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Observing Guide to Rimae Sirsalis, the Longest Rille on the Moon

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#1 Tom Glenn

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Posted 17 November 2018 - 01:16 AM

Rimae Sirsalis is the longest rille on the Moon, at over 400km in length.  Unlike many lunar rilles, Rimae Sirsalis passes almost exclusively over highland terrain.  The feature is best viewed approximately 1-2 days before the Full Moon (although the exact time depends upon libration).  My overview image of the area (below) is from September of this year, while the close-up annotated view was taken in December of last year (next post due to file size limitations).  The Full Moon is increasing declination right now, so if you live in the Northern Hemisphere and happen to get lucky with winter weather, the next few lunar cycles are a good time to view this feature.

 

Sirsalis_overview_TG.jpg

 

Observation Notes:

 

This upcoming week will be the next available time to view Rimae Sirsalis, and the feature will be nearly completely visible starting on November 21, 2018 at 00:00UT (although the very western-most portion will still be in shadow at that time, as will the crater Darwin).  On November 21, 2018 at 12:00UT, the longitude of the terminator will be -73W, which is very similar to that in my images.  And it just so happens that there will also be a libration to the West of over 5 degrees at that time, which will be helpful.  The easiest way to find Rimae Sirsalis is to locate the large lava-filled Grimaldi basin, and then travel to the South and East.  Another landmark to help orient yourself is the dark lava-filled crater Cruger.

 

Note how the main rille of Rimae Sirsalis passes over several older craters, while more recent craters have impacted upon the rille.  Especially noteworthy is where the rille traverses the crater De Vico A, and descends the southwestern wall of the crater, cuts across the floor, and then ascends the northeastern wall.  In contrast, crater De Vico AA is much younger and impacts upon the rille.  Craters Sirsalis F and J also impact upon the rille, and nearly wipe out any trace of it for about 30km.  In several locations, older rilles can be observed to intersect the main rille, including the Rimae Darwin system near the southwestern terminus. 

 

Background Information and Current Research:

 

The precise origins of Rimae Sirsalis are unknown, although it is currently thought to have formed via dike propagation, in which vertical sheets of magma (dikes), exert their influence on the surface.  It is notable that the rille is not associated with mare lava flows in the way that many sinuous rilles are, as it travels over the highlands with no obvious lava flows surrounding it.  Many straight rilles are associated with stress fractures, either from differential cooling, or from large impacts.  Mare Orientale is only about 1000km away from Rimae Sirsalis, although the rille is not oriented either radially or tangentially to the Mare, so it is unlikely to be associated with the Orientale impact.  Additionally, the age doesn’t seem to be consistent with that hypothesis, as Rimae Sirsalis is likely older than the Oriental impact.  Interestingly, the rille is radial to the Imbrium basin, and so could potentially be affiliated with that impact, although whether this is the case remains uncertain. 

 

Another intriguing aspect of Rimae Sirsalis is a strong magnetic anomaly associated with the feature.  The Moon’s surface is largely non-magnetic, and today the Moon has no magnetic field.  However, subsatellites released by Apollo 15 and 16 measured localized magnetic fields over a variety of lunar features (including Rimae Sirsalis) that exceeded the average levels by several orders of magnitude.  The origins of these magnetic anomalies have remained an area of active research, but current research suggests that the Moon at one time had a magnetic field.  This ancient magnetic field was about as strong as the Earth’s magnetic field today, but disappeared several billon years ago.  When molten rock containing iron cools in the presence of a magnetic field, it can become magnetized.  This means that the magnetic material associated with (or beneath) Rimae Sirsalis cooled in the presence of the ancient lunar magnetic field, but does not clarify whether the origin of the material was from an impactor (such as Imbrium) or subterranean dikes.  Further lunar exploration will be required to resolve these questions. 

 

References for further reading on lunar magnetic anomalies and ancient lunar magnetic fields are below (unfortunately some of these are abstracts only without a subscription, but the abstracts are still interesting):

 

http://science.scien...46/6214/1246753
http://advances.scie...nt/3/8/e1700207
http://science.scien...t/335/6073/1212


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#2 Tom Glenn

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Posted 17 November 2018 - 01:17 AM

Please click for full size. 

 

Rimae_Sirsalis_detail_TG.jpg


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#3 dtmracer

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Posted 17 November 2018 - 08:39 AM

Interesting stuff, Tom, Thanks!



