Author

Topic: Identifying a star without the StarFinder

Guillaume

posted February 17, 2023 01:40 PM
Hello!
Quite often during my navigations, I would see one or two stars which I take sights on, without clearly identifying them at first. I can afterwards identify them with the StarFinder but I was wondering how one could dispense from using the StarFinder and calculate or use tables to find the SHA and declination. I have not been able to find that anywhere, can you point me to a book/essay on the topic? I have (and use routinely) the Almanach 2023, Pub.249 vol 123, Pub.229 for all latitudes. Many thanks for your help,
Guillaume
From: France


David Burch

posted February 17, 2023 04:35 PM
The easiest way to identify stars and planets without the star finder is to use the Sky Diagrams from the Air Almanac. This is a free annual publication. We have a copy link at www.starpath.com/celnavbook. There is also an article there on now to use them and a comparison with other methods.
This can also be done with just sight reduction tables. We would of course start with a measured height H, approximate Zn, plus time, and DR Lat and DR Lon.
One easy approach that might work but is not likely a fair answer to your question is to look into the current edition of Pub 249 vol 1, selected stars. We will try that below with a random example, and then go on to more generic solutions.
Let's do an example from Today at Lat 30N, Lon 130W, at 04h00m00s UTC, and we see a star at 18º 20' above the horizon, bearing approximately due east, 088... say we believe it is 088 ± 5º or somewhere between 083 and 093.
First check to see if vol 1 might just tell us this:
We need LHA of Aries = gha Aries  DRLon = 206 52.7  130 = 76 52.7 and our Lat = 30N, and going to that page we see 7 stars with H and Zn given, but not ours.
You can check the actual sky to see that Vol 1 does work for those 7 stars, but not a dependable, general solution for star id and definitely won't help for planets.
Turning to Pub 249, Vol 2 for Lat 30, we can scan the pages, and in less than a minute or so, we know this has to be same name, with dec between N6 and N14, with LHA between about 288 and 284 (or 72 to 76). There is no other stars that will yield 18º 20' with Zn between 083 and 093.
That should be more than enough to ID the star.
I will come back to finish this shortly.
From: Starpath, Seattle, WA


Guillaume

posted February 18, 2023 03:32 AM
Thanks! Following on from what you wrote:
 Dec. between 6° and 15°  it works also at 15° declination and LHA 282, I think  LHA* between 282° and 288° I gather it cannot be be 72° to 78° because of the rule Zn=360Z if LHA<180
Because SHA * = LHA *  LHA Aries, the SHA * must be between 205°07.3' and 211°07.3'
Looking at the 173 Stars in the Almanach pp. 268273, it can only be Regulus, confirmed with the StarFinder for at least the 57 navigational stars.
Can you confirm this is the right way?
This is great, an empirical way to find the result, very quick, and what I was looking for.
Trying to go further, is there a "computational way" where you reach the same result, ie the SHA and Declination by calculus?
Many thanks for your help!
From: France


David Burch

posted February 18, 2023 07:13 AM
That is correct... and indeed all there is to it.
One could likely come up with some equations or numerical procedures to home in on the answer more systematically, but this is not something we can work on now.
Another option is to use the StarPilot where you enter time, date, DR, Height and bearing and it tells you the star or planet. The math you are looking for is in the code of that routine!
I might stress that for manual star id and sight planning, it is very difficult to beat the Sky Diagrams, using them as we outline in the article cited.
From: Starpath, Seattle, WA


Guillaume

posted February 19, 2023 06:15 AM
All great points.
I find that if one wants to find more than the 57 navigational stars (and stars with Declination above 30°), the same method works well with Pub.229. I have tried two examples in N and S latitudes, and am very happy with the result and how quick this is. This will be my method going forward (I just had to pay attention to the Z / Zn computation and the LHA rules depending on the hemisphere, but it works fine)
Thanks again!
From: France


David Burch

posted February 19, 2023 08:37 AM
Using the Nautical Almanac, you are not limited to the 57 navigational stars on the daily pages. There are over 100 more stars listed in the back of the book with monthly data. I forget the exact count.
From: Starpath, Seattle, WA


