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Moon phase with libration in the past

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#1 danmdak

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Posted 15 August 2020 - 04:44 PM

OK so i am really trying to determine the EXACT date I got started in amateur astronomy. Using a 40X Gilbert 2 1/2" reflector and a moon map my 8th grade science teacher gave me, I examined the Moon in late April of 1974 and was hooked. Using the map (I still have it) I thought I "discovered" a new crater and excitedly told my science teacher the next day! I now know it was the crater "Dembowski" near Agrippa and Godin. Agrippa and Godin are on this map, but Dembowski wasnt. Now the fact that I could pick up Dembowski in THAT telescope may be the clue I need to pinpoint the date....maybe. April 29, 1974 or April 30, 1974 are both feasible candidates. I certainly can find the actual phase of the moon, but I need to see exactly where the terminator was on both of those dates in relation to Dembowski and of course libration can severely affect that. Anyone know of a website that takes libration into account where I can type in the date and see what the moon actually looked like from my location on that 47 year ago night? I want to determine this before my 50th anniversary of observing rolls around. I plan also on going to the public library and getting the old newspaper from that date to make sure the weather was clear on the night in question.


Edited by danmdak, 15 August 2020 - 04:45 PM.


#2 ButterFly

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Posted 15 August 2020 - 05:01 PM

http://www.internets...o/MoonCalc.html

 

You can only go from time to selenographic longitude of terminator, for obvious reasons.  Try some dates and times around late April 1974.


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#3 frank5817

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Posted 15 August 2020 - 07:06 PM

Probably April 28th 1974 if it was on or near the terminator. The next day if it was well away from the terminator.

 

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#4 danmdak

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Posted 15 August 2020 - 08:18 PM

Probably April 28th 1974 if it was on or near the terminator. The next day if it was well away from the terminator.

 

Frank

It was a little bit away from the terminator. Plus April 28 was a Sunday. I observed it the same day the teacher gave me the moon map.



#5 danmdak

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Posted 15 August 2020 - 08:37 PM

http://www.internets...o/MoonCalc.html

 

You can only go from time to selenographic longitude of terminator, for obvious reasons.  Try some dates and times around late April 1974.

Based on this the 29th the terminator was 14 degrees away, and on the 30th it was about 26 degrees away. Looks more and more like it was the 29th. Thanks for the link!! I'll find a similar phase/libration and make sure to view the moon that night (unfortunately I cant use the same telescope because I reground it into a little 2 1/2" f/6 scope, then got rid of it).


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#6 Tom Glenn

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Posted 16 August 2020 - 01:03 AM

There are numerous websites that can help you.  The JPL Horizons ephemeris is probably the best source for that data.

 

https://ssd.jpl.nasa.gov/horizons.cgi

 

I used your estimated date of April 29, 1974, since that is a Monday and would be compatible with the school schedule.  Copied below is a portion of the ephemeris data for that day from JPL.  Note the date is April 30, 1974 because the times are listed in Universal Time (UT), and would correspond to viewing times for the First Quarter Moon in the evening from U.S. time zones.  The table shows illuminated %, as well as the sub-observer LAT and LON (which corresponds to libration) and the sub-solar LAT and LON.

 

moon_ephemeris_04301974.jpg

 

Using an estimate of 01:00UT, I used the Lunar Terminator Visualization Tool (LTVT) to produce the view of the Moon you would have using those data.

 

https://github.com/fermigas/ltvt/wiki

 

Note you can download LTVT and use it yourself, although the program requires some study to get working.  

 

LTVT_04301974.jpg

 

LTVT can produce tables of recurring situations.  Because your crater of interest is located near the apparent center of the lunar disk, I would not be worried about libration, because the perspective won't change much at all, and solar illumination is completely independent of libration.  What will change with libration, however, is the illuminated fraction % at which your observation will occur.  the table below shows all instances of the same colongitude that occur until the end of 2021.

 

LTVT_table.jpg

 

Note that all times are listed in Universal Time, and so may not be visible for you.  However, because your crater is centrally located, you have a wide latitude of viewing conditions that will be "close enough".  Another excellent resource for predictions is the NASA "Dial a Moon" website.

 

https://svs.gsfc.nasa.gov/4768

 

You can enter any time during 2020 and get a simulated view of the Moon, with reasonably high resolution, and very accurate shadow simulations.  This website is updated each year, but does not go back in time more than about a decade (you will have to Google for previous iterations of this website). 


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#7 ButterFly

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Posted 16 August 2020 - 05:15 AM

Calsky will show you the terminator for time/location, but doesn't give its selenographic longitude (or colongitude), for some reason.


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#8 danmdak

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Posted 16 August 2020 - 08:08 AM

Wow! thanks guys!! Tom, I have a question for you. You mentioned libration wouldn't matter much. Why not? Wouldn't libration change how far away from the terminator the crater is? For example, lets take a 50% sunlit disk with maximum west libration versus a 50% sunlit disk and maximum east libration.....  a crater near the center would either be in sunlight or not in sunlight depending on the libration. Or am I missing something?


Edited by danmdak, 16 August 2020 - 08:16 AM.


#9 Tom Glenn

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Posted 16 August 2020 - 01:11 PM

Wow! thanks guys!! Tom, I have a question for you. You mentioned libration wouldn't matter much. Why not? Wouldn't libration change how far away from the terminator the crater is? For example, lets take a 50% sunlit disk with maximum west libration versus a 50% sunlit disk and maximum east libration.....  a crater near the center would either be in sunlight or not in sunlight depending on the libration. Or am I missing something?

No, you have it right, but what's the important aspect of your observation?  Is it to see the crater Dembowski under similar lighting conditions to when you observed it in 1974, or is it something else?  If the former, then the only thing that really matters is proximity to the terminator, which is independent of libration.  If you look at the table above, you will see that although the colongitudes are identical for all the listed times, the phase % illuminated varies from 50.239 to 63.141.  This changes with libration, with the 50% value occurring at -7.8  degrees libration in longitude, and the 63% phases occurring at +7.329 degrees.  But the phase % doesn't affect where the terminator is located, and so the lighting on your crater would look the same at both of these times.  If you were really concerned with getting an exact match, then you would need to look at the position of the sub solar point.  But this mainly affects the appearance of features in polar regions, because of the very slight axial tilt of the Moon with respect to the ecliptic plane.  Shadows near the equator will look essentially identical during visual observation.  But if you wanted a match in phase, libration, and solar illumination angle, you would have to observe at intervals corresponding to a Saros period of 18 years, 11 days, 8 hours.  But that is necessarily restrictive just to see your crater in similar lighting to your initial observation! 



#10 danmdak

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Posted 16 August 2020 - 01:22 PM

Ok I get it now. What I am after is where the terminator was in relation to the crater. With that toy telescope i used, and believe me it was a toy (years earlier my brother and I used it as a pretend bazooka when we played army), a partial crater like Dembowski had to be quite near the terminator for me to pick it up. I am now almost sure it is the night of Monday April 29 1974 that I had my first astronomical view. I jammed the "mount" of the telescope on top of 3 hay bales to get the view as the telescope had no tripod. How far things have come! Thanks again.



#11 ButterFly

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Posted 16 August 2020 - 05:51 PM

The day to day motion of the terminator is much greater than the effect of libration.  Libration will get swamped because you have a specific month in mind.




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