BBC Knowledge Asia Edition 3

Disco planet

At night, the Earth becomes a glitterball.

Viewed from the International Space Station,

lights pulsate and flicker across the planet’s

surface. Using a long exposure, NASA

astronaut Don Pettit captured this light

show in June 2012, during a six-month stint

aboard the ISS.

The streaks of yellow are city lights, smeared

by the rotation of the Earth and the motion of

the ISS, while the blue blotches are lightning

bolts, flaring up like flashbulbs. That green

wash of colour in the atmosphere is known as

‘airglow’. It is caused by light being emitted

from oxygen atoms that have been excited by

the Sun’s ultraviolet radiation during the day.

Meanwhile, as the ISS zips around the

Earth, the night sky appears to rotate around

the space station’s axis, creating the star trails

in the top-right.

“My star trail images are made by taking

a time exposure of about 10 to 15 minutes,”

says Pettit. “With modern digital cameras,

30 seconds is about the longest exposure

possible, [so] to achieve the longer exposures I

do what many amateur astronomers do. I

take multiple 30-second exposures, then

‘stack’ them using imaging software.”

PHOTO: NASA