00:00Every instrument on a ship's bridge, from the most advanced electronic gyro to a traditional magnetic compass, is vulnerable to
00:08mechanical drift over time.
00:10To guarantee your heading is true, we can check the compass against something that never drifts, the sun.
00:16By taking a visual observation of the sun at dawn or dusk, we can determine exactly how far off the
00:23compass is.
00:24In this video, we will walk through exactly how to take a bearing, run it through a simple formula, and
00:30extract an exact compass error.
00:32Before we begin, you need five items.
00:35An observed bearing of the sun, the exact Greenwich mean time of the observation, your ship's dead reckoning latitude, the
00:42current year's nautical almanac, and a basic calculator.
00:45Once you master this essential celestial technique, you eliminate the danger of relying solely on drifting instruments, ensuring constant directional
00:54accuracy for your vessel.
00:55Step one is to measure the compass bearing of the sun precisely at sunrise or sunset.
01:01This diagram shows the difference between where the sun actually is and where it appears to be.
01:07Because the Earth's atmosphere bends light, a phenomenon called refraction, the visible horizon sits lower than the theoretical horizon.
01:15To account for this distortion, wait to take your bearing until the sun's lower limb sits roughly two-thirds of
01:22a diameter above the visible horizon line.
01:25The moment you take that bearing, immediately record the exact Greenwich mean time, or GMT, and note the ship's current
01:32dead reckoning latitude.
01:33Taking the observation at this precise height compensates for atmospheric refraction.
01:39Doing this prevents a calculation failure before the math even begins.
01:43Next, open your nautical almanac for the current year to the exact date of your observation.
01:49Find your recorded GMT hour in the table, then cross-reference it to find the corresponding declination of the sun.
01:55Pay close attention to the letter next to that declination value.
01:59You must record whether this direction is north or south.
02:03This declination value is the essential coordinate for the math ahead.
02:07It anchors the physical time of your ship's observation to the exact celestial position of the sun.
02:14Now we calculate the true amplitude.
02:16We use this formula.
02:18The sine of the amplitude equals the sine of the declination divided by the cosine of the latitude.
02:25Let's plug in some example variables.
02:27We will use a latitude of 21.8 degrees and a declination of 11.1 degrees.
02:34Divide the sine of 11.1 by the cosine of 21.8.
02:39To isolate the final amplitude value, take the inverse sine of the equation's result.
02:45In our example, that calculation gives us exactly 12 degrees.
02:49A raw number is useless for navigation without a naming convention.
02:53We format this starting with a prefix.
02:56If you observed the sun rising, assign the prefix east.
03:00If the sun was setting, assign the prefix west.
03:03Then, assign the suffix north or south to exactly match the name of your recorded declination.
03:09For a sunning sun with a southern declination, our final format is west 12 degrees south.
03:15This diagram shows the celestial sphere.
03:18That formatted string we just built defines the sun's exact angular distance from the prime vertical.
03:24To use west 12 degrees south on a physical map, we must convert it into a standard 360 degree true
03:31bearing.
03:31Start at the cardinal direction indicated by your prefix.
03:35Since our prefix is west, we start at 270 degrees.
03:39Step 2. Move along the compass by your calculated degree amount, heading toward the suffix direction.
03:45In this case, moving 12 degrees south.
03:47We simply subtract 12 from 270, which reveals our converted true bearing of 258 degrees.
03:54This number represents the perfect, mathematically true bearing of the sun at the moment of observation, completely free of any
04:01mechanical ship air.
04:02Finally, compare your calculated true bearing of 258 degrees directly against the observed compass bearing you recorded in step 1.
04:13Let's say your compass read 256 degrees.
04:17Calculate the simple numerical difference between the two bearings.
04:22258 minus 256 gives us a total error of 2 degrees.
04:27To determine if that error direction is east or west, use the standard maritime mnemonic, compass best, error west, compass
04:37least, error east.
04:39Apply that rule to our numbers.
04:41Our observed compass reading of 256 is less than our true bearing of 258.
04:48Because the compass is least, our final determined compass error is exactly 2 degrees east.
04:55You have successfully transformed a visual observation of the sunrise into hard, actionable navigational data.
05:04The navigator immediately applies this 2 degree east error to adjust the ship's current heading.
05:10This ensures the helmsman is actually steering the correct true course through the water.
05:15By making amplitude calculations a standard daily watch routine at dawn and dusk,
05:21a navigator never has to second-guess the safety of their heading.
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