Celestial Navigation - Two Questions

[Sailingsailor12]

Question 1 - When plotting a Polaris sight, I end up with a latitude and an Azimuth. The Azimuth is from 0 degrees to 358.3 degrees. Do I need to put a tilt in the line that I plot, or is it negligable?
Question 2 - In the sight reduction table, Hc is sometimes negative. (Declination 0 - 14 Lat 52, Dec 6 degrees, LHA 108). Will somebody explain what is going on and how to deal with it.

Thank you much.
Sincerely,
Mark Rehwaldt

[Benz]

Azimuth for Polaris is pretty negligible--best to ignore unless you're advancing a running fix far into the future.

As for question 2, if I understand what you're asking, you plot your intercept along the azimuth either toward the celestial body's GP or away, depending on the sign. HOLT (HO Larger Toward) or HOSA (HO Smaller Away) are the ditties I use to remember. This has to do with whether your Height Observed as greater or smaller than your Height Computed.

[SeanPatrick]

Originally Posted by Sailingsailor12

Question 2 - In the sight reduction table, Hc is sometimes negative. (Declination 0 - 14 Lat 52, Dec 6 degrees, LHA 108). Will somebody explain what is going on and how to deal with it.

The altitude is negative because for that combination of lat., dec. and LHA, the body would be below the observer's horizon. But remember: if you are indeed using HO-249, the Sight Reduction Tables for Air Navigation, that it is entirely possible in an aircraft at high altitude to shoot a body that is well below your "horizon". For those of us who are a little closer to the ground, this data is essentially useless and will never be needed. That's not to say that HO-249 isn't great for nautical navigation as well ... it is.


As for the azimuth of Polaris: Ben is right - ignore that, too.

[Sailingsailor12]

Benz and Sean:

Thank you much.

Sincerely,
Mark R. aka SailingSailor12

[Adelie]

Originally Posted by Sailingsailor12

...
Question 2 - In the sight reduction table, Hc is sometimes negative. (Declination 0 - 14 Lat 52, Dec 6 degrees, LHA 108). Will somebody explain what is going on and how to deal with it.

2. For HO229 altitudes are tabulated to slightly below 0*, say -1* at most. This is because of refraction. the body (sun or maybe moon) is actually over the horizon (if there was no atmosphere), but refraction has raised the image enough to see so the altitudes were calculated to some negative value for extreme meteorological conditions.

For HO249 altitudes are tabulated to somewhere around -6*. This is because of refraction, as with 229, and height of eye. How can you get another 5* or so over the horizon? 249 is for air navigation. Sometime in the last year I figured out the extra 5* or so is equivalent to about 60-70,00 altitude. Which is a bit higher than almost any military craft flies and certainly higher than anything that would be using celestial.

[Adelie]

In the 1980s version of HO9 there is a table for Polaris that gives corrections to altitude based on LHA to provide Lat. Q tables maybe?
It's a lot quicker than running a regular sight.
Taking Lat directly from Ho can have significant errors since Polaris is not right at the celestial pole.

[SeanPatrick]

Originally Posted by Adelie

In the 1980s version of HO9 there is a table for Polaris that gives corrections to altitude based on LHA to provide Lat. Q tables maybe?

I thought about suggesting the "Q correction" as well. But, when I got to looking at it, I noticed that it does indeed have a separate table for azimuth. I guess I never really noticed it, because I never used it.


I think the OP is using HO-249, and the Q correction is included in that as "Table 6" on page 343 of the current Volume 1 (free download here, BTW - courtesy of Celestaire). I thought he was doing a full reduction of Polaris at first, but I think he is actually using these tables.

[Adelie]

Let's see what he has to say.
Has anybody suggested NavList to him?
Then again he seems to be getting decent answers here and NavList can be a little intense for a newbie.

[SeanPatrick]

You might find these interesting. They use the approximations formulae found in the Nautical Almanac.