Partially Eclipsed Blue Moon, 31st December 2009
Lunar Eclipses Blue MoonsWhile I'm not a particularly avid lunar observer, it does make a very convenient target when you want to check whether the telescope is in good focus! And having got the thing in view you might as well take some pictures. I'm also not as interested in eclipses of the moon as of the sun (as they aren't anything like as dramatic) but if one comes along it seems silly to ignore it!
The best part of the moon to photograph will always be the very edge of the sunlit part: this is called the terminator. In this area the shadows pick out the detail with the greatest definition. Away from the terminator even large craters get "washed out" and hard to see: compare the right and left halves of the pictures below (which are in "naked-eye" orientation i.e. how you would actually see it in the sky).
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| The dark oval area to the right is Mare Crisium, with craters Cleomedes, Burckhardt, Geminus & Messala running up from its top edge. To the left we have Mare Serenitatis (upper) & Mare Tranquilitatis (lower). | To the right is Mare Fecunditatis: the crater with the central peak is Langrenus. The other one with a central peak is Petavius, with Vendelinus in between. The large one the other side of Petavius is Furnerius. |
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A composite of five images running down the terminator of a "first quarter" moon. There are too many craters visible to list any in particular, so I'll just mention that the arc of mountains circling the dark area to the top left are (from the top) the lunar Alps, Caucasus & Apennines. There are two Apollo landing sites visible in this image, Apollo11 about one-third of the way down the right-hand edge and Apollo15 in the far-north part of the Apennines: check out the close-up views below and see if you can find them! |
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An eclipse of the moon happens when the moon enters the shadow cast by the earth, in the same way that an eclipse of the sun happens when the earth enters the shadow of the moon. Lunar eclipses can be total or partial, depending on whether the moon falls completely within the earth's shadow or not, and because the earth's shadow during a lunar eclipse is wider than that of the moon during a solar eclipse (the earth being larger than the moon) they tend to last longer than solar events: totality can last up to 1hr 48mins. The last one this long was in 2000, but there's not another until past 3000AD!
The images here are from a partial eclipse on 7th September 2006. It was in progress at moon-rise, there were (naturally!) clouds about and I should have been at a rehearsal anyway, so I couldn't get a full sequence. I thus used it as a trial run for the total eclipse in 2007: good job too, as there were "exposure issues", let us say! I used the digital camera (as the field of view of the webcam is too small) and let it decide its own exposure, which resulted in a greatly over-exposed moon. However, at least this meant that the eclipsed sector of the moon was visible. Some darker areas can be seen in the eclipsed area in the first image, reflecting the fact that the moon's surface is not uniform. The animation covers a time frame of about 8mins and shows the earth's shadow slowly moving off the moon's disc.
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| Partial lunar eclipse, 7th September 2006 (8pm BST) | The earth's shadow moves across the moon's face |
The next set of images show the total eclipse during the evening of 3rd March 2007. This eclipse had been eagerly anticipated, as the previous one was back in October 2004, and in the end it did not disappoint. As with the occultation the day before (click here to view it), the weather was cloudy until just before the event but then cleared to stay perfect for the entire duration. I mounted the digital camera on my equatorial tripod and set myself up to take one photo every 5mins during the partial phases and an assortment of shots during totality: that'll be just the 4hrs out in the garden then! Good job it wasn't too cold!
As is the case with most eclipses, the moon did not pass exactly through the centre of the earth's shadow so although the eclipse was truly total the moon's surface was not evenly darkened as light scattered by the earth's atmosphere makes the edge of the shadow rather diffuse: this can also been seen in the partial eclipse above. The moon will likewise never be totally dark, the scattered light often turning it dark red or coppery coloured. By comparison, the partial phase of a solar eclipse is sharp because the moon has no atmosphere and so there can be no scattering. The last "central" eclipse was in 16th July 2000 (not visible in the UK) and the next will be on 15th June 2011 (partially visible in the UK).
