NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Exercise #9 Star Time
From: Greg R_
Date: 2008 Jun 5, 00:00 -0700
From: Greg R_
Date: 2008 Jun 5, 00:00 -0700
--- Anabasiswrote: > Your methodology is very strange to me for the sunset numbers, and > gives you the wrong answers. Interesting, because it seems to work for me (at least here in the western longitudes) - I've actually been doing some sunset/twilight calculations lately for practice, and they seem to follow the "real world" situation pretty accurately. I'll have to check this from home tomorrow (I'm at work and don't have my notes in front of me), but I did run my numbers through Navigator and it agreed with me (within a few tenths on some of the calculations, of course). > I got 1907 for NT, but I�m not going to quibble. What is wrong is > that this is the Time in LAT I'm not sure what "LAT" is, but the times in the NA are UT/Z unless I don't remember correctly (?). > which must be converted to ZT before use. Correct. > I got 1907 for NT, but I�m not going to quibble. Hopefully the navigator would be ready to shoot well before then, so our minute of time difference shouldn't matter... ;-) > What is wrong is that this is the Time in LAT which must be > converted to ZT before use. Again, not knowing what "LAT" is I can't discuss this one way or another, but as I understand it all of the times on the NA page (sunrise/sunset/twilights/meridian passage/GHA/etc.) are all in UT/Z. > These times are only good on the Zone reference meridians, including > Greenwich (divisible evenly by 15). Correct, and the arc-to-time conversion changes that time to the UT/Z time at the given DR longitude position. Applying the ZD correction brings the time back to local zone time (for the convenience of the navigator, but not used in the LHA Aries calculation). > Here is where we get really different. The correction from LAT > to ZT should be fairly small (<1 hr) if we are keeping the correct > zone description. Aha - I think the difference here is from the different way we we (apparently) calculate local times for suneset/twilights. > The trouble with the second is that you have to apply the correction > in the correct direction which can be confusing, especially if you > are in East Longitude and are from the Western Hemisphere. Correct - and since events happen earlier in eastern longitudes you'd subtract the (absolute value of) the ZD from the UT/Z time given in the NA to change the time to the local time zone. > I got 146 degrees since we add 22� longitude to 34.7� GHA to get 56.7 > which is close to 60, and we carry the one to 145 to equal 146. I'll have to look at my worksheet tomorrow to see how I arrived at that LHA, but I'm reasonably sure that I remembered to add 1 whole degree to it. > I also use Pub 249 Vol 1 but Epoch 2010 (available online). Aha, wasn't aware that 2010 was out yet (though it might not be in printed form yet) - thanks for the info. > Obviously since my LHA is different from yours, there will be > slightly different results: Agreed. >> correction for precession and nutation > This is correct, but not necessary for precalculation as we > only need an approximate Hs and Zn to find the body. Ah, seems like you mentioned getting Hc and Zn - thought you'd meant to get values for the final sight reduction, but you're correct that an approximate Hs/Zn will work to get the body into the sextant's field of view. > Also, I was taught to only apply this correction in off-epoch years > to the actual fix instead of the LOP�s (unless there is just one LOP > of course). According to the instructions in my copy of Pub. 249, the correction can be applied to either the LOP or the fix itself (and personally I'd rather move just the final fix than the several LOPs used to determine it) - less time, and less prone to errors. > The point of pre-calculating stars at sea is so that you can go out > and just point the sextant in the correct Azimuth, then the star just > pops into view. A tiny adjustment on the micrometer drum and you > take your sight. I've tried to do that from my front/back yard in recent weeks (using an approximate eye-level on the neighbor's houses) but so far I've only been able to make it work for a couple of the selected stars (there isn't much light pollution where I live, but it's still not the same as being out on the open water away from civilization). > I can shoot a round of stars in under 10 minutes, and am just usually > waiting for the dimmer ones to appear. For this round, I > shot 5 bodies in just over 7 minutes. How many shots of each star do you usually take? I've heard conflicting philosophies on this - one says to take ~3 of each star, the other says just to take one of all of them (assuming they're all in view) and any errors will be obvious when the resulting LOP doesn't land reasonably close to where the others cross. -- GregR --- Anabasis wrote: > > -JCA Oh