lunar tetrad Archives

September 6, 2016September 6, 2016 by David Dickinson

An Impalpable Penumbral Eclipse

Hey, how ’bout that annular eclipse last week? Some great images flooded in to Universe Today, as the final solar eclipse for 2016 graced the African continent. This not only marked the start of the second and final eclipse season for 2016, but it also set us up for the final eclipse of the year next week.

The path of next week's penumbral eclipse through the Earth's shadow. Adapted from NASA/GSFC/F. Espenak. — The path of next week’s penumbral eclipse through the Earth’s shadow. Adapted from NASA/GSFC/F. Espenak.

We’re talking about the penumbral lunar eclipse coming up next week on September 16^th, 2016. this sort of eclipse occurs when the Moon just misses the dark inner core (umbra) of the Earth’s shadow, and instead, drifts through its relatively bright outer cone, known as the penumbra. Though not the grandest show as eclipses go, astute observers should notice a subtle light tea-colored shading of the Full Moon, and perhaps the ragged dark edge of the umbra on the northwestern limb of the Moon as it brushes by around mid-eclipse.

The visibility map for next week's eclipse. Image credit: NASA/GSFC/Fred Espenak. — The visibility map for next week’s eclipse. Image credit: NASA/GSFC/Fred Espenak.

The entirety of the eclipse will be visible from the region surrounding the Indian Ocean on the evening of Friday, September 6^th. Viewers in Australia, New Zealand and Japan will see the eclipse transpire at moonset, and the eclipse will get underway at moonrise for observers in western Africa and Europe.

The eclipse runs from first contact at 16:55 Universal Time (UT) to 20:54 UT when the Moon quits the Earth’s shadow almost four hours later. Mid-eclipse occurs at 18:55 UT, with the Moon 91% immersed in the Earth’s outer shadow.

Tales of the Saros

This particular eclipse is member 9 of the 71 lunar eclipses in saros series 147. This saros began on July 2^nd 1890 and runs through to the final eclipse in the cycle on May 1^st 2990. It will produce its very first partial eclipse next time around on September 28^th 2034, and its first total lunar eclipse on June 6^th, 2449.

Why penumbrals? Aren’t they the ultimate non-event when it comes to eclipses? Like with much of observational astronomy, a penumbral lunar eclipse pushes our skills as a visual athlete to the limit. Check out the waxing gibbous Moon the night before the eclipse, then the Moon the night of the event. If you didn’t know any better, could you tell the difference from one night to the next? Often, the camera can see what the eye can’t. Photographing the Moon before, during and after a penumbral eclipse will often bring out the subtle shading on post-comparison. You’ll want to photograph the Moon when its high in the sky and free of atmospheric distortion low to the horizon, which tends to discolor the Moon. Such a high-flying Moon during mid-eclipse favors the Indian Subcontinent this time around. We’ve yet to see a good convincing time-lapse documenting a penumbral eclipse, though such a feat is certainly possible.

See anything... shady going on? Here's the penumbral lunar eclipse from this past March. Image credit and copyright: Neeraj Ladia — See anything… shady going on? Here’s the penumbral lunar eclipse from this past March. Image credit and copyright: Neeraj Ladia

When is an eclipse… not an eclipse? By some accounts, the Moon underwent a very shallow penumbral one cycle ago on August 18^th, 2016, though the brush past the shadow was so slight that many lists, including the NASA’s GSFC eclipse page omitted it. Three eclipses (a lunar partial and a penumbral, or two penumbrals and one solar) can occur in one eclipse season, if the nodes of the Moon’s orbit where it intersects the ecliptic fall just right. This last occurred in 2013, and will happen again in 2020.

And when there’s a lunar eclipse, there’s also a Full Moon. The September Full Moon is the Harvest Moon, providing a few extra hours of illumination to get the crops in. This year, the Harvest Moon falls just six days from the equinox on September 22^nd, marking the start of astronomical Fall in the northern hemisphere and Spring in the southern. The relative ecliptic angle also ensures that moonrise only slides back by a slight amount each evening for observers in mid-northern latitudes around the Harvest Moon.

