Blue Moon eclipsed over the northwest and the Pacific

On January 31 the Moon will be Full, for the second time in this month, and it will be totally eclipsed.

Lunar eclipse 2018 January 31

The moment when the Moon reaches its Full position – opposite to the Sun – is 13:27 by Universal Time.  That is 1:27 PM in Britain; for America it is earlier by 5, 6, 8, 9, and 10 hours for the Eastern, Central, Pacific, Alaska, and Hawaii time zones – therefore 8:27 AM, 7:27, 5:27, 4:27, and 3:27.  So it is already day, with the Moon below the horizon, except for northwestern Canada, Alaska, and Hawaii.

The eclipse is not just that central moment: its stages (called penumbral, partial, and total) extend from 2 hours and 40 minutes before to as long after the central moment.  The real action – the partial and total stages – extends from 102 minutes before the central moment to as long after.

Most of the U.S. and Canada see only stages before totality; the northwestern U.S. and a diagonal band across Canada do see totality but miss some of the closing stages.  Only west of there are all stages completely visible: northwestern Canada, Alaska, most of the Pacific including Hawaii, Japan, and New Zealand, most of Australia, and northeastern Asia.

That may be made clear by these views of the side of Earth facing the Moon, at the beginning of partial eclipse, middle, and end of partial eclipse.  You can call them views of Earth and Sun as seen from the Moon.

The opening and closing forty minutes or so, when the Moon is in the pale penumbra or outer shadow of the Earth, are indiscernible.  A bit of the Sun would still be visible from the Moon.  Think of it: a landscape on Earth when even a speck of the Sun is visible is in daylight.

The exciting moment is 11:48 Universal Time (do the subtraction for your clock time), when the first bite is taken out of the Moon’s left (eastern) edge by the umbra, the truly dark core of the shadow.  This is the huge circular shadow that made the ancient Greeks realize that we live on a globe.

Here is the view from Hawaii after midnight, as the Moon moves into the umbra.  The spectacle is high in the sky because Hawaii is not far from the middle of the hemisphere facing the Moon.

Here is the view from Seattle at the same time.

The Moon will still be above the horizon when total eclipse ends, and will set after the end of the partial phase.  Because it is lower toward the horizon, I can show the picture at larger scale.  The reddish umbra may or may not be so red, and of course the only part of it really visible is the part touching the Moon.  It is larger than the Moon, but not as large as Earth is, because it is a cone of shadow tapering outward between the Sun’s rays converging around it.  In the picture, both Moon and umbra are exaggerated in size by a factor of 2, so their relations are not quite as in reality: the Moon is at this moment entirely inside the umbra.  But that’s all right: the picture gives a more vivid idea of how the Moon slides into the umbra.

As the umbra gradually floods onto the Moon and then swallows it, the most interesting thing to look for is how reddish it seems on this occasion.  It may be anywhere from black (almost invisible amid the darkened Moonless sky) to coppery, with varied brownish shades on different parts.  Why?  No direct sunlight is aiming at the Moon, but some gets refracted into the umbra by the Earth’s atmosphere, represented by the reddish ring around Earth in our second picture.  The variation depends on how cloudy and polluted those regions around Earth happen to be.

The sky, floodlit by the usual Full Moon, has darkened as the Moon went into eclipse, and you may look around for stars.  The Moon is almost exactly on the ecliptic – the Sun’s path around the sky – which is why an eclipse happens; the Moon will ascend across the ecliptic at 18:46 UT, which is why it is passing through the southern half of the umbra.  The revealed constellation in the background is Cancer, whose centerpiece is the wonderful star-cluster called anciently Praesepe, the “Manger,” now more often the Beehive.  Just northwest of the Moon is one of Cancer’s few other naked-eye stars, Delta Cancri, called Asellus Australis, the “southern little donkey” feeding from the Manger.

And what of the Moon being a “Blue Moon” and a “Supermoon,” as the news media have it?  Enough for now; I’ll blog about that as soon as possible after I’ve had some lunch and the electricians, who are trying to get our hot water back, will let me switch my computer on again.

11 thoughts on “Blue Moon eclipsed over the northwest and the Pacific”

  1. Maybe you can clear up a couple of things for me. My understanding is that the “blue moon” is the fourth full moon in a season — which this one was not. Seems that an article in S&T in the 1940s mistakenly defined a blue moon as the 2nd full moon in a month. Then in 1999 S&T discovered its error. There was an article in the New York Times about two decades ago that described the original error and the discovery and correction of same. Now the error appears not only to have recurred, but in spades.

    1. That’s what I had no space or time for in my post and will come to in my follow-up post, when I get time in the day.

  2. You mean, do you, that NYC us NOT the center of the universe, nor even the US of A? Get my smelling salts quickly! Brandy!!!

  3. Here in San Francisco partial eclipse ends at 0712 PST; the Sun rises at 0715, and the Moon sets at 0720. So theoretically one could watch the entire eclipse through the end of the second partial phase. Unfortunately the current weather forecast predicts 100% cloud cover starting before totality and continuing until later in the morning.

    I had planned to take tomorrow off work to catch up on sleep. Now I wish I had also taken today off, so I could head east to see more of the eclipse, albeit by sacrificing any chance of witnessing fourth contact.

    1. Anthony, I think you’ve got this a little backwards. We here on the east coast have less chance of seeing any of this ‘blue moon’ ‘blood moon’ total eclipse of it. Folks in Hawaii get to see it in its entirey and folks on the east coast of Asia get to see some of it as the next sun rises. Get some sleep! lol.

      1. Jack, your NYCentric world view notwithstanding, there’s an entire continent east of me that’s still west of you. If I had traveled 100 miles inland today, I would have had better weather prospects, although I would miss the very end of the second partial phase.

    2. The sky here in SF was lovely for the eclipse. A few thin cirrus clouds only added to the drama, without hiding anything of interest. I walked up Bernal Hill with a folding chair, lap blanket, and image-stabilized 10×42 binoculars, and saw almost the whole show, from penumbral eclipse through partial, total, and most of the second partial phase. The Moon set behind Twin Peaks 5 minutes before fourth contact.

      This eclipse was very dark, mostly grey with only a bit of deep red just inside the edge of the umbra. I would call it L1 on the Danjon scale.

      It was lovely to pan back and forth a few degrees from the eclipsed Moon to delta Cancri and the Beehive cluster. The image-stabilized binoculars are a new toy for me. I like them a lot; they certainly made the walk up and down the hill less cumbersome. But I missed setting up binoculars on a tripod to share the view with passersby. There were a lot of people out on the hill to see the eclipse. Unlike evening eclipses on the hill, which can get rowdy, this crowd was very quiet, one might almost imagine reverential, although I suppose a lot of my fellow eclipsophiles were just sleepy and undercaffeinated.

      I don’t know how the weather service got the forecast so wrong, but I’m grateful. Certainly better than making the opposite error!

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