A lunar tetrad starts Monday with a lunar eclipse!

Lunar eclipses happen fairly regularly, some penumbral, some partial, and some total.  Sometimes the geometry lines up so well we get a run of four total eclipses in a row.  Astronomers call this a “tetrad”, and one of them is starting on Tuesday.  The frequency of tetrads is declining over time as the Earth’s orbit gradually circularizes; billions of years from now, they will no longer happen.  So, like the total solar eclipse resulting from the Moon appearing the same size as the Sun in our sky, someday this phenomenon will be over for good.  (The Moon is receding from the Earth; eventually, all we’ll get is annual solar eclipses, and even those will become less common.)  This is the first tetrad of the 21st century; there will be seven more.


This eclipse is well-placed for viewers in the Americas, but inconveniently early in the morning on Tuesday (April 15).  Unlike solar eclipses, lunar eclipses occur simultaneously for all viewers, since the real action is happening on the Moon; we’re watching our own shadow pass over the Moon.  They’re also a lot more sedate, owing the Earth’s much larger shadow, and the transitions are much more gradual, thanks to the blurring effect of our atmosphere.  This refracts sunlight around the limb of the Earth even into the deepest parts of the shadow, to a different degree depending on the composition of the atmosphere, so it’s hard to predict ahead of time just how dark or how red the Moon will get.  Set your alarm clock Tuesday morning to find out.  😉

There’s an extra treat during this eclipse as well — because Mars just passed opposition, and will in fact be making its closest approach to Earth on Monday, it will be very bright and also very close to the Moon in the sky.  (Lunar eclipses happen, naturally, only when the Moon is in opposition — opposite the Sun in the sky.)  The bright star Spica will be even closer to the Moon, just two degrees away, and it will be much easier than normal to see it, with the Moon’s brilliance veiled.

But you will need an alarm clock.  The Moon will first contact the penumbra at 04:53:57 UT (or 11:53 PM CDT on Monday), then the umbra a little over an hour later at 05:58:19 UT (12:58:19 AM CDT on Tuesday).  Totality starts at 07:06:47 UT (02:06 AM CDT) and lasts an hour and 17 minutes; the Moon starts to emerge from the umbra at 08:24:35 UT (03:24:35 AM CDT), is completely out of the umbra at 09:33:04 UT (04:33:04 AM CDT), and the eclipse is completely over when the last bit of the Moon exits the penumbra at 10:37:37 UT (05:37:37 AM CDT).

The eclipse is visible to anybody who can see the Moon within that window of time, and because Full Moon happens when the Moon is opposite the Sun, that means it has to be nighttime.  Viewers at the extreme eastern edge of the visibility window will see the eclipse begin at moonset/sunrise.  Viewers on the extreme western edge of the viewing area will see only the penumbral portion at moonrise/sunset.  The further inwards you go from those extremes, the better the circumstances will be.


If the Moon enters the umbra from your location during either sunrise or sunset (on the diagram, that’s the area between the U1 and U2 lines on the east and between the U3 and U4 lines on the west), you’ll get an extra little treat.  Although you won’t see the full eclipse, you’ll get to see something special: the Moon in the Earth’s shadow.  Oh sure, that’s what everybody will see, right?  Well, for you it’ll be more obvious, because as the Sun goes down in the west, the Earth’s shadow rises in the east (and vice versa) as a blue-gray band just beneath a pinkish band called the Belt of Venus.  Many people have seen it, but did you realize it’s actually our shadow?  You can only see it if there are no clouds in the way, of course, but it can be quite striking.  And when the Moon is entering eclipse, you see it actually inside that slate colored band, and suddenly the geometry of an eclipse becomes very clear.  Check it out, if you get the chance and the weather cooperates.  😉  And if you miss it, well, try again in October!

For more information, including diagrams and starmaps, check out Fred “Mr Eclipse” Espenak’s page for the eclipse.

And, as an interesting coincidence, this is also the Passover Moon to Jews, and the Paschal Moon to Christians; this particular full moon defines Passover, and therefore to most* Christians it also defines Easter.


* All Christians base the timing of Easter upon the timing of Passover, but due to the inherent difficulties of reckoning time consistently over two thousand years, not everybody calculates it exactly the same way.  Eastern Rite Easter may occur up to a month away from Western Rite Easter.


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