February 19, 2019

RobR_Selenelion7803 (3)

Photographer: Rob Ratkowski 
Summary Authors: Rob Ratkowski; Jim Foster

Featured above is the Haleakala Observatory atop the Haleakala volcano (Maui, Hawaii) as observed at sunset during the lunar eclipse of January 20, 2019. The setting Sun is 180 degrees away from the partially eclipsed Moon, at extreme far left. In addition, the conical shadow of Haleakala can be seen just below the Moon, projected on the distant atmosphere by dust particles that scatter sunlight. 

The term used when both the setting Sun and eclipsed rising full Moon are observed at the same time is selenehelion. At first glance, this seems an impossibility since for an eclipse to occur the Sun, Earth and Moon must all line up, and if we can view both the full Moon and the Sun simultaneously, one or the other must be offset. Thanks to atmospheric refraction, however, this phenomenon can occasionally be seen because light refracting through layers of our atmosphere acts to make a celestial object appear slightly higher in the sky than is the case.