Only part of the Moon enters the umbra
Earth casts two nested shadows into space. The pale outer penumbra reduces some direct sunlight. The much darker inner umbra blocks direct sunlight almost completely. A partial lunar eclipse occurs when the alignment carries only a portion of the Moon into that central umbra.
The result looks like a dark, curved bite moving across the lunar disk. That curved boundary is Earth’s shadow. Unlike the ordinary phase terminator, which divides lunar day from night, the eclipse shadow can appear unusually soft, dark, and sometimes faintly colored near its edge.
What “magnitude” means
Lunar-eclipse predictions often list an umbral magnitude. For a partial eclipse, the number describes the fraction of the Moon’s apparent diameter immersed in the umbra at maximum. A magnitude near zero means a very shallow encounter. A magnitude near one means almost the entire Moon is covered, which can look nearly total even though a bright sliver remains outside the umbra.
Watching a partial eclipse
No eye protection is required. Start observing before the partial phase so the change is easier to appreciate. Binoculars reveal color and texture near the shadow edge, while a camera on a stable support can record its progress.
Check more than the headline date. Eclipse catalogs normally use UTC, and your local date may be the previous or following calendar day. The Moon must also be above your horizon. A location-aware calendar translates the maximum into local time and shows whether moonrise or moonset cuts into the event.
