+- Forums (https://thenauticalalmanac.com/Forum)
+-- Forum: Main Forum Area (https://thenauticalalmanac.com/Forum/forumdisplay.php?fid=1)
+--- Forum: General Topics Here (https://thenauticalalmanac.com/Forum/forumdisplay.php?fid=2)
+--- Thread: Apparent time (/showthread.php?tid=213)
Apparent time - Rumata - 02-05-2021
Gentlemen, hope you can help me in this little "predicament."
At first , a few general terms:
1. Apparent time is a measure of the sun' true position. To get Greenwich apparent time at any instant we just convert GHA of the sun into time (hh/min/ec)., We need apparent time to find when the sun will be on the observer's meridian. Local apparent noone, when the observations for latitude are made. We know that it is different for each meridian, and unless two places are on the same meridian- apparent time is different.
2. On the Mercator chart every parallel is expanded in the ration of the secant of its latitude.
To avoid local distortion every meridian of a Mercator chart must also be expaned in the vicinity of each parallel by multiplying it by the secantr of the latitude of the parallel.
My question:
When the vessel is located above let's say 50 deg Lat. and we are trying to find LAN the natural curvature of meridians would make GHA slightly shorter than Mercator chart projections ( straight, parallel lines). And it means LAN calculated would be different from LAN real , considering expansion ratio of scant of the latitude. And it means the moment we are catching the sun in a sextant is not exactly the time the sun is in Zenith. Difference should not be significant, but I just want your expert opinion about it.
Please, advice.
Thank you
RE: Apparent time - P.Rutherford - 02-05-2021
Rumata,
My thought is that the moment of LAN as observed in your sextant is correct but this must be qualified by the following;
- Sextants and ship movement combine to effect imprecise determination of LAN.
- Making a series of observations before MP and after MP based on equal altitudes will mitigate inaccuracies but not exactly. Equal Altitudes, of course, will give you longitude and an imprecise latitude. Plotting these observations and then drawing a curve through them can provide a fair assessment of latitude.
These things kept in consideration, a plot isn't necessary to determine LAN- just get zenith and do the calculations to obtain latitude.
See here- https://thenauticalalmanac.com/Formulas.html#Noon_sight-_Determine_Latitude
Yes, the lines of longitude converge is a problem which can be accounted for by adjusting the amount on the UPS (Universal Plotting Sheet) between the lines of latitude. At 50°, as you mentioned, it's not that much....but it is an amount that can be easily measured. Again, this isn't a problem of determining LAN- it would be relative to determining longitude.
Oh, the UPS, generally, is only laid out in 1° lines of latitude and, of course, 1° lines of longitude. And at the Poles it's a real mess since the lines of longitude are so close on the UPS.
Anyone have another idea?
Paul
RE: Apparent time - Rumata - 02-05-2021
Thank you, Paul. Will follow. Re: UPS. Tried a few times to construct a specific area of a chart using meridional parts method. It is much more precise and a lot of headache with calcs and proper scales for longs and lat. At least I tried it. Will I use it? Don't think so.
RE: Apparent time - pabrides - 02-06-2021
Ive never tried to ascertain longitude or latitude with a noon shot, but I am aware that LAN is not always LAN. The sun, as I recall, is sometimes advanced or retarded of clock noon if I'm thinking about this correctly. The difference between zenith and clock noon is shown in the daily pages as equation of time or Meridian passage.
A plotting chart has a true compass rose and is also marked in degrees of latitude. Draw a line from the center out to your latutude along the circle. Using dividers pick off miles from the meridian (central vertical line), then lay it along the diagonal you drew. Draw a vertical line down to the parallel and thats the equivalent short longitude. Im at 10 degrees north so there is only a slight difference between equatorial and local divisions, but I draw the diagonal every time just so I don't get lazy. I guess this might be termed the graphic method. Works for me.
First thing I do with a plot sheet is draw in my dr bracketing meridians to either side of the circle intersecting the latitudes north and south. Then I draw the diagonal as mentioned which scales minute and second for that latitude. I never use the associated longitude scale.
I hope were all talking about the same thing. Lol
I'd like to know more about meridional parts and how to use them.
BTW - sometimes to assist accuracy I rescale the ups to reflect half degrees both lat and long... it makes the mile tick marks into half mile. With some planning you could even quarter or double the ups. My ups are all 8.5x11. Rescaling saves me the expense of large plotting sheets.
Cheers.