Can’t wait til the next eclipse? Well, 2017 has four of ’em: an annular on February 26th favoring South America, two lunars (another penumbral on February 11^th and a partial on August 7^th) and oh yeah, there’s a total solar eclipse crossing the United States on August 21st. And the next total lunar eclipse? The dry spell is broken on January 31^st, 2018, when a total lunar eclipse favoring the Pacific Rim occurs. Yeah, we got spoiled with four back-to-back lunar eclipses during the Blood Moon tetrad of 2014-2015…

Read Dave Dickinson’s eclipse-fueled sci-fi tales Exeligmos, Shadowfall, The Syzygy Gambit and Peak Season.

September 24, 2015December 23, 2015 by David Dickinson

Sunday Night: Getting Ready For a ‘Super-Harvest-Blood-Moon Total Lunar Eclipse’

So, heard the one about this weekend’s impending ‘Super-Harvest-Blood-Moon eclipse?’ Yeah, us too. Have no fear; fortunately for humanity, the total lunar eclipse transpiring on Sunday night/Monday morning is a harbinger of nothing more than a fine celestial spectacle, clear skies willing.

This final eclipse of the ongoing lunar tetrad has some noteworthy events worth exploring in terms of science and lore.

The Supermoon Total Lunar Eclipse of September 27-28 2015 from Michael Zeiler on Vimeo.

The Specifics: First, you almost couldn’t ask for better timing. This weekend’s total lunar eclipse occurs during prime time Sunday night for North and South America, and early Monday morning for Europe, Africa and most of the Middle East. This means the Atlantic Region and surrounding areas will see totality in its entirety. This eclipse occurs very near the northward equinoctial point occupied by the Sun during the Northern Hemisphere Spring equinox in March. The date says it all: this eclipse coincides with the Harvest Moon for 2015, falling just under five days after the September equinox.

Early cloud cover prospects for Sunday night over the contiguous United States. Image credit: The National Weather Service

For saros buffs, Sunday’s eclipse is part of lunar saros series 137, member 28 of 81. This saros started back in 1564 and produced its first total lunar eclipse just two cycles ago on September 6^th 1979. Saros 137 runs all the way out to its final eclipse on April 20^th, 2953 AD.

And yes, this upcoming total lunar eclipse occurs very near the closest lunar perigee for 2015. How rare are ‘Supermoon’ lunar eclipses? Well, we took a look at the phenomenon, and found 15 total lunar eclipses occurring near lunar perigee for the current century:

You’ll note that four saroses (the plural of saros) are producing perigee or ‘Proxigean’ total lunar eclipses during this century, including saros 137.

Does the perigee Moon effect the length of totality? It’s an interesting question. Several factors come into play that are worth considering for Sunday night’s eclipse. First, the Moon moves a bit faster near perigee as per Kepler’s second law of motion. Second, the Moon is a shade larger in apparent size, 34’ versus 29’ near apogee. Lastly, the conic section of the Earth’s shadow or umbra is a bit larger closer in; you can fit three Moons side-by-side across the umbra around 400,000 kilometers out from the Earth. Sunday night’s perigee occurs 65 minutes after Full Moon at 2:52 UT/10:52 PM EDT. Perigee Sunday night is 356,876 kilometers distant, the closest for 2015 by just 115 kilometers, and just under 500 kilometers short of the closest perigee that can occur. This is, however, the closest perigee time-wise to lunar totality for the 21^st century; you have to go all the way back to 1897 to find one closer, at just four minutes apart.

Now, THAT was and eclipse!

This all culminates in a period for totality on Sunday night of just under 72 minutes in duration, 35 minutes shy of the maximum possible for a central total lunar eclipse. An eclipse won’t top this weekend’s in terms of duration until January 31^st 2018.

Here are the key times to watch for on Sunday night:

Penumbral phase begins: 00:12 UT/8:12 PM EDT (on the 27^th)

Partial phase begins: 1:07 UT/9:07 PM EDT

Totality begins: 2:11 UT/10:11 PM EDT

Totality ends: 3:23 UT/11:23 PM EDT

Partial phase ends: 4:27 UT/00:27 AM EDT

Penumbral phase ends: 5:22 UT/1:22 AM EDT

Note that one 18 year 11 day and 8 hour saros period later, saros 137 will again produce a perigee eclipse nearly as close as this weekend’s on October 8^th, 2033.

The classic hallmark of any total lunar eclipse is the reddening of the Moon. You’re seeing the combination of all the world’s sunsets, refracted into the inky umbra of the Earth and cast upon the surface of the Moon. To date, no human has stood upon the surface of the Moon and gazed upon the spectacle of a solar eclipse caused by the Earth.

