Is There A Math Formula For Deriving Sun's GHA @ UT?

[charliehows]

i"ve worked out how to use a common spreadsheet to convert degree inputs to radians and then apply the spherical trigonometric equations in order to derive computed altitude and azimuth. This means i dont need the huge sight reduction tables.
My question is; is there a mathematical formula for the sun's GHA @ UT or is this necessarily based on observation?

[El Pinguino]

Yes there is and it a shame you didn't ask this question before I left the campo last week... I'm away from my books now.

However... they can be found in this book and elsewhere.... Astro Navigation by Pocket Computer | Trade Me

Very simple they are to...

[barnakiel]

Yes. There is a formula from a number of factors. Google your friend.

Otherwise how do you think they make printed almanacs?

b.

[charliehows]

thx, I'll go see mr magoo(gle)

[wsmurdoch]

Try Cruising World Magazine, March 1996, page 48. https://books.google.com/books?id=yJ...urdoch&f=false

[jbinbi]

Originally Posted by charliehows

i"ve worked out how to use a common spreadsheet to convert degree inputs to radians and then apply the spherical trigonometric equations in order to derive computed altitude and azimuth. This means i dont need the huge sight reduction tables.
My question is; is there a mathematical formula for the sun's GHA @ UT or is this necessarily based on observation?

Would you be willing to post the formula, or upload the sheet?

Sent from my SM-G900V using Cruisers Sailing Forum mobile app

[wsmurdoch]

I put this together years and years ago. It has been a long while since I have used it. I am not sure that it makes the date to time calculation correctly after the year 2000, but you can try it. It is what is behind the Cruising World article I mentioned earlier.

[barnakiel]

The formulas are multifactor but for quick and dirty solutions (e.g. in survival navigation) you can consider declination as a sine function with period equal to a year and extremes at tropics. Easy with any calculator BUT you do have to memorise some sine values if you want to calculate ad hoc in the field, with ball pen and paper.

https://en.wikipedia.org/wiki/Position_of_the_Sun


GHA can similarly be calculated from a linear approximation. Your main corrective factor comes from equation of time:

https://en.wikipedia.org/wiki/Equation_of_time

I think this place explains things in a neat and not too complicated way:

Part 3: Calculating Solar Angles | ITACA

etc.

b.

[kish]

Originally Posted by charliehows

i"
My question is; is there a mathematical formula for the sun's GHA @ UT or is this necessarily based on observation?

Do not reinvent the wheel!

You can buy the official Nautical Almanac, Nautical Publication Number: NP 314 The Nautical Almanac, which tabulates all material needed for astro.

You can download a free Nautical Almanac look alike from:

"The Online Nautical Almanac" which tabulates GHA and other essential data.

The site Navigator Software is a very useful and reliable source for astro navigation information.

[Adelie]

Post the question to the NavList folks: http://www.fer3.com/arc/


A house is but a boat so poorly built and so firmly run aground you would never try to refloat it.

[KJThomas]

All you need is the number called "the equation of time" for the day. Calculate Gha from the difference between the equation of time and your clock time.
Declination is almost the same at the same times of the year.
However Nories tables and the current nautical almanack gives everything you need to reduce sights. Including trig tables for when your calculator blows up.

[charliehows]

My main reason for poking at this is the online sunsight reduction tool that I've been playing with doesnt have a downloadable version - bit annoying having to connect up just to do a bit of quick and dirty math, my version does pretty much the same, but the next obvious step is deriving GHA, LHA dec. etc. I think i've found a reasonable formula that doesnt require delving into VSOP tables, which will be accurate enough. As for putting up a downloadable version - if i can make it effective and presentable i'll have a go but unfortunately i dont have a website i can park it at so may not happen. As for the almanac - of course i have them on my laptop also but a spreadsheet shifts the whole process into time overdrive - the business of crawling through the huge ho229 tables can be replaced by half a dozen cells of formulae and executed in no time.

[StuM]

You may find some useful information here:
Computing planetary positions

[StuM]

I think you may find the Excel formulas you need in this NOAA spreadsheet:

http://www.esrl.noaa.gov/gmd/grad/so...ations_day.xls

Details here: ESRL Global Monitoring Division - Global Radiation Group

[jbinbi]

Originally Posted by charliehows

but a spreadsheet shifts the whole process into time overdrive - the business of crawling through the huge ho229 tables can be replaced by half a dozen cells of formulae and executed in no time.

+ Million!!!! couldn't have said it better than this.