map app is a useful tool for locating dark sky areas where light pollution won't affect your observation of the night sky. The app displays light pollution-related content over Microsoft Bing base layers (road and hybrid Bing maps). The app is easy to use and has a good interface, with a free version that is useful and not just a ploy to get you to purchase the full version.

The map uses NASA VIIRS NTL (2012-2023), VIIRS trend, World Atlas 2015 overlays, observatories, and SQM overlay managed by users. When you click on a point on the map, a ratio is listed, which is the ratio of artificial brightness from pollution over the natural brightness of the night sky. The app also provides a darkness countdown timer, local temperature and cloud coverage indicators, and settings to customise the app to your needs.

You can enable widgets on your home screen to show aurora information, cloud cover percentage, temperature info, astronomy photo of the day, and the time total darkness begins and end. The app also has a night mode to save battery life or put out less light pollution when stargazing.

The Light Pollution Map app also provides local Aurora Borealis (Northern/Southern Lights) alerts, meteor shower alerts, supermoon alerts, and lunar eclipse alerts. You can save and load your favourite dark sky locations and navigate to them using the Apple Maps app.

Adjusting the intensity slider

For instance, moving the slider to 100% means no change, while 50% indicates a 50% reduction, and 500% means a 5-fold increase in the original value. This adjustment can help you visualise the impact of light pollution on a particular area and make informed decisions about lighting conditions.

Additionally, the "Map layers" section in the right menu offers controls to select overlays, features, and base maps. The default opacity is set at 60%, but you can use the slider to change it. The Bing Road and Hybrid base maps, provided by Microsoft, offer flexibility in visualising road details and satellite imagery.

In specific applications like Daz 3D, the "Environment Intensity" slider controls the intensity of environmental lighting in all four modes, including 'Dome Only', 'Dome and Scene', 'Sun-Sky Only', and 'Scene Only'. The "Environment Map" slider, on the other hand, controls the intensity of lighting from the map itself. These sliders can be used together to achieve the desired lighting effects.

By adjusting the intensity slider on a light pollution map, you can tailor the visualisation to your needs, making it a powerful tool for understanding and addressing light pollution.

Using the Bortle scale

The Bortle scale is a way to measure the brightness of the night sky for a particular location. It uses astronomical observations to determine the amount of light pollution in a given location. The Bortle scale was created by John E. Bortle in 2001 and published in Sky & Telescope magazine. Its goal was to help amateur astronomers understand the darkness of a particular observing site.

There are nine classes on the Bortle scale, with each class having a corresponding NELM (naked-eye limiting magnitude) and SQM value. NELM represents the faintest star(s) visible to the naked eye under the conditions being measured. For example, a Class 1 Bortle sky means that one can observe M33 (Triangulum Galaxy) without the aid of any equipment.

For some classes, there can be drastic differences from one class to the next, for example, Bortle 4 to 5, because the scale's SQM's at the lowest are usually the same as the highest for the next class. The Clear Outside smartphone app can be used to determine your current Bortle scale class.

Light pollution limits the visibility of the Milky Way to the unaided eye, the visibility of nebulae and galaxies seen in telescopes, and raises the noise on CCD astrophotographs. Only the observation of planets and double stars is unaffected. Low light pollution conditions, or dark skies, are important for a good astronomical observing site.

Locating dark sites

• Adjust the intensity slider: When using an online light pollution map, adjust the intensity slider to view both light pollution colours and road map details clearly. This will help you identify areas with varying levels of light pollution.
• Pan and zoom: Use the pan and zoom functions to navigate the map. By holding down the shift key and dragging your mouse, you can explore different regions and zoom in on specific areas of interest.
• Compare colours: Set the intensity to maximum to compare the colours on the map to the provided colour scale or legend. Each colour typically represents a different level of light pollution, ranging from dark skies to heavily light-polluted areas.
• Filter and search: Some maps allow you to filter SQM (Sky Quality Meter) points by name or date. You can also search for specific observatories and assess their light pollution levels. Utilize these features to narrow down your search for dark sites.
• Create a list of locations: Take note of the locations with low light pollution, indicating potential dark sites. Create a list of these locations on the map for quick access later. This will help you keep track of promising areas for astronomical observations.
• Explore additional features: Depending on the map, there may be additional features to aid your search. For example, you can find the closest dark site to your current location or add map markers to pinpoint specific areas of interest.

By following these steps and utilizing interactive light pollution maps, you can effectively locate dark sites that offer optimal conditions for astronomical observations and stargazing experiences. Remember to cross-reference the map data with other sources, such as local astronomy club recommendations and weather conditions, to make the most informed decision possible.

Using the interactive world light pollution map

The interactive world light pollution map is a useful tool for understanding light pollution levels in different areas. The map uses NASA VIIRS NTL (2012-2023) data, VIIRS trend data, World Atlas 2015 overlays, observatory information, and SQM overlay data managed by users to provide a comprehensive view of light pollution.

To use the map effectively, you can start by adjusting the intensity slider to view both the light pollution colours and road map details. Clicking on the top right corner will allow you to view the map in full screen. You can pan and zoom using the shift key and mouse drag to navigate to specific locations.

The map also offers additional features to customise your view. By clicking on the "cog wheel" icon, you can access various options. For example, you can filter SQM points by name or date, and use the observatories option to find observatories and assess their light pollution levels. The map also provides a legend to help you understand the currently visible overlay and feature information.

Another feature is the ability to create a list of locations that you can quickly access later. By clicking on the "New" button (green round + button), you can create a new entry, which captures your current map position. You can then name it, reposition it by clicking and dragging, or delete it if needed. You also have the option to add a pushpin to desired coordinates by clicking the pushpin icon. Remember to click "Edit" again to exit the edit mode when you're done. Your list of locations will be saved on your device.

Additionally, the map allows you to work with different data projections and formats. For example, to display the data as overlays on top of Bing maps, the data needs to be projected to a Spherical Mercator projection. The original VIIRS data has a higher resolution of 15 arc seconds per pixel compared to the World Atlas at 30 arc seconds per pixel. When using the measurement tools and zooming in closely, you will notice a faint red grid representing individual data pixels.

Frequently asked questions