NavList:

Peter,

 

I had neglected to note in my log the actual temperature and pressure at the moment of the sight so was left with merely the correction of +0.2.  Due to the Hs being below 20 degrees, I add this correction by using the table in the front of the nautical almanac.  You enter the table with temperature and pressure and it gives you a correction for low angle observations.  Instead of making people look it up, I merely gave it to them.  For the actual Temp and Pressure, my rough guess is 78 deg F and 1014 MB which should give you about that correction.

 

Lower transits are as easier than upper transits to reduce, just far more difficult to find the opportunity to shoot them.

 

Indeed LHA at lower transit is 180 degrees.  At this moment, just as when LHA is 0, we can use an abridged reduction method to acquire latitude.  You simply add the polar distance (co-declination) to your Ho and you get Latitude.  Your Latitude name will be equal to the declination of the body.  Kochab is the easiest body in the lower regions of the northern hemisphere to observe, and Acrux is the easiest in the south.  I missed Acrux when I was south of Africa, but made up for it my last days at sea in the North Atlantic. 

 

I haven't a clue what my longitude was at that moment, but the latitude was 31 deg 27.5' N by GPS.  My observation gave me a Lat of 31 deg 29.0' North, so a 1.5 nm error in latitude, due mostly to a poor horizon at 4 AM. 

 

Your answer is a bit closer to mine and the difference is obviously by the method you used.

 

This is an "exotic" sight and this is the first one I've ever shot in my 10 years at sea.

 

Thanks for reducing it!

 

Jeremy

 

Jeremy,

I did not know how to interpret the Temp/pressure correction, so I just used standard conditions.  This is apparently another piece of terminology that I have yet to learn...

This way I obtain the following coordinates of your vessel at the moment of observation:
N 31 deg 28.7'
W 74 deg 11.8'

At that moment the GP of Kochab was:
GHA: 254 deg 11.8'
Dec: N 74 deg 07.1'

The (west) longitude is given by the GHA minus 180 for lower transits.

The altitude of the Pole (i.e. latitude) is the sum of the Ho (15 deg 35.8') and Kochab's colatitude (polar distance = 90 - Dec = 15 deg 52.9').


Peter Hakel

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From: "Anabasis75@aol.com" <Anabasis75@aol.com>
To: NavList@fer3.com
Sent: Saturday, September 19, 2009 3:08:53 PM
Subject: [NavList 9806] Exercise: Lower Transit

A ship is sailing in the North Atlantic. on 15 September 2009.
At 04h 09m 44s local time a lower meridian transit of the star Kochab was
taken by moonlight.  Hs is 15 deg 50.3'

 

Height of eye is 103 feet
Index correction is -1.2'
Temp/pressure correction is +.2
ZD is +4
The 1930 Star fix from the previous evening is

Lat 30 deg 48.9' North
Long 073deg 22.2' West
Crs is 304  Spd 8.2 Knots

 

Determine a 0409 Latitude.

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