Not all eclipses are created equal when it comes to hue and color. The amount of dust and aerosols suspended in the atmosphere can conspire to produce anything from a bright, yellowish-orange tint, to a brick dark eclipse where the Moon almost disappears from view entirely. The recent rapid fire tetrad of four eclipses in 18 months has provided a good study in eclipse color intensity. The deeper the Moon dips into the Earth’s shadow, the darker it will appear… last April’s lunar eclipse was just barely inside the umbra, making many observers question if the eclipse was in fact total at all.

We the describe color of the eclipsed Moon in terms of its number on the Danjon scale, and recent volcanic activity worldwide suggests that we may be in for a darker than normal eclipse… but we could always be in for a surprise!

Old time mariners including James Cook and Christopher Columbus used positional measurements of the eclipsed Moon at sea versus predictions published in almanac tables for land-based observatories to get a one-time fix on their longitude, a fun experiment to try to replicate today. Kris Columbus also wasn’t above using beforehand knowledge of an impending lunar eclipse to help get his crew out of a tight jam.

And speaking of the next perigee Moon total lunar eclipse for saros 137 on October 8th, 2033… if you catch that one, this weekend’s, and saw the September 16^th, 1997 lunar eclipse which spanned the Indian Ocean region, you’ll have completed an exeligmos, or a triple saros of eclipses in the same series 54 years and 33 days in length, an exclusive club among eclipse watchers and a great word to land on a triple letter word score in Scrabble…

Exeligmos is also the title of one of our original scifi tales involving eclipses, along with Shadowfall.

Here’s another neat challenge: the International Space Station makes two shadow passes during the lunar eclipse over the contiguous United States. The first one occurs during totality, and spans from eastern Louisiana to central Maine from 2:14 to 2:20 UT; the second pass occurs during the final partial phases of the eclipse spanning from southern Arizona to Lake Superior from 3:47 to 3:54 UT. These are un-illuminated shadow passes of the ISS. Observers have captured transits of the ISS during a partial solar eclipse, but to our knowledge, no one has ever caught a transit of the ISS during a total lunar eclipse; ISS astros should also briefly be able to spy the eclipsed Moon from their orbital vantage point. CALSky will have refined passage times about 48 hours prior to Sunday.

Clouded out? Live on the wrong side of the planet? The good folks at the Virtual Telescope Project have got you covered, with a live webcast of the total lunar eclipse starting at 1:00 UT/9:00 PM EDT.

And as the eclipse draws to an end, the question of the hour always is: when’s the next one? Well, the next lunar eclipse is a dim penumbral on March 23^rd, 2016, which follows a total solar eclipse for southeastern Asia on March 9^th, 2016… but the next total lunar won’t occur until January 31^st, 2018, which also happens to be the second Full Moon of the month… a ‘Blue Blood Moon Eclipse?’

Sorry, we had to go there. Hey, we could make the case for Sunday’s eclipse also occurring on World Rabies Day, but perhaps a ‘Rabies Eclipse’ just doesn’t have the SEO traction. Don’t fear the Blood Moon, but do get out and watch the final lunar eclipse of 2015 on Sunday night!

July 27, 2015December 23, 2015 by David Dickinson

Blues for the Second Full Moon of July

An artificially created 'Blue Moon,' using the white balance settings on the camera. Image credit and copyright: John Chumack

Brace yourselves for Blue Moon madness. The month of July 2015 hosts two Full Moons: One on July 2^nd and another coming right up this week on Friday, July 31^st at 10:43 Universal Time (UT)/6:43 AM EDT.

In modern day vernacular, the occurrence of two Full Moons in one calendar month has become known as a ‘Blue Moon.’ This is a result of the synodic period (the amount of time it takes for the Moon to return to a like phase, in this case Full back to Full) of 29.5 Earth days being less than every calendar month except February.

In the ‘two Full Moons in one month’ sense, the last time a Blue Moon occurred was on August 31st, 2012, and the next is January 31^st, 2018. The next time a Blue Moon occurs in the month of July is 2034, and the last July Blue Moon was 2004.

We say “once in a blue Moon,” as if it’s a rarity, but as you can see, they’re fairly frequent, occurring nearly once every 2-3 years or so.

