Full moon Sept. 17 marks Canada's third eclipse of the year - here's ...

Canada is about to experience its third eclipse of 2024 during the Sept. 17 full moon — actually two eclipses in one.

Viewers of the night sky, however, might have a hard time noticing either of them.

That’s because it’s a partial lunar eclipse — with emphasis on “partial.”

“It’ll be really hard to see the peak of the eclipse. It will be just like a teeny, tiny little bite of the moon that’s shadowed,” explained Samantha Lawler, an associate professor at University of Regina’s Campion College, with research interests in observational astronomy, planetary science and dynamical modelling of exoplanets.

The eclipse is caused by the moon going into the main part of Earth's shadow, called the umbra.

The rest of the moon will still also be in shadow, but in a secondary shadow cast by the Earth called the penumbra. And a penumbral part of an eclipse can sometimes be hard to see because it’s so faint.

There was a full penumbral eclipse for the full moon back in March, which foreshadowed the much, much more anticipated spectacular solar eclipse that followed on April 8.

And that’s the way it works: solar eclipses and lunar eclipses always come in pairs (and sometimes threes).

So to sum it up, this Tuesday night, a hardly noticeable edge of the moon may be in Earth’s umbral shadow and therefore shadowed out. The rest of the moon, to quote Lawler, “will be inside the outer part of Earth’s shadow (the penumbra) which means that all of the light from the sun that’s hitting it has kind of filtered through Earth’s atmosphere,” explained Lawler. “It might look a little strange.”

In her experience, penumbral eclipses “are quite variable just depending on weather patterns on Earth and whatever’s happening around the edge of the Earth, where the sunlight’s travelling through (Earth’s atmosphere).

“It might be that the moon looks like a funny colour during the eclipse or it might be totally not noticeable at all.”

Her advice: “This isn’t a spectacular one, but it will be noticeable if you get binoculars, if you’re paying close attention.”

Here’s the timing of the eclipses from Toronto from timeanddatecom (but rounded to the nearest minute):

Penumbral eclipse begins: Sept. 17 at 8:41 p.m.Partial eclipse begins: at 10:13 p.m.Maximum eclipse: 10:44 p.m.Partial eclipse ends: 11:16 p.m.Penumbral eclipse ends: Sept. 18, 12:47 a.m.Second of four full supermoons

What might make it a bit more spectacular is that this will mark the second of four supermoons in a row. And this month’s supermoon is the second closest to the Earth this year, while October’s will be the closest.

Here’s the distance of the four supermoons as we had listed in last month’s story on the full moon (as listed by both earthsky.org and space.com), keeping in mind the moon’s average distance is 384,400 kilometres:

Aug. 19: 361,969 kmSep 18: 357,485 kmOct. 17: 357,363 kmNov. 15: 361,866 km

The distance changes because the moon’s orbit around Earth is not circular, but elliptical, so sometimes it’s closer and sometimes it’s farther — and when a full moon occurs when it’s a lot farther away than average, it’s called a micro moon.

Once in a blue moon …

Another distinction, with this moon — known in many traditions as the harvest moon (since it's closest to this year's Sept. 22 autumn equinox) — is that it’s a bit of a rarity, being the fourth full moon of the season (as it’s still summer!). This occurrence is called a blue moon, but for some reason the designation goes to the third full moon in the series — not the fourth one, so last month's August full supermoon was the blue moon.

Not to make it more confusing, but tradition has also grown to bestowing a blue moon designation to when the same month has a second full moon.