On Saturday night, in time for the Fourth of July, we will have the opportunity to see a penumbral lunar eclipse. This occurs when the alignment of the sun, Earth and moon causes the moon to pass through a portion of Earth’s shadow.
Of all lunar eclipses–total, partial and penumbral–that we see, this will be the most subtle, limited to a darkening of a corner of the full moon. The penumbral eclipse will be visible over most of North and South America, and parts of western Europe and Africa in places where skies are fairly clear.
What causes a penumbral eclipse
The penumbra is Earth’s outer shadow, which the moon will pass through beginning at 11:07 p.m. EDT on July 4, and exit at 1:52 a.m. on July 5. The best view will be the midway point around 12:30 a.m. The subtle shadowing of a corner of the moon’s surface will be evident without using a telescope. The full moon in July is also known as the Thunder Moon or Hay Moon.
In a total lunar eclipse, the moon passes through the deep dark inner shadow (umbra) and virtually disappears, often turning reddish or coppery (blood moon), as all direct sunlight is blocked. Diffuse sunlight that reaches the moon is heavily scattered, leaving mostly red wavelengths in the light spectrum illuminating the lunar surface. If there are more particles than usual in the air, the colors may be orange, yellow or brown.
Penumbral eclipses comprise about 35 percent of all lunar eclipses.