Now, we’ll let you in on a secret. Like its modern internet meme cousin the ‘Super-Moon,’ astronomers don’t sit in mountain top observatories discussing the vagaries of the Blue Moon. In fact, astronomers rarely like to observe during the weeks surrounding the light-polluting Full Moon, and often compile data from the comfort of their university offices rather than visit mountaintop observatories these days…

The modern Blue Moon is now more of a cultural phenomenon. We’ve written previously about how an error brought us to the current ‘two Full Moons in one month definition.’ A more convoluted old timey definition was introduced in ye ole Maine Farmer’s Almanac circa 1930s as “the third Full Moon in an astronomical season with four.”

Legend has it that the Maine Farmer’s Almanac denoted this pesky extra seasonal Full Moon with ‘blue’ instead of black ink… to our knowledge, no examples exist to support this intriguing tale. Anyone have any old almanacs in the attic holding such a revelation out there?

The ghostly glow of the gibbous moon in Jean-Francois Millet's The Sheepfold. Image Credit: Public Domain — The ghostly glow of the gibbous moon in Jean-Francois Millet’s The Sheepfold. Image Credit: Public Domain

We’ve also laid out the occurrences for both types of Blue Moons for the remainder of the decade, as well as its New Moon cousin and internet meme to be, the Black Moon.

Untitled — The rising waxing gibbous Moon on the night of September 23rd, 1950. Image credit: Stellarium

Of course, the Moon most likely won’t appear to be physically blue, no matter what friends/family/co-workers/anonymous persons on Twitter say. The Moon can actually appear blue, as it did on September 23^rd, 1950 for much of the eastern United States and Canada through the haze of several forest fires in western Canada. The Moon was actually at waxing gibbous phase on the evening of this phenomenon, and as far as we can tell, no photographic documentation of this event exists. Spaceweather, has, however gathered a gallery of blue moon eyewitness reports over the years, including a few images. This occurs when moonlight is filtered through suspended oil drops about a micrometer in diameter which scattered yellow and red light, leaving a Moon with a ghostly indigo glow.

So there’s definitely another challenge to catch and photograph a truly ‘Blue Moon’ under such rare atmospheric circumstances… and remember, the Moon doesn’t have to be near Full to do it!

Watch that Moon, as we’ve got a few red letter dates coming up through the remainder of 2015. First up: the Supermoon season cometh in August, as we have a series of three Full Moons falling less than 24 hours from perigee on August 29^th, September 28^th, and October 27^th. Our money is on that middle one as having the potential to generate the most online lunacy, as it’s also the last total lunar eclipse of the current tetrad of four total lunar eclipses for 2014 and 2015, a ‘super-blood moon eclipse’ anyone? Though the dead won’t rise from the grave to mark such an occasion, you can be sure that many a sky aficionado will stumble zombie-like into the office the next day after pulling an all-nighter for the last good North American total lunar eclipse until 2018.

And it’s worth noting the path of the Moon, as it reaches its shallow mid-point in the last half of 2015. The Moon’s orbit is tilted about five degrees relative to the ecliptic, meaning that it can ride anywhere from 18 degrees—as it does this year—to 28 degrees from the celestial equator. This cycle takes about 19 years to complete, and a wide-ranging ‘long nights Moon’ last occurred in 2006, and will next occur in 2025.

A 'mock Blue Moon...' — A ‘mock Blue Moon…’ induced by use of a military flashlight filter. Image credit: Dave Dickinson

So don’t fear the Blue Moon, but be sure to take a stroll under its light this coming Friday… and perhaps enjoy a frosty Blue Moon beer on the eve of the sultry month of August.

March 3, 2015December 23, 2015 by David Dickinson

The Mini-Moon Cometh: Catch the Smallest Full Moon of 2015 This Thursday

Supermoons. Blood Moons. Moons both Black and Blue… by now, you’d think that there was nothing new under the Sun (or Moon, as it were) when it comes to new unofficial lunar terminology.

Sure, the Moon now seems more colorful than controversial viral dress shades. Love it or loathe it, the Internet can sure set a meme in motion. And this week’s Full Moon on Thursday evening offers up one of our faves, as the most distant Full Moon of 2015 occurs on March 5th. Yup, the Mini-Moon is indeed once again upon us, a time when the Full Moon appears slightly smaller than usual as seen from the Earth. But can you really tell the difference?

