Planning for twilight phenomena observation

[c_davidson]

Gentlemen, I have a question regarding planning for twilight phenomena observation. But before some wordy stuff, sorry.

The situation: In a morning I plan for sunset phenomena observation.  I understand that below is the way to do it.:

1.  From NA find LMT for sunset, civil and nautical based on the  MOST RECENT FIX OR DR LATITUDE. ( emphasize by caps).
2. Find GMT by converting most recent fix or DR longitude into time and add/subtract it to LMT
3. Find from NA GHA of Aries for this GMT and calculate LHA of Aries
4. Pub.249 gives a list of stars as a function of LHA Aries and latitude OF THE MOST RECENT FIX OR DR.

i think that more correct approach would be:

1. Calculate assumed distance travelled considering time of civil twilight as the time of a new DR ( v* (time of last fix/DR - time of civil twilight).
2. Plot it and find latitude and longitude at the new assumed DR.
3. Using these latitude and longitude do items 1-4 from the paragraph above.

I suppose that in this case latitude and longitude would really correspond to the twilight phenomena and to LHA Aries than the values taken at the current location, presumably 10 hours earlier.

There is my question: does my guess make some sense?

Thank you

Rumata,

Being I'm an old dog that learns no new tricks your idea is a good one.  GMT is what the Sunrise, Sunset, twilight figures are based on in The Nautical Almanac.  Those figures are for the events at Greenwich but can be acceptably used for any LMT (taking into account DST or ST).

I assume you're trying to get ready for evening twilight observations and making all of your preparations in the morning (local time) prior to the event. 

1- Yes, determine in advance, approximate DR Longitude and Latitude of where you expect to be when Sunset, Twilight etc. occur but calculate it based on GMT adding or subtracting your DR Longitude from or to it.

2- Determine GHA Aries of the time you figured in my paragraph 1 above.

3- Determine LHA Aries based on your expected DR Longitude of my step 1 above.  No matter what you determine in advance your figures will not be accurate but will be usable for when you shoot.

4- Use Pub. No. 249 to look up the stars that are usable at the LHA of my step 3 above.

Your approach is identical to mine but maybe mine will help clarify?

In truth Sunset, Sunrise & all Twilights are very loose figures as we are the ones using our eyes to look at it.  Those figures are mathematically determined to position the Sun a certain number of degrees and minutes below the horizon.  That's good but what about a hazy horizon?  The Nautical Almanac SAYS nautical or civil twilight ends now!  But there's no horizon to see!  Nuts!


Does that help you any, what I wrote?  I hope I understood your question correctly.

Clen