The Cuba Eclipse

Let’s be ready for the night between January 20 and 21 (Sunday and Monday).  I call this the Cuba Eclipse because that’s where the totally eclipsed Moon will be overhead at the middle time: which means it will be high in the sky for most of both Americas and much of Europe.

See the note at the end about enlarging images.

This picture shows the stages of the eclipse.  The Moon glides through Earth’s shadow, a process of more than five hours.

It’s the middle stages that are wonderful; the opening and closing are almost dispensable.  The wide outer ring of the shadow, called the penumbra, gets some direct sunlight and therefore the Moon is scarcely dimmed.  So the first and last moments (“touches penumbra” and “quits penumbra”) are indiscernible.  Maybe half or three-quarters of an hour after the beginning, you might notice a pale grayness – the inner penumbra – on the Moon’s advancing edge.

The exciting moment is: “touches umbra.”

At this moment, or seconds later, you spot on the advancing edge a dark chip.  It grows, becomes a curve, the definite edge of the inner shadow, the umbra.  You are compelled to realize that it is the huge shadow of the globe you stand on, a globe that hangs between Sun and Moon.

The Moon moves deeper and, more than an hour later, is completely inside the umbra (“totality begins”).  It is inside for about another hour

Though it goes deep, it does not touch the umbra’s center, so this is not among the very longest of total eclipses.  “Totality ends”: a bright spot touches the advancing edge.  Then the dwindling of the total shadow; the pale gray inner penumbra; the indiscernible closing stage.

The exciting “touches umbra” moment is 3:34 by Universal Time, on January 21.  That’s 3:34 AM in Britain, but is back in January 20 by North American clocks: 10:34 PM in the Eastern zone, 9:34, 8:34. 7:34 in the Central. Mountain, and Pacific zones.

Don’t forget that though the Moon moves eastward (leftward) in relation to the Sun, it and the Sun and all else in the sky are moving westward as the night goes on.

The spectacle of the eclipsed Moon is at different positions in the sky for different places.

These are views from the Moon as the Earth slides between it and the Sun.  They are arranged from right to left because that’s the direction in which Earth seems to slide  You can see which countries, at each stage, have the Moon overhead, or are moving into sight of it or rotating out of sight of it.

For the U.S., the Moon during the early stages climbs from the eastern horizon, and by the middle stages it is in the middle of the sky.

This view is from latitude 40° north, longitude 90° west – roughly the middle of the contiguous United States – and is about at mid eclipse – Jan. 21 5 UT, which is Jan. 20 11 PM CST.

Over in Britain, the eclipse doesn’t begin till well after midnight (2:35 AM on Jan. 21).  The touching of the umbra is at 3:34.  This is the view for mid eclipse at latitude 50° north, longitude 0.

By the end of totality the Moon is almost setting in the west-northwest.

Another excitement of a total lunar eclipse is the dramatic change – if conditions are good – from a sky awash with the glare of Full Moon to a black sky full of stars.  Guideposts among them are Castor and Pollŭ near to the Moon, and Sirius toward the south.  Mars is low in the west for America in the early half of the eclipse, but the other bright planets are on the Sun’s side of the sky.

The stars are always with us, the eclipsed Moon is not.  Some sunlight is refracted through Earth’s skin of atmosphere into the umbra.  How much, and what color, depends on conditions – clouds, pollution – in the ring of atmosphere around the sunrise-sunset line on the Earth at the time.  At some eclipses the Moon goes black – disappears.  At some, it shows brownish and reddish shades, varying across its face, usually darker near the center and lighter near the edge of the umbra.

The Danjon Scale is a way of quantifying your judgment of the Moon’s appearance, preferably near the middle of the ecliopse (“L” stands for “luminosity”).

L0: very dark eclipse, Moon almost invisible, especially in mid-totality.

L1: dark eclipse, gray or brownish coloration, details distinguishable only with difficulty.

L2: deep red or rust-colored eclipse, umbra usually having a very dark center and relatively bright outer rim.

L3: brick-red eclipse, umbra usually having a yellow or bright gray rim.

L4: strikingly bright copper-red or orange eclipse, with very bright bluish tint where umbra and penumbra meet.

Fractional estimates can be used, such as 1.3; and different parts of the Moon may have different values.

Here is a general picture of the eclipses of 2019.

The Earth is shown at the dates when the Moon is inward from it (New Moon) or outward (Full).  They and the Sun are greatly exaggerated in size.  The plane of the Moon’s orbit is blue, paler for the half lying south of the ecliptic.  This plane gradually rotates backward.  There is an eclipse if the Moon is Full or New when it is in or near the ecliptic plane, which happens at intervals of rather less than half a year.

This year there are five eclipses, instead of the most usual four, because a third eclipse season begins before the end of the year.  But eclipses are not equal.

Our January 21 event may be the best of this year’s eclipses.  The other lunar eclipse, of July 16, is only partial, and seen mainly over Africa.  Of the three solar eclipses, the January 6 one was only partial, seen in the northern Pacific; the December 26 one, across southern Asia, will be of the annular kind, in which, as with partial solar eclipses, the sky does not go very dark; the good one will be the total solar eclipse of July 2, for which you would have to go half way down Chile or Argentina, or be on a ship in the southern Pacific.

Because this eclipse was the one to look forward to, I devoted to it the whole back cover of the last Astronomical Calendar, for 2016.  So if you still have your copy of that you can see the eclipse described in a whole other way, including a large star chart and an imaginary comet.

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DIAGRAMS in these posts are made with precision but have to be inserted in another format.  You may be able to enlarge them on your monitor.  One way: right-click, and choose “View image”  Or choose “Copy image”, then put it on your desktop, then open it.  On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping with three fingers).  I am grateful to know of what methods work for you.