Celestial navigation question

Hi, I’m trying to teach myself celestial navigation. I’m at the stage where I’m trying to familiarise myself with the almanac and have been calculating the position of stars and going out and checking them, but I think I’m making some real dumb error.

Here’s what happened. Tonight I worked out the GHA and dec for Dubhe at 2015. They were about 151deg and 64 deg. (I verified these on the US navy’s online almanac too). So, I go outside and expect Dubhe to be at 151deg. But it’s not. It’s at something like 20deg west of N.

What’s my very basic error here? The GHA is the longitude, right? So the GHA of a star should be where it is on the compass rose, no?

I’m in the UK, btw

[quote=StarSailor;19000]Hi, I’m trying to teach myself celestial navigation. I’m at the stage where I’m trying to familiarise myself with the almanac and have been calculating the position of stars and going out and checking them, but I think I’m making some real dumb error.

Here’s what happened. Tonight I worked out the GHA and dec for Dubhe at 2015. They were about 151deg and 64 deg. (I verified these on the US navy’s online almanac too). So, I go outside and expect Dubhe to be at 151deg. But it’s not. It’s at something like 20deg west of N.

What’s my very basic error here? The GHA is the longitude, right? So the GHA of a star should be where it is on the compass rose, no?

I’m in the UK, btw[/quote]

The Zn or azimuth of the star is where it is on the compass.

Hi StarSailor

GHA does not indicate the Azimuth (direction) of a Star. GHA simply tells you how many degrees of Longitude a Star is to the West of Greenwich Meridian. If you convert GHA to LHA (by applying your own Longitude to GHA) you get a rough idea of where it is going to be, but not exactly. Similarly, Declination has a very little part in altitude of the star.

You need to use the Azimuth calculation methods (A, B, C)to work out the bearing and calculation of Zenith distance to work out the Altitude.

Good luck

Capt Nadeem Anwar

Hi Capt Anwar, I’m at 3 deg W so applying my long to the GHA doesn’t alter it by very much.

Time to go back to my textbooks I think.

might want to review a graphic representation of the relationships between the “3 celestial spheres” and “navigational triangle”…by all means learn the “math” for the sake of any testing…but from a practical stand point also include the use of the “star finder” among your “skill sets”.

[quote=StarSailor;19000]Hi, I’m trying to teach myself celestial navigation. I’m at the stage where I’m trying to familiarise myself with the almanac and have been calculating the position of stars and going out and checking them, but I think I’m making some real dumb error.

Here’s what happened. Tonight I worked out the GHA and dec for Dubhe at 2015. They were about 151deg and 64 deg. (I verified these on the US navy’s online almanac too). So, I go outside and expect Dubhe to be at 151deg. But it’s not. It’s at something like 20deg west of N.

What’s my very basic error here? The GHA is the longitude, right? So the GHA of a star should be where it is on the compass rose, no?

I’m in the UK, btw[/quote]

Using your data, If Dubhe fell STRAIGHT to earth it would hit it’s Ground Point (GP) at Long 151W/Lat 64N, in Alaska. So if you’re looking at Dubhe from England it would look like about 340ish. You seem to have the wrong interpetation of the correct data.
It’s comendable you’re trying to get a holistic understanding of Celestial Navigation, that said, star Idetification/finding isn’t commonly done this way. Checkout a site called quitebay at this link; http://gcaptain.com/maritime/blog/celestial-navigation-101-star-identification/
Also, checkout the use of the Star Finder 2102D. And explore calculating the Azimuth and Altitude for the 57 Navigational Stars and Planets.

(another cool site I found: http://www.smithsofweston.com/NavMike.php

One of the simplest methods is by using the Rude Star Finder. It comes with very simple instructions and only costs about $30US.

The Star finder is the way to go if you’re looking to set up for your evening sights. The Rude SF is easy to use once you’re familiar with it. I found a good piece of free software that may help with identifying constellations and give you a good idea where to look for specific stars in addition to the Starfinder. http://www.stargazing.net/astropc/download.html
Download and install the full version cdcin276.zip
(and the update for win vista cdcin276c.exe), it’s pretty useful to get familiar with the night sky anywhere. It can be set for any observation point.

[QUOTE=StarSailor;19000]Hi, I’m trying to teach myself celestial navigation. I’m at the stage where I’m trying to familiarise myself with the almanac and have been calculating the position of stars and going out and checking them, but I think I’m making some real dumb error.

Here’s what happened. Tonight I worked out the GHA and dec for Dubhe at 2015. They were about 151deg and 64 deg. (I verified these on the US navy’s online almanac too). So, I go outside and expect Dubhe to be at 151deg. But it’s not. It’s at something like 20deg west of N.

What’s my very basic error here? The GHA is the longitude, right? So the GHA of a star should be where it is on the compass rose, no?

I’m in the UK, btw[/QUOTE]
For navigational stars, you need to determine the Local Hour Angle (LHA) of Aries (LHA Aries = Total GHA Aries + East longitude or -West longitude) for the GMT of your observation. Use the LHA Aries with a star finder to determine the star’s Altitude and Azimuth (true direction) from your position for the GMT you want to observe it.

It’s often helpful to work backwards. Identify the star you are trying to work out (either by simple visual identification or with a Rude Star Finder) at the time you computed then get a bearing. Now you have the answer to your problem… try to work backwards to find your error.

Obviously you have already done this, but be sure to play around with the E vs W and other common mistakes to identify the problem. Often finding the error on your own gives you a better understanding than going back to the books… but it takes time.

Or you could always use an electronic sextant with gyroscopic input :wink:

You can also use Pub. 249 aka the “Air Almanac” to get pre-computed stars.

Or you can purchase the Sharp Celesticomp V Pocket Computer and it does it all for you…wish I could have used mine on the Oceans test…

do they still sell that old fossil??you can purchase software for you palm, pocket computer or laptop that are the next generation!!

Now that your ask, I really don’t know if they still sell it or not…mine still works great except that it has always been consistantly .2nm off on sunlines. I have not seen one advertised in a while. I am hoping they still sell them as the almanac on mine only goes to 2011.

Who are you calling an old fossil?

I started when this was the coolest thing available:

This picture is smaller than actual size.

mine got wet when my briefcase went for a swim a few years back…history…went to buy a new one could only find a used one from celestaire…pretty handy but don’t think there are many/if any around now… everything I got now is pc software…got to lug that thing arround now anyway!!

[quote=jdcavo;19291]Who are you calling an old fossil?

I started when this was the coolest thing available:

This picture is smaller than actual size.[/quote]

Holey Smoley…that there is big. Does that REALLY say smaller than actual size?

I’m still surprised at how many people do not know how to use the rectangular>polar functions on a regular TI-33. Talking about making life easy for certain tasks. I taught myself from the ground up and that was one of the little pleasant surprises I learned along the way. In the “Formulae for the Mariner”, that is one of the first things that Richard Plant emphasizes. I found myself teaching it to several instructors at Star Center and MITAGS.:confused:

Jim,

myself for sure…I am dinosaur…genus: seadawgsauras!!

hey…just noticed I made TC statis…does that mean I get to run with the big dawgs…if so can’t I stay on the porch w/ the puppies and the bitches…fefee smells sooo sooo goodd!!

[quote=seadawg;19294]Jim,

myself for sure…I am dinosaur…genus: seadawgsauras!!

hey…just noticed I made TC statis…does that mean I get to run with the big dawgs…if so can’t I stay on the porch w/ the puppies and the bitches…fefee smells sooo sooo goodd!![/quote]

Fefee has fleas…!:wink:

Smaller, but not by much. With it’s wooden case, it was the same length/width as a nautical almanac.