CELESTIAL NAVIGATION
HOW TO SAIL BETTER
Making a ‘mer pass’ observation is one of the simplest, yet most useful navigation tools for sailors to possess, explains Jay Renton
Even day sailors who rarely lose sight of land could enjoy learning the skill to fix a position if your chart plotter or electrical system decide to join the choir eternal.
And for those sailors who find yourself in blue water with terra firma beyond the horizon, making a celestial observation of the sun at meridian passage (aka ‘mer pass’) is one of the simplest, yet most useful navigation tools to possess.
With just a sextant, a chronometer and a nautical almanac, you can get a very good estimate of latitude and a fairly good estimate of longitude. Taking a night-time observation of Polaris (the North Star) is another straightforward way to determine latitude alone.
This simplified primer and accompanying pro forma are designed for locations with latitude >23.5°N and westerly longitude. While this mer pass solar observation is both simple and useful, the big limitation is that the sun must be visible at solar noon.
Names explained
It’s difficult to sail effectively until you know the difference between a sheet and a halyard; likewise, celestial navigation is hard to grasp until you have an understanding of the basic nomenclature:
Angles are measured using degrees, minutes and tenths of a minute (eg 22° 45.2’). Time is measured in hours and minutes (eg 14h 38m). Please note that ‘minute’ can refer to either an angular quantity or a time quantity.
Dec / Dec° Declination: the angle measured north or south of the equator, used to describe the GP (geographical position) of a celestial body. The sun’s Dec° varies between 23.5°N (at the summer solstice) and 23.5°S (at the winter solstice), so the hourly change of the sun’s Dec° is quite small. We obtain the sun’s Dec from a nautical almanac. See figure 1, opposite.
The earth rotates through 360° of longitude in 24 hours, so it follows: the earth rotates through 15° of longitude in 1 hour; the earth rotates through 1° of longitude every 4 minutes; the earth rotates through 15’ (a quarter of a degree of longitude) every minute >>> 15° longitude = 1h // 1° longitude = 4m // 15’ longitude = 1m <<<
The circumference of the earth along any meridian ≈ 21,600nm: 21,600nm / 360° = 60nm / 1° = 60nm / 60’ = 1nm / 1’ >>> 1° latitude = 60nm // 1’ latitude = 1nm <<<
EO