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| Total lunar eclipse, 3rd March 2007, showing the distinctive coppery colour and the area of lighter shadow | To demonstrate that it's the moon that moves, not the shadow, this animation shows the motion of the moon during totality relative to two stars (one each side). The time interval between images is 5mins. |
| As the moon moves in a straight line through the earth's curved shadow, the area which is lightened due to being near the edge of the shadow changes. This montage of two images, from the beginning and end of totality, shows this effect well: the lighter arc moves about 60deg anti-clockwise round the moon's circumference in an hour. |  |
Photographing the full extent of a lunar eclipse is quite difficult, as the brightness of the subject varies enormously, as does the contrast between the eclipsed and non-eclipsed regions. I thus experimented by using a different exposure setting for the "inward" and "outward" phases. From the images I took I constructed two animations showing the advancing and retreating shadow line: they are quite large files (300 and 200kbytes respectively), so I've left them as "links" rather than placing them directly in this page. I also made movies from the same sequences (AVI format) which resulted in much smaller file sizes (50 and 40kbytes) so I've included them as well - take your pick! The exposure used for the inward phase turned out to be better, so this animation/movie is technically superior, but I present both sequences here as they show very clearly the different angle at which the eclipse departs from which it arrives - a consequence of the curvature of the shadow disc mentioned above.
 | Animation | Movie |
|---|---|---|
| Inward | Click here | Click here |
| Outward | Click here | Click here |
This eclipse took place in the early hours of the morning - from just before 2am to just after 5am, in fact - so little sleep was in prospect! Weather conditions in the previous week would have been perfect (clear and cold) but during the period in question cloud cover had been much greater. It tended to be fairly high and thin though so when a bright full moon was visible through a haze in the early evening hopes were rising. This did not last, unfortunately, with the cloud continuing to thicken throughout the night. At 1am the sky was quite overcast so by the time the eclipse started (1:43am) visibility was not good and getting worse. I took a few shots of the advancing shadow line but they were decidedly indistinct. By just after 2am the moon had more-or-less disappeared - eclipsed by cloud though, not the earth's shadow! Occasional glimpses allowed one to keep track of progress in binoculars but photography was hopeless.
During totality itself (3:01 to 3:51) there were a few periods of lighter cloud but nothing actually visible in the sky. I decided to fire off some time exposures in the right general direction and was astounded to find that the camera picked up the dark moon! Not only that, it saw Saturn and the star Regulus as well. This wasn't consistent though, so I think I was just lucky. Just after maximum eclipse (3:26) the sky did almost clear so I was able to take a few shots with the moon actually in the viewfinder but this didn't last long. By 4am dense cloud had returned so I gave up a little after this and sought the comfort of my bed!
My photographic endeavours were thus not entirely in vain but it was not possible to get enough good images to replicate the animations of the 2007 eclipse above. However, they do show nicely that while the track of the 2007 eclipse was above the centre-line of the earth's shadow (so the lighter areas were to top and left) this eclipse was below it (with lighter areas to right and below). It's difficult to say whether the colouration was any different, as of course the cloud could have affected this, but perhaps it was rather more browny-red than orangey-red this time.
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| Total lunar eclipse, 21st February 2008. This image is not very sharp as it is a stack of three 8sec time exposures at 400ASA, illustrating how dark the moon actually was as a result of both the eclipse and the cloud cover. Taken between maximum and end of totality. | Images taken just after the start of totality and right at the end. Note the fuzziness (particularly in the backgrounds) caused by the cloud cover. |
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| A composite of the two smaller images above, showing that the proper motion of the moon across the sky (and therefore through the earth's shadow) causes the lighter area to move round its limb as totality proceeds: compare it with the similar image from 2007, above. The bright spot to the left is the planet Saturn, that to top right the star Regulus. | |
And that's it for totals until 21st December 2010: an unusually long wait. Were it not for cloud (again!) the very large partial (81%) on 16th August 2008 would have been visible in the UK [maximum at 22:10 BST], just one half lunar-orbit after the total solar eclipse on 1st August. Anyone who has read my pages on eclipses might like to note that this partial was 177days after the total in 2008, as was the total in 2007 after the partial in 2006. The two totals illustrated above are separated by 354days (=2*177). And what happpened to the eclipse opportunity 177days after 3rd March 2007? There was actually a total eclipse then but it wasn't visible in the UK!
