A light rising and dipping: Sandy Hook on a clear night

What you do on the bridge

1. Up to the monkey island
2. It rises
3. Bob the light
4. It rises again
5. Fix by rising range and bearing

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. What are the nominal, luminous and geographical ranges of a light?Often asked

   A good answer covers:

   • Nominal range: the luminous range in a meteorological visibility of 10 miles; the range on the chart in most countries
   • Luminous range: how far the light can be seen in the visibility at the time, from its intensity; found from the luminous range diagram
   • Geographical range: how far it can be seen over the curve of the earth, from the height of the light and the height of eye
   • You see it at the lesser of the luminous and the geographical range

   Show me on the bridgeFine on the starboard bow, Sandy Hook Light is still below the horizon. Watch for it to rise.

2. How do you work out the rising range of a light?Often asked

   A good answer covers:

   • Distance to the horizon from the light plus distance to the horizon from the eye
   • Each is about 2.08 times the square root of the height in metres, in nautical miles, with normal refraction
   • Or from the geographical range table in the List of Lights
   • Here: 2.08 times root 26.8 plus root 15.6, about 19.0 miles from the monkey island
3. How far can you trust a rising or dipping range for a fix?

   A good answer covers:

   • Only roughly: abnormal refraction can move it by a mile or more
   • The light's height is above high water (MHWS, or MHW on US charts), so at lower tides it rises a little sooner
   • Only if the luminous range is greater than the geographical range in the visibility at the time
   • Bob the light: lower your eye, and if it dips it is just on the horizon
   • Confirm with another position line as soon as you can
4. What is the loom of a light?

   A good answer covers:

   • The glow of a powerful light reflected off the haze or cloud above it before the light itself is above the horizon
   • It shows the light's bearing early, but not its range: wait for the light itself before using a rising range
   • A loom can be seen well beyond the geographical range, so it is a help to identification, not a position line by range

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   We're closing Sandy Hook from the south east on a very clear night. The chart shows Sandy Hook Light, fixed white, twenty six point eight metres, nineteen miles.

   In this visibility it will first be seen at its geographical range. Two point oh eight times the square root of twenty six point eight, plus the same for our height of eye: from the monkey island, fifteen point six metres, about nineteen miles. From the bridge wings, twelve point nine metres, about eighteen point two.

2. Up to the monkey island

   Go up to the monkey island, key nine. The higher the eye, the sooner the light rises. Look fine on the starboard bow, about two nine zero true.

3. It rises

   Watch the horizon. When the light appears, aim the azimuth mirror at it and press T. Note the time: that is when she was at the rising range. Remember this is a compass bearing.

4. Bob the light

   Now go straight down to a bridge wing, key seven. Your eye is nearly three metres lower, so the light has dipped below the horizon again. That is bobbing the light: it proves the light is right on the horizon.

5. It rises again

   Stay on the wing and keep watching. In a few minutes it rises again at the lower height of eye. Take a gyro bearing with the azimuth ring the moment it shows.

6. Fix by rising range and bearing

   Now fix her. On the ECDIS, add a range line from the light at the rising range for that height of eye, and the true bearing taken at the same moment, both timed when it rose. Select them and press Fix. Treat it as approximate: refraction and the tide both move it.

The rules and guidance behind it

• NOAA chart and US Coast Guard Light List: Sandy Hook Light