Answer: it depends. I had the great fortune to encounter two different, but spectacular, green rims in the last week.
If you are an astronomer, you might look for the green rim on the trailing (upper) side of the sun as it sets. It helps to be at high altitude on a clear day with a good set of binoculars. This green rim, which can be seen as a green flash reflected across the ocean as the sun sets over water, is a result of the way our atmosphere scatters light of different wavelengths. Scattering of light is also why the sky appears blue on earth. Shadows on Mars are red because their atmosphere scatters light differently.
After at least three tries at observing sunset with binoculars from the top of Mount Lemmon, last night I finally saw the green crescent that lasted for the final seconds as the sun sank below the horizon! I was at the Mount Lemmon Sky Center with a number of Tucson area science teachers and MLSC Director Alan Strauss. It was a fantastic night to observe the sky, animals’ tracks in the snow, and to explore future collaborations to bring more K-12 students up the mountain to observe these same things.
Speaking of snow, I believe that was the attraction that packed the highway to the top of Mount Lemmon yesterday. Mount Lemmon, one of an archipelago of Sky Islands, provides a taste of weather rarely experienced in the Sonoran Desert. Sky Islands are mountain peaks sporting environments, plants, and animals more expected in the Rocky Mountains or Canada than Mexico. The Madrean Archipelago of Sky Islands, a collection of mountain ranges whose diverse landscape provide a network of interconnected habitats for montane species, stretches from the Sierra Madres in the south to the Mogollon Rim in the north.
The Mogollon Rim creates a very different kind of Green Rim. This line across eastern central Arizona is the edge where the Colorado Plateau falls off into the Sonoran Desert. One week ago, I drove east and north on Highway 77 through the towns Show Low and Pinetop to ski in a foot of new powder at Sunrise Ski Resort. As we climbed in elevation, bare ground shrank as shrubs and grass cover increased. Saguaro cactus and agave were replaced with pinion pine and juniper trees, and, ascending into the mountains, taller, more majestic conifer forests.
The variable environments provided by the Mogollon Rim, the Colorado Plateau and Sonoran Desert, and the archipelago of Sky Islands, increases the biodiversity of this spectacular region.