Planning for twilight phenomena observation

[CelNav57]

Rumata had posted this question on another thread but I've moved and made a specific thread for his question as follows;



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

[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

[EdCa]

Rumata- you've got a great idea which isn't that complex to solve, just like you wrote about above.

My guess is you could determine, very, very roughly, longitude by the time of sunset or twilight. Granted, it would be a very rough figure but you might be able to make a pretty good approximation. Maybe +/- 5° ?? And then you could further narrow the figure to a more precise longitude figure.

Good idea you have!

Ed

[Rumata]

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

Clen , thank you for the detailed reply.  I think we're on the same page. And yes, in general youyr and my approaches similar. Your reply made my post more accurate by including your paragraph 1. Thanks again

---

Rumata- you've got a great idea which isn't that complex to solve, just like you wrote about above.

My guess is you could determine, very, very roughly, longitude by the time of sunset or twilight.  Granted, it would be a very rough figure but you might be able to make a pretty good approximation.  Maybe +/- 5°  ??  And then you could further narrow the figure to a more precise longitude figure.

Good idea you have!

Ed

Thank you, Ed. It  "...you could determine, very, very roughly, longitude by the time of sunset or twilight." was my intention.  And, more importantly,  I think the method we're discussing, is more logical.

[P.Rutherford]

Gentlemen,

Perhaps for a refinement in determining longitude by, say, Sunset, you could use the Altitude Correction Tables- https://thenauticalalmanac.com/Altitude%...0Table.pdf

....using page 2 of the Table.

Also, in each of The Nautical Almanacs on this site CelNav57 put in an explanation of The Nautical Almanac. I think it's on page 7 & 8 of each Almanac

"Sunset- the approximate GMT/UT when the Sun is 0° 50' (semi-diameter plus refraction) below the
horizon. First locate your approximate Latitude in the Lat. column and then follow across horizontally to
the right to find the time."

The problem might be clearly determining exactly when Sunset occurs. "Below the horizon" will probably trick our eyes as refraction is pretty large. "On the hard" will be difficult as a landscape horizon is so variable. Sea horizon is more cooperative.

Nice work Rumata!

Paul (it's too hot to sail today)

But....in difficult straits it might be the only way you could get a clear idea of the time.

[Rumata]

Gentlemen,

Perhaps for a refinement in determining longitude by, say, Sunset, you could use the Altitude Correction Tables- https://thenauticalalmanac.com/Altitude%...0Table.pdf

....using page 2 of the Table.

Also, in each of The Nautical Almanacs on this site CelNav57 put in an explanation of The Nautical Almanac. I think it's on page 7 & 8 of each Almanac

"Sunset- the approximate GMT/UT when the Sun is 0° 50' (semi-diameter plus refraction) below the
horizon. First locate your approximate Latitude in the Lat. column and then follow across horizontally to
the right to find the time."

The problem might be clearly determining exactly when Sunset occurs.  "Below the horizon" will probably trick our eyes as refraction is pretty large.  "On the hard" will be difficult as a landscape horizon is so variable.  Sea horizon is more cooperative.

Nice work Rumata!

Paul (it's too hot to sail today)

But....in difficult straits it might be the only way you could get a clear idea of the time.

Thank you, P. Rutherford.