Current Sky: Nantucket's Dark Skies
Regina Jorgenson •
For some folks who are lucky enough to experience life on Nantucket as a resident or visitor, it can be easy to take for granted the brilliant dark skies that fill our nights. But for many, the night sky, littered with jewel-like stars, holds just as much charm as the sun-drenched beaches. Indeed, this same night sky inspired the life and work of the first American woman astronomer, Maria Mitchell, who was born here in 1818 and discovered a comet that brought her world-wide fame.
The remoteness of the island, often touted as a selling point for a vacation destination, is also a selling point for anyone who loves stargazing. At 30 miles out to sea, far enough away from major metropolitan areas, the island is relatively sheltered from the negative impacts of urban light pollution. Visitors to the island often remark that it’s here on this island that they first saw the magnificent Milky Way stretching across the night sky.
In this respect, Nantucket is a special and, alarmingly, increasingly rare place. According to the World Atlas of Artificial Night Sky Brightness published by the Light Pollution Science and Technology Institute,[1] “more than 83% of the world’s population experiences light-polluted night skies, including more than 99% of all Europeans and Americans.” And the situation is changing at an alarming rate. A study[2] of community-scientist-collected data over the past decade has indicated that in some places, the level of light pollution is increasing as quickly as 9.6% per year, which is equivalent to doubling the sky brightness every eight years!
So, what is light pollution? According to the definition given by the International Dark Skies Association (IDA), “light pollution is the inappropriate or excessive use of artificial light” and “it can have serious consequences for humans, wildlife and our climate.” Light pollution is typically categorized into four different types: glare, skyglow, light trespass, and clutter. Detailed explanations of each of these can be found on IDA’s website[3], but it’s important to note that light pollution affects every one of us, not just a small group of astronomers interested in studying the night sky (although it affects them as well!).
The effects of light pollution are far reaching and ultimately touch all aspects of life on Earth. In brief, besides disconnecting us from the universe and threatening humanity’s cultural and historical relationship with the night sky, light pollution is devastating to wildlife, adversely effects human health, can negatively affect nighttime safety, and is a massive waste of energy and money.
Yet despite all these negative impacts of light pollution, there is a significant silver lining – it is a solvable problem! And moreover, it is one that can be solved quickly, inexpensively, and relatively easily. With all the serious issues facing the world today, it’s nice to know that at least some problems, like human caused light pollution, are fixable. There’s really no downside to making sensible lighting choices.
To measure the current state of light pollution on Nantucket Island, the Maria Mitchell Association (MMA) has teamed up with the all-volunteer community advocacy group Nantucket Lights.[4] Nantucket Lights is exclusively dedicated to preserving and protecting Nantucket's nighttime environment and heritage of dark skies by raising awareness about light pollution on the island and advocating for environmentally responsible outdoor lighting. For the past year, with the help of a dedicated group of volunteers, the team has been regularly measuring the night sky brightness at eight locations across the island.
Measurements are taken with a special light meter called a Sky Quality Meter, or SQM, that measures the brightness of the night sky in magnitudes per square arcsecond. Magnitudes are an astronomical unit of brightness, that (unintuitively) are logarithmic and backwards, meaning that the larger the number, the smaller the brightness.
The group has been taking measurements every month during the week of the new moon – when the moon will not contribute any brightness to the night sky – to determine a baseline sky brightness level for assessing the current level of light pollution at each of the eight locations across the island. After nearly one year of measurements, the average value across the island is 20.63 magnitudes per square arcsecond. For context, values below approximately 20.2 are when the Milky Way becomes difficult to impossible to discern in the night sky.
Moving forward, these measurements will support the lengthy and rigorous application process for Nantucket Island to be designated an IDA Dark Skies Community. If awarded this designation, Nantucket would be the first, and only, Dark Skies Community in the state of Massachusetts, and one of only a handful of official dark sky sites in the entire Northeast.
For more information and for a host of helpful resources, including a Guide for Residences and tips on responsible outdoor lighting, visit the Nantucket Lights webpage (https://nantucketlights.org). To enjoy a guided tour of Nantucket’s dark skies and peer through the MMA’s telescopes, register to attend an Open Night at the MMA’s Loines Observatory. Register here: https://www.mariamitchell.org/open-nights
[1] https://www.darksky.org/80-of-...