The 2010 total should provide some unusual photo opportunities as totality begins just a few minutes before moon-set. A dark red moon against landscape will be very weird so start investigating those unusual viewpoints now!
I had been looking forward to this eclipse for some time, not because it was spectacular (far from it!) but because it was unusual - it was a partially eclipsed Blue Moon. Those of you who have read my article on the subject will know that a Blue Moon is the second Full Moon in a month, and does not happen very often (about once every three years). An eclipsed Blue Moon happens much less frequently and, currently, a partially-eclipsed Blue Moon is even less common! Click here to find out more about Blue Moons.
| Things were not looking good early in the evening but, just for once, the sleet-clouds parted at exactly the right moment to allow me to get some good images when the eclipse was at its maximum extent (which was, however, only 8%!). The clouds prevented me from getting a sequence of images for a full animation, so I present here just one picture of the eclipse "as viewed". I was, however, able to demonstrate that the shadow did indeed move across the moon - place the mouse pointer over the picture to see this. The frames, deliberately over-exposed to bring out the shadowed area better, were taken 13mins apart before maximum eclipse. The images also show that the shadow edge is curved. This is only to be expected of course, as it is the shadow of a round object - the earth! |  |
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| Although the animation makes it appear that the shadow is moving, this is not the case: it is the moon that moves. In this composite I have offset the later image by the correct distance the moon moved in the 13mins and you can see that the shadowed sections join up to form a continuous curve. | By reducing the previous composite in size and adding a circle to represent the edge of the earth's shadow (to the correct scale), the fact that the moon was just cutting the very edge of the earth's shadow during the eclipse becomes obvious. Given that the moon is moving towards bottom left, it is also clear that the images were taken just before maximum eclipse, as stated. |
As I said above, partially-eclipsed Blue Moons are rather unusual so if you missed this one I'm afraid you've got another 120yrs to wait, as the next one isn't until 31st July 2129!! | |
This eclipse was unusual for two reasons - it was on the day of the northern winter solstice and it happened just as the moon set. There has only been one other eclipse on the solstice since 1 A.D., and that was in 1638. You won't have to wait quite so long for the next one though - that will be in 2097. An eclipse at moon-set was rather less auspicious, as the probability of cloud interfering with proceedings would be rather great. Still, I set the alarm for 6:30am, dressed warmly (two pairs of trousers, four top layers, a woolly hat & gloves), grabbed the camera and tripod and set off to see if I would be in luck.
Venus was shining brilliantly in the pre-dawn sky as I crunched through the frosty snow but prospects were not good - cloud on the horizon and much more all around. I caught a glimpse of the moon between clouds, showing a distinct dark crescent taken out of its left-hand side, but not much more. It seemed possible a gap might be heading in the right direction though so I carried on to my viewing spot and set up the tripod. I was eventually rewarded with about 10mins of visibility, in which I took a few shots, but the hazy conditions meant they weren't particularly good. The cloud then set in solidly, so although I waited until the time of sunrise I didn't see anything else (except for two cats and a rather surprised small dog, that is!).
Despite the lack of success, it was interesting to experience the full geometry of the eclipse, with the rising sun behind me throwing the earth's shadow towards the moon in front of me. Night-time eclipses are easier to see, but you don't get quite the same feeling of celestial alignment.
As I said, the pictures weren't brilliant, but here's two just for the record. They were taken only 5mins apart (at around 7:20am GMT) but the decrease in the non-eclipsed crescent is quite marked. Note also that the sky is beginning to lighten as dawn approaches. It is clear from the images that the eclipse was going to be total, as the eclipsed moon looks just like a "normal" crescent moon, as opposed to just having a bite taken out of it as in the cases of Sep. 2006 and Dec. 2009 above.
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The conditions for the 15th June 2011 total lunar eclipse, which was the mirror opposite in that it happened at moon-rise, were even worse unfortunately - total cloud, in fact! I had hoped for a good photo-opportunity but it was not to be. Naturally, had the eclipse been the day before or the day after things would have been fine! 'Twas ever thus.
The next total visible from the UK is on 10th December 2011 but the actual total phase will be over well before moonrise - there will only be a slight partial to see, therefore. After that, it's a long wait until 28th September 2015 when totality runs from 2:11am to 3:23am.
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