Archive - Airglow, Andromeda Galaxy and Cassiopeia

June 19, 2021

6a0105371bb32c970b0192abf697a7970dEvery weekend we present a notable item from our archives. This EPOD was originally published August 5, 2013.

Photographer: Miguel Claro
Summary Author: Miguel Claro

August 2013 Viewer's Choice This serene view showing the night sky near Lake Alqueva, Portugal, in the Alqueva Dark Sky Reserve, was captured on June 15, 2013. Cassiopeia is at the left top corner of the image directly above the double cluster NGC 884 and NGC 869. The Great Andromeda Galaxy (M31) is visible at top center and right is the constellation of Pegasus with its familiar square asterism. Click here to see an annotated version.

The strong chartreuse glow above the lake is airglow. This light results from atmospheric chemiluminescence -- emission from sodium atoms in a layer approximately 57 mi (92 km) above the Earth's surface. Just above this yellowish light, greenish light from oxygen atoms can be detected in a layer about 55-62 mi (90-100 km) above the surface. This emission layer is visible from Earth orbit on nighttime visible satellite imagery. However, neither the green nor yellow emissions can be seen with the naked eye.

Photo Details: Canon 60Da camera; ISO 1600; 35mm lens at f/2; 15 sec. exposure; a mosaic of 7 images; taken at 03:45 a.m.