#4 Tyson M

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Posted 17 November 2018 - 10:31 PM

Excellent image and write up!  I viewed the moon last night but with no specific targets in mind.

 

Hopefully I can get clear skies around the 21st, I shall look for the Rille. 



#5 Tom Glenn

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Posted 19 November 2018 - 12:34 AM

Thanks for the comments dtmracer and Tyson.

 

Tyson, I wish you good luck with the weather.  I wanted to make this post before the upcoming opportunity to view this region.  I've always found that it's more enjoyable to observe the Moon if you have specific targets in mind.  If you don't luck out with the weather, the next opportunity will be December 21, 2018 at 00:00UT.  Slightly before or after is OK, as the feature is very long in length.  I'm setting a terminator position of -72W as the approximate time at which the western rim of the crater Darwin is in sunlight.  Up to 12 hours before this time you can still get a partial view of Rimae Sirsalis, although the western half of it will still be hidden.  January 19, 2019 at 15:00UT would also be similar.  


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#6 Tom Glenn

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Posted 21 November 2018 - 03:00 AM

A quick update.  I had clear skies and was able to view Rimae Sirsalis tonight at 07:00UT (Nov. 21).  I did take an image but I don't know the quality yet (will post if it's decent).  But I also wanted to see what the region looked like at 60x in my 85mm spotting scope.  Well, I can report that some of it is visible at 60x, but it's not an ideal view.  At 60x, you can see very nice texture on the floor of Grimaldi, and finding Cruger and Darwin is easy.  From there, the most obvious sign of Rimae Sirsalis was near its western terminus where it travels through an unnamed crater east of Darwin.  In this region, the rille is rather wide (over 3km), and tonight this region was very near the terminator so it was high contrast.  Aside from this region, it was very difficult to see any other parts of the rille at 60x.  The crater De Vico A was obvious, but I couldn't see the rille going through it in the spotting scope.  Occasionally I could see a glimpse of other parts of the rille to the northeast, but these were fleeting and not as obvious as the western-most portion.  I would think that at 100x and above, one could achieve a pretty decent view of the region even in small scopes, although I didn't have the time tonight to also set up my 4.5 inch scope to test this.  Hopefully some others got to see the region tonight (and into tomorrow, as it should be very good lighting for the next 12 hours, and even 24 hours from now you should see something although it will be reduced in contrast).  


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#7 Tom Glenn

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Posted 21 November 2018 - 06:31 PM

Here is the overview of the terminator region including Rimae Sirsalis from last night.

 

Sirsalis_112118_TG.jpg


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#8 Kutno

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Posted 22 November 2018 - 12:03 PM

 

Another intriguing aspect of Rimae Sirsalis is a strong magnetic anomaly associated with the feature.

Hmmm.  Maybe someone moved the monolith.  smile.gif 

 

All kidding aside, I really enjoyed the reads and pictures.  Thank you.



#9 aeroman4907

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Posted 23 November 2018 - 09:40 AM

Thanks for posting Tom.  I found I captured a decent image of this rille the other night.



#10 Tom Glenn

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Posted 23 November 2018 - 11:23 PM

Thanks for posting Tom.  I found I captured a decent image of this rille the other night.

Thanks.  When I saw your image in the other forum I immediately thought of Rimae Sirsalis because I knew you had the right phase.  



#11 Tom Glenn

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Posted 20 December 2018 - 04:19 AM

Just a heads up that Rimae Sirsalis is coming into view right now, and will be optimal for observation for about the next 24 hours.  In addition, the near-Full Moon is very high in the sky for northern observers at this time of year, even at high latitudes.  


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#12 aeroman4907

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Posted 21 December 2018 - 01:00 PM

Hi Tom,

 

Here is a reprocessed color image from last month.

 

Rimae-Sirsalis.jpg

 


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#13 aeroman4907

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Posted 21 December 2018 - 01:02 PM

Uggg… curse of the dark presentation.  Here is a brighter version.

 

Rimae-Sirsalis.jpg


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#14 Tom Glenn

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Posted 22 December 2018 - 12:04 AM

Nice images Aeroman.  Thanks for contributing.  I would love it if this thread stayed alive every so often with additional contributions to document the appearance of Rimae Sirsalis at a variety of lunar phases and with varying equipment.  


Edited by Tom Glenn, 22 December 2018 - 12:05 AM.


