Penumbral lunar eclipse: the subtle one

The shadow that is not dark

Earth's shadow has a dark core, the umbra, and a pale halo around it, the penumbra. From within the penumbra the Sun is only partly hidden by Earth, so the Moon still receives a good share of sunlight. When the Full Moon travels through just this outer zone, there is no sharp edge, no bite and no red colour.

That separates it from a partial or total lunar eclipse, where the Moon enters the umbra and the shadow is unmistakable.

What you can actually notice

At the deepest point one side of the Moon takes on a subtle grey or smoky tint, as if someone had breathed on a window. The shading is slight, and the part nearest the umbra looks the darkest. Many casual watchers never realise anything is happening.

Experienced observers compare the two halves of the Moon, or take photographs and then boost the contrast to bring out the gradient.

Tips for seeing it

• Pick the right time. The table gives the time of greatest eclipse in your time zone; look then, and a half hour either side.
• Compare. Glance at the Moon before the event so you have a reference.
• Use a camera. A tripod photograph with adjusted contrast makes the shading plain.
• Check the location. The Moon must be above your horizon, and observatories list the local times.

Why they are still interesting

Although they are faint, penumbral events are part of the same eclipse seasons and the same cycles as the dramatic ones, and they often occur just before or after a bigger eclipse. Seeing the gentle dimming helps you appreciate how the three shadow zones fit together.

As with any lunar eclipse, no protection is needed at all. For more on how the seasons work, see the eclipses hub.

Tradition

Because they pass almost unnoticed, ancient chroniclers rarely recorded them. Some astrologers still note them as quiet echoes of a season, usually less important than other eclipses. That is a symbolic reading, nothing more.