NavList:

I have really enjoyed the recent round of posts about Lunars, which also served as a reminder that I have not tried shooting a lunar in about a year.

Last night I thought I would try my hand at shooting a set of Regulus-Moon (Near) lunars with the added twist of shooting the altitude of the Moon and Regulus with a bubble attachment before- and after- measuring Lunar Distances.  I used the before- and after- measurements to simulate a situation in which I did not have a chronometer.   The thought was to use the before- and after altitudes of the Moon and Regulus in Frank's Lunar App for comparison with just taking the lunar distance and having the App calculate the altitudes.  I was certainly surprised at the difference in results.

    Using just the average of four lunar distances (see details below) I was very pleased to an error in Lunar of -0.03' and and error in Longitude of 0.9'  - certainly the best lunars that I have ever shot and probably just a stroke of luck.

    By comparison, when I used the average altitudes of Regulus and the Moon in Frank's App, I got the more disappointing result of an error in Lunar of 0.7' and an error in Longitude of 19.8'

This result lead me to wonder how navigators or map makers (thinking of Thomson or Powell here) would have taken these shots if they were by themselves and did not have assistants simultaneously shooting the altitude of the star/sun and the moon?

Does anyone in this group know what the best practice was when Lunars were popular (guessing that Frank will know!!).   About 20 years ago I remember shooting lunars by interleaving shots as: star/sun altitude then moon altitude then lunar distance then repeat.  I am going to dig that old note book out and see if I get between consistency.

Incidently: this was my first time using Frank's App.   I must say that I think it is fantastic and I will be much more inclined to add lunars to my celestial practice again going forward.  That being said, I also want to try my hand at using Thomson's tables and hand calculating at least one time just to broaden my horizons. 

I welcome any feed back.

Best regards,

Greg

Here are the raw data from my shots last night (times in GMT) for those who might be interested:

    Shooting from Rio Rancho, NM, Lat 35° 18.0' N  Long 106° 35.0' W

     The air pressure in Rio Rancho (ca. 5280 ft above Sea Level) is about 0.83x of Sea Level.   I used 29.8x0.83 = 24.73 inches of pressure and 45°F (temp when I was shooting last night) in Frank's Lunar App. 

    Lunar distance measured with a Cassens and Plath Standard Sextant with a 6x30 scope, IC=-0.2'

    Altitude of Regulus and the Moon measured with an Celestaire Astra Pro with a Cassens and Plath Professional Bubble Horizon attachement, IC=21.6.  I usually get within ca. +/-2' or so with this unit averaging seven shots, but I used less shots last night in      the interests of expediency (had to get up and good to work early today).  This is also why I shot the moon at such a low altitude, which I would normally avoid. 

    Regulus  (pre, adjusted for IC)

       02:42:35   57° 59.4

       02:43:59   58° 07.4

       02:45:01  58° 14.6

     Moon LL (pre, adjusted for IC)

       02:49:03   16° 35.4

       02:50:09   16° 49.2

       02:52:35   17° 27.1

     Regulus-Moon (near) NOT adjusted for IC

       02:57:20   45° 40.85

       02:58:35   45° 41.1

       02:59:41  45° 41.3

       03:00:53  45° 41.6

     Moon LL (post, adjusted for IC)

       03:02:38   18° 57.3

       03:03:47   19° 31.0

       03:05:06   19° 46.0

       03:06:10   19° 47.2

     Regulus (post, adjusted for IC)

       02:42:35   57° 37.8

       02:43:59   57° 45.8

       02:45:01  57° 53.0

       02:45:01  57° 53.0

Note that I used sets of three shots for the pre-lunar altitudes and sets of four shots for the post-lunar altitudes.  This was not on purpose: I originally meant to maintain a symmetry in these shots to center the average of altitudes near the average time of the lunars.   This may have skewed my results a bit.