Do navigation lights move faster when near ?

[capecodda]

If the lights you see have a constant bearing to you, you are on a collision course.

Let's try the following case. A large motor driven vessel (ship) is on a collision course with you. Let's try and simulate what we would see. Corrections welcome

In this specific case (an interesting one I would think ), the lights do not appear to move except as mentioned as a large ship's red and green side lights might appear to move away from each other horizontally (assuming they are positioned on the sides of a beamy ship) and masthead light(s) (2 if over 50 meters in length) appear higher and higher as the bow of the ship gets dangerously close. Before you've seen this motion, you've hopefully altered course appropriately according to the rules (and maybe the other guy has too if you are lucky and he knows the rules). You've altered your course to starboard.

After you've changed course according to the rules, his green light should go away and the red light should be visible along with the 2 mast head lights (over 50 meters). As the ship goes by, the 2 mast head lights would reveal themselves to be one behind the other, with the aft most light higher than the forward light. You'll observe these 2 lights, along with the red side light. Once he goes by even further, both the mast heads and red side light go away, and all you see is the stern light. Now you can really breath easy.
The speed at which all these changes occurs would be faster if your were closer.

[jrd22]

Lapworth- I guess I'm not getting the point of all this. Is it to try and determine how far away the other boat is from you? Without knowing their speed I don't see how you could determine distance?

[lapworth]

jrd22- No real point just something two play around with. I also think I can make a good estimate on how fast another sailboat is moving just by checking my speed. There is no real point for me to use a sextant on the bay but sometimes I do. If you really think about it why even sail.

[jrd22]

You got me thinking about if there is a way to determine distance at night based on the lights (without looking at the radar). Wasn't sure if that was what you were trying to do or just a mental exercise. If it's dark and you don't know what the other boat is (sail, power, etc) and without knowing the speed I don't know how you would be able to make a determination about distance. If you were in an area where there was a shoreline and lights and you knew the distance to the shore you might be able to make some kind of guesstimate, not sure how accurate it might be though. Usually at night if I see the lights moving I don't waste much time thinking about them, it's the ones that don't seem to be moving that I pay attention to. I always have the radar on at night so I've never really thought about this.

[jackdale]

You can judge distance if you know the height of your eye and the height of a light as it appears on the horizon (measured in feet)

1.17 * sqrt ( Height of the Eye ) + 1.17 * sqrt ( Height of the Light ) = distance in NM

Knowing the height of the light is the issue.

[jackdale]

Originally Posted by capecodda

If the lights you see have a constant bearing to you, you are on a collision course.

Use a hand bearing compass get your bearing. For larger vessels, close by, the bearing change should be substantial.

[svHyLyte]

A vessel at 2000 yards making 5 knots "apparent" will traverses an angle of 10º in approximately 2 minutes. At 4000 yards it will take a bit over four minutes. I use the term "apparent" because when looking at a running light only, one cannot be sure that the vessel is making way perdendicular to the included angle and very likely is not. If the ship's course is 45º to the axis of the included angle, it's actual speed would be about 7.1 knots. At 60º its actual speed would be 10 knots. The apparent aspect of the light is an important factor although so long as the bearing is changing relative to ones own ship there is no risk of collision provided everyone maintains course and speed, eh?

[capecodda]

Originally Posted by jackdale

Use a hand bearing compass get your bearing. For larger vessels, close by, the bearing change should be substantial.

Absolutely Right!