The moon will shine just a little less brightly tomorrow night, but don’t blame the clouds or air pollution.
It passes through a half shadow cast by the earth from about midnight and will dim slightly as a result.
The moonlight will change while going through what astronomers call a “penumbral eclipse”. It won’t be passing through the darkest part of the earth’s shadow, the umbra, but through the half shadow either side of this, a region of half light-half shadow called the penumbra.
Astronomy Ireland has encouraged everyone to go out and have a look between midnight and 2am on Thursday.
While the timing of the penumbral eclipse is certain, less clear is how the moon will look.
“We are not sure what people will see,” stated Astronomy Ireland chairman, David Moore. “Mid-eclipse is around 1.39am on Thursday morning and perhaps for half an hour either side of this time the lower edge of the moon will dim somewhat. We just don’t know.”
The moon is in total eclipse when the sun, earth and moon are perfectly aligned. The lunar disc is always therefore full and is considerably dimmed when it passes into the umbra.
The dimming is much less when the moon passes through the penumbra. A total penumbral eclipse is rare but happens when the moon dips totally into this region of half shade.
As this event is only partial it may be a challenge to actually see any change in the strength of the moonlight. Astronomy Ireland has encouraged anyone who does stay up late to observe the phenomenon to report back and let them know what you see.