#15 Tom Glenn

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Posted 22 December 2018 - 02:03 AM

I took this image on December 19, 2018 (December 20 in Universal Time).  West is at top and North is to the right.  Gassendi is in the lower left, and Rimae Sirsalis is at the terminator near the top-center, and was only partially illuminated at this time.  Sirsalis F and J are prominent near the center of the exposed portion of the rille, and the feature fades into the lunar night before reaching De Vico A.   I was unable to image the following night, so this will be my only contribution for the month.  

 

Rimae_Sirsalis_122018_TG.jpg


Edited by Tom Glenn, 22 December 2018 - 02:04 AM.

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#16 Howard Fink

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Posted 13 November 2019 - 02:33 PM

Here is a synthetic image of the southern area below Cruger crater generated from topographic data collected by the LRO and Kaguya missions that show Rimae Darwin and half of Rimae Sirsalis.

 

Howard Fink


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#17 Tom Glenn

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Posted 04 February 2020 - 04:44 AM

I haven't revisited this tread in a while, but I realized that what is missing here is a list of times that people might observe Rimae Sirsalis.  Below is a list of times, generated in LTVT, in which the lighting conditions present in the image in post #2 are approximately recreated, through the end of 2021.  Because the feature is so long in length, there is quite a bit of flexibility here, unlike some other lunar features.  Anytime slightly before or after these listed times (by about a day or so) would still be suitable times to see something of the feature.  The next available time is coming up in three days on February 7, 2020.  Note that the phases at which these times occur vary from 96% to 99% illumination, depending on libration, and provide an interesting target to try and observe just before the Full Moon.  

 

Rimae-Sirsalis-times-2020-2021.jpg


Edited by Tom Glenn, 04 February 2020 - 04:46 AM.

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#18 astrochef

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Posted 05 February 2020 - 04:49 PM

     It's a region that I always enjoy observing.  Even when I don't get good views of Rimae Sirsalis, there is always eye candy in the area.  I enjoy the duo of Billy and Hanstein.  Gassendi is always a pleasure.  One evening I enjoyed watching shadows from peaks along Grimaldi's East rim slowly stretching out onto it's floor outstretched fingers.  I could go on and on.  

     It's nice to see this thread get a bump.  I won't have to dig so far for it now.  I am not a real imager but one of these days I'm going to get a decent cell phone pic, including the rille, to add to this thread.  

If these clouds ever part.

Regards,

Ty


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#19 Tom Glenn

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Posted 09 February 2020 - 01:24 AM

Here is the region from yesterday, February 8, 2020 at 07:02UT, colongitude 81 degrees.  The sun is a few degrees higher than in my previous images, but you can still clearly see Rimae Sirsalis.  This also blows apart the myth that you can't see any interesting details on the Moon near the Full Moon.  The illumination fraction was 98.5%, and the terminator was still full of details.  

 

Rimae-Sirsalis-02-08-20-0702UT-TG.jpg


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#20 B l a k S t a r

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Posted 09 February 2020 - 10:33 AM

Wondering how the co part of colongitude came to be. Perhaps when used it's just indicating the longitude "cohabitating" the degrees longitude at the moment? ie colongitude 42°.  So co = at ?



#21 astrolexi

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Posted 15 February 2020 - 10:01 AM

This is an image showing Rimae Sirsalis from last week, February 7, 2020 at 21:40 UT.

The illumination fraction was 97.4%.

 

Captured with an ASI178MM and a 685nm IR-pass filter on a C8. Binning was 2x2.

800 frames stacked.

 

Although they are not very spectacular I especially like the dark floors of Cruger and Billy.

 

Best wishes

Klaus

 

Moon_224045_Rimae_Sirsalis.jpg

 


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#22 astrochef

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Posted 06 April 2020 - 01:50 PM

Well,I finally captured this Rille on my cell phone.  Last night, April 5, around 10:45 pm EDT.  The Seeing had been in and out for a couple hours and I had just about given up on getting a pic.  Had some really nice views over that time visually.  Rimae Sirsalis as well as the Billy and Hanstein pair.  A little further down the terminator, Schickard and Wargentin stealing the show.  Even had a pretty nice view of Mons Rumker up North.  It's just a single frame cell phone shot so I really should post this in the cell phone thread but I've been trying so long to contribute to this one I just had to put it here.  It was just so nice to end the observing drought that I've endured this Winter.  

 

Regards,

Ty

 

 

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  • Rima Sirsalis.jpg

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