Sky and Telescope - May 2015 - 36

Measuring Light Pollution

A CITY'S PALL Chicago ranks among the most
overlit cities in the world. Here its streets and
buildings burn brightly under blankets of clouds.

© JIM RICHARDSON

CHRISTOPHER KYBA

curs with Bennie and Elvidge that some of the decreases
measured by satellites result from better-shielded lamps
that direct their light onto the ground and not into the
sky. Less light is getting beamed into the sky.
However, looking down from space alone doesn't tell
us the whole story, Kyba says. One limitation of satellites
is that they measure light streaming directly upward,
whether beamed in that direction or reflected off the
ground. But this light is not what contributes most to

BAD TO WORSE The spotlights illuminating Berlin's main
television tower (left in photo) are far less obvious from the air
than those lighting up the hotel at right. Upward-pointing lights
are the type most easily detected from orbit.

36

May 2015 sky & telescope

skyglow. Rather, as shown in the simulation on page 38,
worst is the light emitted just above horizontal, which
can be scattered by air molecules, airborne dust, and
water vapor for hundreds of kilometers in all directions.
For years researchers have struggled to calculate
skyglow indirectly using the upward-directed lighting
levels measured by relatively crude images from Defense
Meteorological Satellite Program (DMSP) satellites.
In 2001, Italian astronomers Pierantonio Cinzano and
Fabio Falchi published a highly useful global atlas of
artificial night-sky brightness (is.gd/DMSP_atlas) - but
those maps plot the satellite-derived (not directly measured) skyglow intensities seen at ground level.
Newer spacecraft like the Suomi NPP and its visibleinfrared imager (VIIRS) provide improved sensitivity
and resolution, but there's a catch. Hundreds of millions
of streetlights and security lights worldwide utilize highpressure-sodium (HPS) bulbs, which emit a peachcolored light that's skewed toward red wavelengths.
Future installations, both new and as HPS replacements,
will be dominated by smaller, cheaper, and more energyefficient light-emitting diodes (LEDs).
For astronomers, this could become a worrisome development. Most LEDs emit a large amount of blue light,
which scatters much more readily than red light does and
thus causes more skyglow. "To make things worse," Kyba
explains, "both DMSP and VIIRS are not sensitive to blue



http://www.is.gd/DMSP_atlas

Sky and Telescope - May 2015

Table of Contents for the Digital Edition of Sky and Telescope - May 2015

Contents
Sky and Telescope - May 2015 - Cover1
Sky and Telescope - May 2015 - Cover2
Sky and Telescope - May 2015 - 3
Sky and Telescope - May 2015 - Contents
Sky and Telescope - May 2015 - 5
Sky and Telescope - May 2015 - A
Sky and Telescope - May 2015 - B
Sky and Telescope - May 2015 - 6
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