Ex-Meridian by Formula

The Ex-Meridian can be done by formula for improved precision over Bowditch vol.2 tables #29 and #30. (current edition table #24)

Formula :

(a) = 1.9635 x COS Latitude x COS Declination / SIN (Lat. ± Dec.*)
*RULES (+) if Lat. & Dec. are contrary in name. (-) if Lat. & Dec. are the same name.


Ex-Meridian Correction in minutes of arc to be added to Sun (Ho) near LAN (upper transit) :

Ex.Correction' = (a)( Time from LAN in minutes and tenths)^2 / 60

or

Ex.Correction' = (.267)(a)( Meridian Angle in degrees and tenths)^2

Example:

Lat.34°N Dec.20°S (Sun observed 20 minutes 30 seconds before LAN)

(a) = 1.9635(Cos 34)(Cos 20)/Sin (54) = 1.89
Ex.Corr.' = (1.89)(20.5)(20.5)/60 = 13.24' added to (Ho)

or

Ex.Corr.' = (.267)(1.89)(5.125)(5.125) = 13.24' added to (Ho) *

The corrected (Ho) is then reduced as a normal LAN to get the Latitude at the time of observation.


*Try a formula variant that uses meridian angle vs. time
.267 (a)(Meridian Angle)^2 = Ex.Correction' file:///Users/greg/Desktop/factors.html

Symmedian Point

Shift a celestial navigation LOP triangular plot center slightly toward the short side (sides) for an improved probable position (symmedian point)

http://en.wikipedia.org/wiki/Symmedian

Horizontal Sextant Angle Plot Made Easy

Make short work of plotting horizontal sextant angles by determining the arc radius using a chart plotting board, radar plotting sheet, or the simple formula (0.5 mark A to mark B)/ sin(horizontal sextant angle) = radius. Once the radius is known then a chart plot is performed by arcing the radius off navigation marks A & B then plot the arc of position from the radius point intersection to determine an estimated position. The chart plotting board image above shows a radius solution for a horizontal sextant angle of 45° and a distance between marks A and B of 16.6 nautical miles.

Unique Short Table Sight Reduction

Combine Pub. 208 Dreisonstok with a modified 2102-D star finder for quick determining of the Sun's azimuth and intercept. Use the star finder to first derive azimuth which is then preplotted through the assumed position. Follow up with a calculated altitude using Pub. 208 Dreisonstok to get the intercept.

Set the 2102-D Star Finder without a Nautical Almanac

Set the 2102-D star finder without using a Nautical Almanac by scaling a radar plotting sheet as depicted by the above image.

1. Set Aries zero to month and day (clockwise)
2. Set blue latitude template to zone time of twilight (counter clockwise)
3. Select stars for twilight viewing.

The image example is for 12 October 2010 zone time civil twilight 1800 at 35°N 120°W

Try penciling in the Sun onto the 2102-D base disc using the slotted red template.

1. Set Aries to month and day.
2. Set red template to time of meridional passage.
3. Mark Sun's declination through the slot onto the base disc.

The twilight sky can now be set by rotating the blue latitude template so the Sun is 5° to 10° below zero altitude. For the southern hemisphere reverse the zone time direction to clockwise.

The Mechanical Watch

Image by Greg Rudzinski

There are many places where watch batteries are not readily available which makes the mechanical watch an important backup to the very accurate quartz watch. I recommend the Seiko 7s26 automatic mechanical movement which is affordable, simple , sturdy, and reliable. The best part about the 7s26 movements is that they can be easily regulated to better than ± 5 seconds per day.

Go to the link below for an excellent description on how to regulate a modern mechanical watch movement:

http://en.wikibooks.org/wiki/Budget_Watch_Collecting/Regulating

Voyaging Tip #26: Icebox Space Management

Nest a 5 gallon, 3 gallon, and 2 gallon bucket into the icebox putting ice and beverages in the bottom 5 gallon bucket, frozen goods in the middle 3 gallon bucket, and dairy in the top 2 gallon bucket. This system will save space, conserve ice, and assist with icebox access.