Astro calculations

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davidej

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To while away wet and windy Monday evenings, I have signed up for the Astro course at the yacht club.

I get most of the theory behind it, but being accurate and reliable with all those calculation is beyond my ancient brain.
I thus tried to write an Excel spreadsheet program to do them. In principle is is not too difficult, but it seems to be very easy to make mistakes with North south east and west and all the signs involved with them.

Has anyone succeeded with this and if so, can I have a copy?

Alternatively anyone is welcome to see my "work in progress" and improve on it.
 
davidej said:
To while away wet and windy Monday evenings, I have signed up for the Astro course at the yacht club.

I get most of the theory behind it, but being accurate and reliable with all those calculation is beyond my ancient brain.
I thus tried to write an Excel spreadsheet program to do them. In principle is is not too difficult, but it seems to be very easy to make mistakes with North south east and west and all the signs involved with them.

Has anyone succeeded with this and if so, can I have a copy?

Alternatively anyone is welcome to see my "work in progress" and improve on it.
Click to expand...

Why do people try to reinvent the wheel, when HMSO print proforma's used for years by the Mercantile Marine?
 
Dealing with N,S,E and W requires the use of a consistent and reliable sign convention for the angles concerned. If you make N and E angles positive and W and S angles negative, you will find that the mathematics works. This convention works in all circumstances IF applied accurately and rigorously.
 
davidej said:
To while away wet and windy Monday evenings, I have signed up for the Astro course at the yacht club.

I get most of the theory behind it, but being accurate and reliable with all those calculation is beyond my ancient brain.
I thus tried to write an Excel spreadsheet program to do them. In principle is is not too difficult, but it seems to be very easy to make mistakes with North south east and west and all the signs involved with them.

Has anyone succeeded with this and if so, can I have a copy?

Alternatively anyone is welcome to see my "work in progress" and improve on it.
Click to expand...

Yes, it's call a gps.

Surely the point of astro these days is to not to use computers /calculators.... What happens when your electrics get taken out by lightning?
 
temptress said:
Surely the point of astro these days is to not to use computers /calculators.... What happens when your electrics get taken out by lightning?
Click to expand...

Its only a bit of adding up and taking away.

Have three pencils. One in use, one spare and one to snap when you realise youve added up where you should have taken away and have to start again.
 
davidej said:
I get most of the theory behind it, but being accurate and reliable with all those calculation is beyond my ancient brain.
Click to expand...
Well worth making up your own proforma sheet makes you think through each step much more . Also , opencpn has a good astro plugin if you want to get some practice in without working out each sight by hand and not knowing where any errors are - there's a comprehensive page of data for each sight you could compare with your own, though unfortunately mostly decimal -

Almanac Data For SunGeographical Position (lat, lon) = -16.0179 25.9043
GHAAST = 195 34.2'
SHA = 138 31.5'
GHA = 334 5.7'
Dec = S 16 1.1'
SD = 16.1'
HP = 0.1'


Formulas used to calculate sight


Index Error is 0.0333 degrees


Eye Height is 2.0000 meters
Height Correction Degrees = 1.758*sqrt(2.0000) / 60.0
Height Correction Degrees = 0.0414


Apparent Altitude (Ha)
ApparentAltitude = Measurement - IndexCorrection - EyeHeightCorrection
ApparentAltitude = 27.8617 - 0.0333 - 0.0414
ApparentAltitude = 27.7869


Refraction Correction
x = tan(Pi/180*ApparentAltitude + 4.848e-2*(Pi/180) / (tan(Pi/180*ApparentAltitude) + .028))
x = tan(Pi/180*27.7869 + 4.848e-2*(Pi/180) / (tan(Pi/180*27.7869) + .028))
x = 0.5289
RefractionCorrection = .267 * Pressure / (x*(Temperature + 273.15)) / 60.0
RefractionCorrection = .267 * 1020.0000 / (x*(20.0000 + 273.15)) / 60.0
RefractionCorrection = 0.0293


Sun selected, Limb Correction
ra = 0.9913, lc = 0.266564/ra = 0.2689
Lower Limb
LimbCorrection = -0.2689


Corrected Altitude
CorrectedAltitude = ApparentAltitude - RefractionCorrection - LimbCorrection
CorrectedAltitude = 27.7869 - 0.0293 - -0.2689
CorrectedAltitude = 28.0265


Sun selected, parallax correction
rad = 0.9913, HP = 0.002442/rad = 0.0025
ParallaxCorrection = -180/Pi * asin( sin(Pi/180 * HP ) * cos(Pi/180 * CorrectedAltitude))
ParallaxCorrection = -180/Pi * asin( sin(Pi/180 * 0.0025 ) * cos(Pi/180 * 28.0265))
ParallaxCorrection = -0.0022


Observed Altitude (Ho)
ObservedAltitude = CorrectedAltitude - ParallaxCorrection
ObservedAltitude = 28.0265 - -0.0022
ObservedAltitude = 28.0287
Click to expand...
 
GHA said:
Well worth making up your own proforma sheet makes you think through each step much more . Also , opencpn has a good astro plugin if you want to get some practice in without working out each sight by hand and not knowing where any errors are - there's a comprehensive page of data for each sight you could compare with your own, though unfortunately mostly decimal -
Click to expand...

EEK!
 
capnsensible said:
Your #13.
Click to expand...
What about it?
Some corrections & body almanac data in there to check your own if you want, a moment on the web to convert to ddd:mm . Well you know how it works already so you won't want ;)

If anyone else wants a quick method to check the sextant readings without all the calc then it's there for free, if not then you don't have to. :cool:
 
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