The third Full Moon of the year occurs this week on Thursday, March 5th. Also known as the Worm or Sap Moon by the Algonquin tribes of New England, the moment of Full phase occurs at 18:07 Universal Time (UT) or 1:07 PM Eastern Standard Time (EST). This is also just over 10 hours after apogee, which occurs at 7:36 UT/2:36 AM EST. This month’s apogee is also an exceptionally distant one, measuring 406,385 kilometres from the center of the Earth to the center of the Moon. This is just 80 kilometres shy of the most distant apogee of 2015 on September 14^th, which occurs when the Moon is near New phase.

Can you spy Jupiter next to the waxing gibbous Moon *before* sunset tonite? Credit: Stellarium.

Apogee for the Moon ranges from 404,000 to 406,700 kilometres distant, and the Full Moon appears 29.3 arc minutes across near apogee versus 34.1’ across near perigee as seen from the Earth.

This is also the closest apogee near a Full Moon time-wise until January 27^th, 2032.

What is a Mini-Moon? As with a Supermoon, we prefer simply defining a Mini-Moon as a Full Moon which occurs within 24 hours of apogee. That’s much more definitive in our book rather than the cryptic and often cited ‘within 90% of its orbit’ refrain for Supermoons.

And speaking of which, we’ve got three ‘Super’ Full Moons in 2015, with the very closest Super (Duper?) Full Moon occurring within an hour of perigee on September 28^th during the final total lunar eclipse of the ongoing tetrad… what will the spin doctors of the Internet make of this? A ‘Super Duper Blood Moon,’ anyone?

The path of the Moon this week also takes it towards the Fall equinoctial point in the astronomical constellation of Virgo, as it crosses Leo and nicks the corner of the non-zodiacal constellation Sextans. The Moon reaches Full two weeks prior to the Vernal Equinox, which falls this year on March 20^th. Keep an eye on the Moon, as the first eclipse of 2015 and this year’s only total solar eclipse also occurs just 13 hours prior to the equinox for observers in the high Arctic. (More on that next week).

Can’t wait til Thursday? Tonight, observers across Canada, northern Maine, and Europe will see a fine occultation of the star Acubens (a.k.a. Alpha Cancri) by the 94% illuminated waxing gibbous Moon:

The ‘shadow footprint’ for tonight’s occultation of Acubens by the Moon. Credit: Occult 4.0.1.

Alpha Cancri is 175 light years distant, and folks living along the U.S./Canadian border will be treated to a fine grazing occultation as the double star plays hide and seek along the limb of the Moon. This is number 17 in an ongoing series of 21 occultations of the star by the Moon stretching out until June 20th, 2015. There’s a wide separation of 11” between the star’s A and B components, and there are suspicions from previous lunar occultations that Alpha Cancri A may itself be a double star as well.

We caught a similar occultation of the star Lambda Geminorum by the Moon this past Friday:

Ever feel sorry for moonless Venus? This Wednesday night also offers a chance to spy Venus with a brief ‘pseudo-moon,’ as +6^th magnitude Uranus passes just 15’ — less than half the apparent diameter of a Full Moon — from brilliant -4^th magnitude Venus. Neith, the spurious 18th century moon of Venus lives! From the vantage point of Venus on March 4^th, the Earth and Moon would shine at magnitudes -2.3 and +1.5, respectively, and sit about 4 arc minutes apart.

The rising Full ‘Mini-Moon’ of March 5th. Credit: Starry Night Education Software.

Does the rising Full Moon look smaller to you than usual this week? While the apparent change in diameter from apogee to perigee is slight, it is indeed noticeable to the naked eye observers. Remember, the Moon is actually about one Earth radius (6,400 kilometres) more distant on the local horizon than when it’s directly overhead at the zenith. The Moon is also moving away from us at a current rate of 1-2 centimetres a year, meaning that Mini-Moons will get ever more distant in epochs hence.

Already, annular solar eclipses are currently more common than total ones by a ratio of about 11 to 9. The first annular eclipse as seen from the Earth went unheralded some time about 900 million to a billion years ago, and 1.4 billion years hence, the last total solar eclipse will occur.

The rising waxing gibbous Moon against the daytime sky. Photo by author.

Be sure to get out and enjoy the rising Mini-Moon later this week!

-Send those Mini-Moon pics in to Universe Today.

-Looking for eclipse sci-fi? Check out Dave Dickinson’s eclipse-fueled tales Exeligmos and Shadowfall.