A penumbral eclipse of the moon is quite a tough observing challenge; it is much harder to see than a total lunar eclipse. In a total eclipse, the moon’s surface is buried deep in the shadow cast by Earth blocking the sun’s light from reaching the lunar surface.
This total eclipse is caused by a deep shadow known as an umbra.
However, there are two kinds of shadow cast by a large light source such as the sun. The umbra, as I said above, is the deep shadow we usually think of which causes a total eclipse, it’s where all the light from the sun is blocked by our planet. The penumbra, on the other hand, is an outer shadow where the light source (the sun) is only partially blocked.
If you were on the moon’s surface during a penumbral eclipse, you would witness a partial eclipse of the sun.
This image is taken from the excellent website TimeandDate.com, and shows you the general principle behind umbral and penumbral eclipses. We do not get Antumbral eclipses of the moon. Their article goes into much more depth on shadow types.
As you might expect, penumbral lunar eclipses are harder to observe because the shadow cast by a penumbral shadow is not as dark as total clip eclipse.
The eclipse begins at 17:07 universal time (UTC) on Friday, 10 January. Greatest eclipse happens two hours later and the whole event concludes after four hours at 21:12 UTC.
UTC is the same as GMT in the UK. Mainland Europe will remove either one or two hours from these times to get their local time. Alaska is 9 hours after these times, so the event occurs early on Saturday 11 January. For Nova Scotia, look to observe four hours after these times.
It’s visibility is complete for almost all of Europe, Asia, and Africa. There is partial visibility for Alaska and Quebec, Newfoundland & Labrador, New Brunswick and Nova Scotia in Canada. In the Canadian and Alaskan far north there is full visibility.
There is no visibility of this eclipse for all of South America (except the eastern tip of Brazil) and the contiguous states of the USA (with the exception of northeast Maine).
The chart below (source) shows the path of the penumbral eclipse and where it will be visible from. Click it for a larger version.
I mentioned above that if you were to stand on the lunar surface at the moment of a penumbral eclipse what you would observe is a partial eclipse of the sun. Obviously, some partial eclipses are greater than others. For example, a 90% eclipsed sun results in a darker shadow than a 20% eclipsed sun.
There are four penumbral eclipses in 2020, of which this one on 10 January is the first. This is also the deepest penumbral eclipse of the four – the moon almost skims the Earth’s umbra – which gives this one the most noticeable change.
Sadly, we won’t see the moon turning red as we often do during a total lunar eclipse. Instead, we’ll see its surface darkening for a couple of hours around the height of the penumbral eclipse. This is visible with the naked eye but it is a very subtle effect and worth watching as the event progresses to see the creeping darkness which will begin around the 8 o’clock mark on the moon (if we imagine a clock face on the lunar surface). At maximum eclipse, almost the whole moon will be in shadow before a gradual lightening takes hold from the 10 o’clock mark, which works its way west and down. The last part of the shadow leaves the moon’s disc at the 4 o’clock mark.