Light pollution is robbing us of the night sky
From: Association of Universities for Research in
Astronomy (AURA)
January 19, 2023 -- A
startling analysis from Globe at Night -- a citizen science program run by
NSF's NOIRLab -- concludes that stars are disappearing from human sight at an
astonishing rate. The study finds that, to human eyes, artificial lighting has
dulled the night sky more rapidly than indicated by satellite measurements. The
study published in the journal Science showcases the unique contributions that
citizen scientists can make in essential fields of research.
From the glowing arc of
the Milky Way to dozens of intricate constellations, the unaided human eye
should be able to perceive several thousand stars on a clear, dark night.
Unfortunately, growing light pollution has robbed about 30% of people around
the globe and approximately 80% of people in the United States of the nightly
view of their home galaxy. A new paper published in the journal Science
concludes that the problem is getting rapidly worse.
New
citizen-science-based research sheds alarming light on the problem of 'skyglow'
-- the diffuse illumination of the night sky that is a form of light pollution.
The data for this study came from crowd-sourced observations collected from
around the world as part of Globe at Night, a program run by NSF's NOIRLab and
developed by NRAO astronomer Connie Walker. The research reveals that skyglow
is increasing more rapidly than shown in satellite measurements of Earth's
surface brightness at night.
"At this rate of
change, a child born in a location where 250 stars were visible would be able to
see only abound100 by the time they turned 18," said Christopher Kyba, a
researcher at the German Research Centre for Geosciences and lead author of the
paper detailing these results.
Light pollution is a
familiar problem that has many detrimental effects, not only on the practice of
astronomy. It also has an impact on human health and wildlife, since it
disrupts the cyclical transition from sunlight to starlight that biological
systems have evolved alongside. Furthermore, the loss of visible stars is a poignant
loss of human cultural heritage. Until relatively recently, humans throughout
history had an impressive view of the starry night sky, and the effect of this
nightly spectacle is evident in ancient cultures, from the myths it inspired to
the structures that were built in alignment with celestial bodies.
Despite being a
well-recognized issue, however, the changes in sky brightness over time are not
well documented, particularly on a global scale.
Globe at Night has been
gathering data on stellar visibility every year since 2006.* Anyone can submit
observations through the Globe at Night web application on a desktop or
smartphone. After entering the relevant date, time and location, participants
are shown a number of star maps. They then record which one best matches what
they can see in the sky without any telescopes or other instruments.
This gives an estimate
of what is called the naked eye limiting magnitude, which is a measure of how
bright an object must be in order to be seen. This can be used to estimate the
brightness of skyglow, because as the sky brightens, the fainter objects
disappear from sight.
The authors of the
paper analyzed more than 50,000 observations submitted to Globe at Night
between 2011 and 2022, ensuring consistency by omitting entries that were
affected by factors including cloud cover and moonlight. They focused on data
from Europe and North America, since these regions had a sufficient
distribution of observations across the land area as well as throughout the
decade studied. The paper notes that the sky is likely brightening more quickly
in developing countries, where satellite observations indicate the prevalence
of artificial lighting is growing at a higher rate.
After devising a new
method to convert these observations into estimates of the change in skyglow,
the authors found that the loss of visible stars reported by Globe at Night
indicates an increase in sky brightness of 9.6% per year over the past decade.
This is much greater than the roughly 2% per year global increase in surface
brightness measured by satellites.
"This shows that
existing satellites aren't sufficient to study how Earth's night is
changing," said Kyba. "We've developed a way to 'translate' Globe at
Night observations of star visibility made at different locations from year to
year into continent-wide trends of sky brightness change. That shows that Globe
at Night isn't just an interesting outreach activity, it's an essential
measurement of one of Earth's environmental variables."
Existing satellites are
not well suited to measuring skyglow as it appears to humans, because there are
no current instruments monitoring the whole Earth that can detect wavelengths
shorter than 500 nanometers, which corresponds to the color cyan, or greenish
blue. Shorter wavelengths, however, contribute disproportionately to skyglow,
because they scatter more effectively in the atmosphere. White LEDs, now
increasingly commonly used in high-efficiency outdoor lighting, have a peak in
emission between 400 and 500 nanometers.
"Since human eyes
are more sensitive to these shorter wavelengths at nighttime, LED lights have a
strong effect on our perception of sky brightness," said Kyba. "This
could be one of the reasons behind the discrepancy between satellite
measurements and the sky conditions reported by Globe at Night
participants."
Beyond wavelength
differences, space-based instruments do not measure light emitted horizontally
very well, such as from illuminated signs or windows, but these sources are
significant contributors to skyglow as seen from the ground. Crowd-sourced
observations will therefore always be invaluable for investigating the direct
human effects of sky brightness.
"The increase in
skyglow over the past decade underscores the importance of redoubling our
efforts and developing new strategies to protect dark skies," said Walker.
"The Globe at Night dataset is indispensable in our ongoing evaluation of
changes in skyglow, and we encourage everyone who can to get involved to help
protect the starry night sky."
* From 2006 to
2010, Globe at Night data were collected based on a paper
rather than an online form, so they were incompatible and were not included in
this analysis.
https://www.sciencedaily.com/releases/2023/01/230119141539.htm
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