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Old 07-16-2019, 06:59 PM   #16
Fidgety
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Thanks, I did see that save button before but didnt know it would reload nav marks or super marks on the map
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Old 07-19-2019, 04:53 PM   #17
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oops
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Old 07-20-2019, 12:34 AM   #18
Nathaniel B.
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Sorry I didn't respond earlier. I somehow missed your posts. If you're still looking for some answers ...



Quote:
Originally Posted by Fidgety View Post
OK I have something thats bugging me and I cant figure it out yet from 3 websites I have been to.


On this tutorial > https://www.subsim.com/radioroom/sho...d.php?t=206103



first we need to find Zenith Distance = 90 - 42.54 we convert the 90 degree to 89.60 - 42.54 = 47.06 is the Zenith Distance



From everything I have read so far, the zenith distance is always 90 degrees minus the altitude angle of the celestial object you measure. I cannot figure out why in this tutorial he converts the 90 to 89.60 before subtracting the altitude angle.


This is just a way of simplifying the calculation. 89°60' is equal to 90°00' (there are 60 minutes in a degree). When subtracting 42°54' from 90°00', you would have to do this anyway - you can't subtract the 54' from 00', so you would borrow 60' from 90°. Another practice that was common during the time celestial navigation was used was to apply a simple correction to a sun sight of +12' for a lower limb sight or -20' for an upper limb sight. This was a "combined correction" including refraction, semi-diameter and dip of the horizon. In reality, these values vary with different conditions (altitude of the sun, height of eye of the observer, etc.), but these simple corrections were considered "close enough" for the average sight.



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Id also love my question answering in previous post about the radio beacon technology if you know anything about that.



I do not know enough about that to provide a good answer, sorry.



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One other thing I'd like to know is when you are logging when the sunrise occurs, do you take the time that the sun first peaks above the horizon or do you go from when the sun has fully past the horizon line? I tried logging the time earlier but from the time it first peaked above horizon to being fully visable took roughly 1 hour and that messed me up from not knowing if I should use GMT or local time. I was in +1 time zone so it was just unfortunate. If I was in +4 or +5 I coulod have figured it out.



The generally accepted definitions of "sunrise" and "sunset" are when the upper limb (the "top") of the sun meets the horizon. This is the definition that the "official" Nautical Almanac uses. However, it is interesting to note that when you see the sun very near the horizon, it is actually well below it. This is due to the effect of refraction of the atmosphere. The positions of the sun tabulated in the Nautical Almanac are for the apparent center of the body. The average semi-diameter of the sun is 16'. Refraction can be calculated using the following formula:


-0.0167°/tan(H+7.32/(H+4.32))


... where "H" is the altitude of the body. At 0° altitude, we find that the value for refraction equals -0.0167°/tan(0°+7.32/(0°+4.32)) = -0.5645...° or -0°34'. Add the -16' of semi-diameter to this (to account for the difference between the apparent center and the upper limb) and we find that, at the "moment of sunrise/set", the sun is actually 0°50' below the horizon. That's nearly two apparent sun diameters.


The time of sunrise/set for a particular latitude and longitude can be calculated using the formula for a "time sight":


cos(LHA)=(sin(Ho)-sin(Dec.)·sin(Lat.))/(cos(Dec.)·cos(Lat.))


Once the LHA (local hour angle) is determined, it can be corrected for longitude by using the standard 15°/hour "movement" of the sun (in 24 hours the Sun appears to move 360° around the Earth - 360° ÷ 24hr. = 15° per hr.), and for the "equation of time" [EoT] - the difference between the "apparent" and "mean" places of the sun. The EoT is due to the differing speed of the Earth in its orbit around the Sun and varies from about -14 minutes of time to about +16 minutes of time*. These corrections are necessary because clocks generally keep track of "mean zone time" as opposed to "apparent local solar time".


If you would like me to expand on any of these concepts, just ask. I would be more than happy to.


*The EoT varies in a somewhat irregular way due to the [slightly] elliptical orbit of the Earth around the Sun. This is the source of the "analemma" diagram sometimes found on globes and sun dials.
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Last edited by Nathaniel B.; 07-20-2019 at 01:10 AM.
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Old 07-20-2019, 03:05 AM   #19
Fidgety
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Hi thanks for responding and answering my questions. I already figured the first question out myself from reading things on external sites but haven't got round to looking sunrises up yet. thanks you have saved me a lot of time I think.


I will shortly send you a PM if you dont mind taking a peak.
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