BETHLEHEM, Pa. — Brad Klein reviews upcoming astronomical highlights with Bethlehem’s ‘Backyard Astronomy Guy,’ Marty McGuire.
This week, a look at the lunar phenomenon known as ‘earthshine.’
You’ve likely seen it when looking at the crescent moon, but may not have asked yourself, ‘Why is the dark portion of the Earth-facing side of the moon faintly illuminated?’
The only source of bright visible light in space is from stars, and the earthshine that falls on the dark side of the moon originates with our own star, the sun.
As seen from the moon’s surface, the Earth has phases, just as the moon appears to grow larger and smaller as seen from Earth.
“The surface of the moon is illuminated by a full earth, bouncing sunlight back to the moon,” according to McGuire. And the phenomenon is most visible at the times of the crescent moon, waxing or waning. That’s when the Earth is shining most brightly as seen from the lunar surface.
That light shines on the lunar surface and is reflected back to our own eyes, looking up into the night sky.