boy Greg, this is going to take a bit of writing. Your > methodology is very strange to me for the sunset numbers, and gives > you the wrong answers. I will try to work my way through it and > explain my methods. > > Using the handy Arc to Time converter in the NA, 145deg 22' E yields > a > time correction of 09h 41m 28s. > ----------------------- > -JCA We agree thus far. > ----------------------- > Interpolating the Sunset, Civil Twilight, and Nautical Twilight times > for N 10deg and N 20deg latitude for the 3-day period including 7 May > 2008 (at Greenwich): > SS: 18:18 Z > CT: 18:41 Z > NT: 19:08 Z > ---------------------- > -JCA. I got 1907 for NT, but I�m not going to quibble. What is > wrong > is that this is the Time in LAT which must be converted to ZT before > use. These times are only good on the Zone reference meridians, > including Greenwich (divisible evenly by 15). > ---------------------- > Applying the Arc-to-Time correction to get local time: > SS: 20:37 (Local) > CT: 21:00 (Local) > NT: 21:27 (Local) > ---------------------- > -JCA Here is where we get really different. The correction from LAT > to ZT should be fairly small (<1 hr) if we are keeping the correct > zone description. Sunset should not jump 2 hours between LAT and ZT. > There are 2 methods to do this, apply the arc to time of the whole > longitude to bring the time back to UTC and then apply the reverse > zone description to find zone time; or 2), find the difference in > Longitude (Dlo) between the DR Longitude and the reference Longitude. > The trouble with the second is that you have to apply the correction > in the correct direction which can be confusing, especially if you > are > in East Longitude and are from the Western Hemisphere. I prefer > using > the whole arc to time of Longitude to get GMT (which will be used to > get the GHA Aries later). > > So we apply the -9h 41m 28s to each to get the following UTC times. > We know that we are ahead of UTC in East Longitude, so we subtract > this number: > SS: 08h 36m 32s Z > CT: 08h 59m 32s Z > NT: 09h 25m 32s Z > Add 10 hours for ZD and you get the following local ZT: > SS: 18h 36m 32s L > CT: 18h 59m 32s L > NT: 19h 25m 32s L > ------------------ > And applying the ZD-10 correction to get Z time for this location: > SS: 10:37 Z > CT: 11:00 Z > NT: 11:27 Z > Now we need to calculate LHA Aries for the time we're going to be > doing > the star sights. Since the time for Civil Twilight falls neatly on an > exact hour, we can just look up GHA Aries from the NA daily page (and > don't have to do the correction for minutes/seconds) > GHA Aries: 30deg 39.6' > -------------------------- > -JCA The method is sound, the math is obviously incorrect. I too > chose 1900L (0900 UTC) as my star time, but I got a nice easy GHA of > Aries of 000deg 34.7�. > -------------------------- > Using an AP of 14deg N / 145deg 20.4' E, we add GHA Aries to our > Assumed Longitude to get: > LHA Aries: 176deg > ------------------------- > -JCA I got 146 degrees since we add 22� longitude to 34.7� GHA to > get > 56.7 which is close to 60, and we carry the one to 145 to equal 146. > -------------------------------- > Now using Pub 249 Vol. 1 (Epoch 2005.0), for Latitude 14deg N and LHA > Aries 176deg we get this list of stars to shoot (ALL CAPS = first > magnitude stars, [x] = stars suitable for a 3-star fix): > Star Hc Zn > ------------------------ > Dubhe 41deg 44' 354 [x] > ARCTURUS 53deg 19' 077 > SPICA 54deg 24' 134 [x] > ACRUX 12deg 25' 175 > Suhail 22deg 25' 210 > PROCYON 29deg 16' 268 [x] > POLLUX 33deg 10' 295 > -------------------------------------------- > > -JCA I also use Pub 249 Vol 1 but Epoch 2010 (available online). > Obviously since my LHA is different from yours, there will be > slightly > different results: > 1) Dubhe 40deg 10� @012 deg > 2) Arcturus 24 deg 59� @075 deg > 3) Spica 29deg 32� @112 deg > 4) Suhail 31deg 57� @ 188 deg > 5) Sirius 36deg 18� @ 237 deg > 6) Betelgeuse 33deg 39� @ 270 deg > 7) Capella 26deg 11� @ 315 deg > ------------------------ > There's also a correction for precession and nutation of 03' / 110deg > T > to be applied to the LOPs and/or fix. > ------------------------- > -JCA. This is correct, but not necessary for precalculation as we > only need an approximate Hs and Zn to find the body. Also, I was > taught to only apply this correction in off-epoch years to the actual > fix instead of the LOP�s (unless there is just one LOP of course). > The point of pre-calculating stars at sea is so that you can go out > and just point the sextant in the correct Azimuth, then the star just > pops into view. A tiny adjustment on the micrometer drum and you > take > your sight. I can shoot a round of stars in under 10 minutes, and am > just usually waiting for the dimmer ones to appear. For this round, > I > shot 5 bodies in just over 7 minutes. > JCA > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---