# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Message:αβγ
Message:abc
 Add Images & Files Posting Code: Name: Email:
Re: Longitude by Sunrise example
From: Gary LaPook
Date: 2010 Feb 05, 15:24 -0800

```Zero altitude sights (Hs = 0) can work out well but the problem is that
you don't know if this sight is an accurate one or an inaccurate one due
to the vagaries of refraction that the observer won't know about.
Because of this you have to allow a larger uncertainty band around any
LOP taken this way. I have taken such observations in flight and they
work out well considering the lower accuracy inherent in flight
navigation and I have carried a table in my wallet for 30 years for this
purpose, it is posted here:

http://www.fer3.com/arc/img/104652.ho%20table.jpg

On my recent Atlantic crossing I took three zero altitude sights. The
semi-diameter of the sun was 16.1', parallax in altitude was .1'  and
standard refraction correction was 34' . The temperature for all the
sights was about 70 F but I do not know the actual atmosphere pressure.
So using the average pressure from the pilot chart of 1017.5 mb the
refraction correction for non-standard conditions is + 2.3'. So for the
upper limb observations this totals to a -47.7' for all of the
observations not including the dip correction .

The first was sunrise on October 23, 2009 at 06:53:25 Z at GPS position
37 11.2 N, 9 32.1 W. Height of eye was 24 feet so dip was -4.7' so the
observed altitude (Ho) was -52.4'. Using the GPS position and then using
the navy website at:

http://aa.usno.navy.mil/data/docs/celnavtable.php

the computed altitude (Hc) was -52.3' resulting in an intercept of .1 NM

The second was at sunset the same day, October 23, 2009 at 17:49:58 Z at
GPS position 35 15.3 N, 8 43.4 W. The height of eye was the same so the
Ho was also -52.4'. Hc was -50.6' resulting in an intercept of 1.8 NM away.

The third was sunset on October 24, 2009 at 17:46:27 Z at GPS position
33 42.9 N, 7 34.6 W. Height of eye was 26 feet giving a dip of -4.9'
resulting in an Ho of -52.6'. Hc was -54.9' resulting in an intercept of
2.3 NM toward.

These all provide useful LOPs  keeping in mind the larger band of
uncertainty.

gl

Greg Rudzinski wrote:
>
> Jeremy,
>
> I worked your upper and lower limb sunset examples using a palm
> program to derive a standard LOP azimuth and intercept with the given
> GPS position of 16°33'N 129°7'E as A.P.,Height of eye 100ft., Pressure
> /Temperature correction from A4 of +3'. No index error applied.
>
> Sun L.L. 18:58:16 ZT
> Hs 0°0'
> Azimuth 294.3°
> Intercept 3.9' away
>
> Sun U.L. 19:00:48 ZT
> Hs 0°0'
> Azimuth 294.5°
> Intercept 2.2' away
>
> This shows that your observations with the naked eye were very good.
> The sunset Longitude solution should work out if a precise table or
> program is used.
>
> GRudzinski
> ----------------------------------------------------------------
> NavList message boards and member settings: www.fer3.com/NavList
> Members may optionally receive posts by email.
> To cancel email delivery, send a message to NoMail[at]fer3.com
> ----------------------------------------------------------------
>

```
Browse Files

Drop Files

### Join NavList

 Name: (please, no nicknames or handles) Email:
 Do you want to receive all group messages by email? Yes No
You can also join by posting. Your first on-topic post automatically makes you a member.

### Posting Code

Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
 Email:

### Email Settings

 Posting Code:

### Custom Index

 Subject: Author: Start date: (yyyymm dd) End date: (yyyymm dd)