Bortle is a scale used to measure the darkness of the night sky at a particular location. It was created by John E. Bortle in 2001 and has become a widely used reference in astronomy and astrophotography. The Bortle scale ranges from 1 (the darkest sky) to 9 (the most light-polluted sky).

The Bortle scale takes into account factors such as the visibility of stars, the Milky Way, and other celestial objects, as well as the amount of light pollution, skyglow, and artificial light sources present in the area.

The Bortle scale is useful for astronomers and astrophotographers in choosing observing or imaging locations that offer optimal conditions for their purposes. For example, someone who wants to observe or photograph faint deep sky objects will likely want to choose a location with a low Bortle scale rating, while someone who is observing or photographing bright planets or the Moon may not need to worry as much about light pollution.

Here’s a breakdown of the Bortle scale ratings and what they represent:

1. Excellent dark sky: A sky that is completely free from light pollution, with the Milky Way visible from horizon to horizon and a wealth of deep sky objects visible to the naked eye.

2. Very good dark sky: A sky that is almost completely free from light pollution, with the Milky Way still visible from horizon to horizon and many deep sky objects visible to the naked eye.

3. Rural sky: A sky in a rural area with minimal light pollution, with the Milky Way still visible but not as bright or expansive as in a darker sky. Many deep sky objects are still visible to the naked eye.

4. Suburban sky: A sky in a suburban area with moderate light pollution, where the Milky Way is no longer visible to the naked eye and only the brightest deep sky objects are visible.

5. Bright suburban sky: A sky in a suburban area with significant light pollution, where only the brightest stars and deep sky objects are visible.

6. Suburban/urban transition: A sky in an area with a mixture of suburban and urban lighting, where the sky is noticeably bright and only the brightest stars and objects are visible.

7. Urban sky: A sky in a heavily light-polluted urban area, where only a handful of stars are visible and no deep sky objects are visible to the naked eye.

8. Inner-city sky: A sky in the heart of a major city with extremely high levels of light pollution, where no stars or deep sky objects are visible.

9. Bright inner-city sky: The brightest level of light pollution, found in areas with bright streetlights and other sources of artificial light, where the sky is indistinguishable from the city itself.

It’s worth noting that the Bortle scale is not a perfect measure of observing conditions, and individual locations may vary in their sky quality even within a given Bortle rating. However, the scale does provide a useful framework for understanding and comparing different observing locations.

A dark site is a location that has very low levels of light pollution, making it an ideal spot for stargazing and astrophotography. In the UK, dark sites are typically located far away from major cities and other sources of light pollution, in rural areas with few artificial lights.

Some popular dark sites for stargazing and astrophotography in the UK include:

1. Galloway Forest Park in Scotland: This park has been designated as a Dark Sky Park by the International Dark-Sky Association, and offers some of the darkest skies in the UK.

2. Northumberland International Dark Sky Park: Another Dark Sky Park, this park in northern England offers stunning views of the Milky Way and other deep sky objects.

3. Exmoor National Park: This park in southwestern England has been designated as an International Dark Sky Reserve, and is home to some of the darkest skies in the country.

4. Snowdonia National Park: Located in Wales, this park offers some of the best dark skies in the UK, with excellent views of the Milky Way and other celestial objects.

5. Brecon Beacons National Park: Another Dark Sky Reserve, this park in Wales is home to some of the darkest skies in southern Britain.

These are just a few examples of the many dark sites available for stargazing and astrophotography in the UK. It’s always a good idea to check local weather and sky conditions before heading out, and to be aware of any special regulations or restrictions in place at the location you plan to visit.