"It should be dark at night"
Earth at Night
NASA has published this famous picture of what the dark side of the Earth looks like from space. This picture shows hot bright the cities of the world are at night, and how much of the developed world is lit up 24/7. (A really large version of this map, 10MB, is here. [biggest, 40MB, here.]
How many features can you see of the Earth without sun light? Find the Nile River, the Hawaiian Islands, the border between N and S Korea.
A lot of places 'left the lights on'.
Light PollutionAll of these lights cause light pollution, light that obscures the night sky.
Here is an interactive map of world light pollution. Light pollution has been put on a color scale. Red is like Times Square, NYC. Black is Point Nemo. Where do you live? Orange, Yellow, maybe Green. [Map from Falchi et. al.]
[Good background, nice tweet, good references for mapping light pollution on the ground.]
Years ago a scale was developed for the apparent magnitude of stars in the night sky. The scale is logarithmic (read the link for all the maths :) ). The edge of human seeing is 6.5 on this scale. Just over 9000 stars can be seen by the human eye if the sky is dark enough (article has a lot more detail on the exact count and where this count is valid).
Add light pollution to obscure the dim stars, and the number of stars you could see at 4th magnitude drops to only HUNDREDs.
Here's a great set of charts showing how the constellation Cynus the Swan changes from 0 magnitude to 6 magnitude viewing.
Excellent demonstration of the difference of two areas with different night pollution.
Milky WayHave you ever seen the Milky Way, horizon to horizon? Seeing it makes you understand why the Ancient Greeks called it gala, 'milk'.
All ancient civilizations have stories on how the Milky Way was created or what it represents. In the modern world, with our night skies, that connection is lost.
Citizen Science, Apps, No Telescope NeededThere is a great mobile phone app "Loss of the Night" (iTunes here). You become a citizen scientist. The program guides you through locating dimmer and dimmer stars until it determines how dark is your sky. It takes 15 or so 'sightings' to get enough data for a reasonably accurate measurement. The authors have a blog, shows the results, good stuff. They are developing a map of how the night sky is changing, literally from the ground up. See this for more details.
Another, more manual, citizen science project that has been running longer than 'Loss of the Night' is Globe at Night. Simple 5 step directions here.
Dark Sky Reserves and ParksThe International Dark Sky Association has started declaring areas a Dark Sky Reserve. It is a rigorous process to get an area designated such a site. Only 11 such sites are listed.
A lower designation is a Dark Sky Park. The US has many of these, associated with its National Park system.
Finally, a Dark Sky Sanctuary is like a Dark Sky Reserve, but remote, with limited access.
It is a fact of modern times, one must create a park to see the night sky as it was ONLY 100 or so years ago, just outside living memory.
Man Made Lights and Lighting ScienceThe culprit in the loss of the night sky is the electric light. IDSA does have an excellent page on outdoor lighting and what communities can do to "bring back the night".
One result of light pollution is the brightening of the night sky, Sky Glow.
Another excellent website on lighting is The Lighting Institute at Rensselaer Polytechnic Institute The Lighting Institute conducts a two day course on outdoor lighting, and much more. A great resource for municipal public works departments, planning and zoning committees, as well as departments of transportation at the state level (Yes, USA centric terminology).
Look Up, Measure Your Night Sky, Report It, Get Involved
Lots of links in this article. Lots of people to talk to.
Learn about outdoor lighting.
Learn where you can educate local government officials on what is a loss of a NATURAL RESOURCE, the night sky. They can control lighting issues with local zoning.
Many US states are passing laws regarding the lighting of highways and roads, state buildings, etc.
Bring the Milky Way back, for good!