earthterminator_iss002_920The terminator, that is, the line between day and night, on the airless moons pictured here from time to time, is a firm line. No such sharp boundary marks the boundary between day and night in this picture of ocean and clouds on Earth. Instead, the shadow line is diffuse and shows the gradual transition to darkness as twilight falls. The Sun illuminates the scene from the right, and the cloud tops reflect gently reddened sunlight filtered through the troposphere, the lowest layer of the atmosphere. The upper atmosphere scatters blue sunlight and fades into the blackness of space.

This picture actually was taken in June, 2001, from the International Space Station orbiting at an altitude of 390 km.

Image Credit: NASA

Perseid Meteor Shower As Seen From the ISS

Folks on Earth watched last year’s Perseid meteor shower by looking up into the bright moonlit night sky. But this remarkable view was captured on 13 August 13 of last year by Ron Garan looking down on a Perseid meteor. Garan was onboard the International Space Station orbiting at an altitude of about 380 km while the Perseid meteors streaked below. The glowing dust grains left over from comet Swift-Tuttle are traveling at about 60 km/s through the atmosphere around 100 km above the surface of the Earth’s. The foreshortened meteor flash is just right of the center of the picture—below the curving limb of the Earth and a layer of greenish airglow.

This year’s Perseid shower should peak this weekend. Given a clear sky, viewing should be better than last year with less interference from a waning crescent Moon rising a few hours before the Sun.

BTW, the white speck between the green airglow and the earth’s limb near the meteor trail isn’t dust on your screen. It’s the star Arcturus.

Image Credit: NASA