Why are penumbral lunar eclipses difficult to notice?

A penumbral lunar eclipse occurs when the Moon passes through the penumbra—the Earth's partial shadow—rather than the umbra, which is the darkest, central part of the shadow where light is totally excluded.123

These eclipses are difficult to notice because the dimming of the Moon’s illumination caused by the penumbra is so slight that it is scarcely perceptible to the human eye. Because the light from the Sun is only partially blocked in this region, the reduction in brightness is not dramatic enough to be easily distinguished from the standard appearance of a full moon. Consequently, penumbral eclipses are rarely watched compared to partial or total lunar eclipses.42