The May 25th penumbral lunar eclipse will be practically imperceptible for creatures viewing from Earth. Most folks won’t even bother to try to see this eclipse and hardcore amateurs with telescopes will probably scoff at the idea of unpacking and setting up their telescopes for this event. That still doesn’t change the fact that however slight the moon’s grazing of the Earth’s penumbral shadow, and this one is about as scant as they get, it still technically counts as an eclipse.
So why is this such a non-event? Well there are three parts that make up a bodies shadow. The penumbra, the umbra and the antumbra. The penumbra is the weak or pale part of an object’s shadow, in this case the Earth’s shadow. From within the penumbra, the Moon is only slightly cast in shadow as in the case of a partial eclipse. This contrasts with the umbra, where the Moon is completely within the Earth’s shadow which sometimes results in a total eclipse. If it’s a total solar eclipse then the Moon’s shadow is cast on the Earth. On the other hand if it’s a total lunar eclipse the Moon passes through the Earth’s shadow, and more specifically, the Earth’s umbra. You can think of the umbra as a cone getting smaller as it retreats from an object. The antumbra is that portion of the shadow past where the umbra’s cone of influence ceases. If you’ve seen an annular eclipse you’ve witnessed the antumbra first hand.
So what we have on May 25th is the Moon ever so slightly creeping into the Earth’s penumbra. The partial eclipse begins at 10:53 p.m. CDT and lasts 33 minutes and 45 seconds, the big non-event will be visible (or not) for all in North America.