Do navigation lights move faster when near ? - SailNet Community
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post #1 of 18 Old 01-07-2011 Thread Starter
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Do navigation lights move faster when near ?

I was coming home last night and noticed a plane flying by and he was going fast over the horizon. I know with a motor boat this would make as much sense because they can change there speed. So what I was wondering is could you get a better feel for the distance of a boat from you by how fast they are moving over the water ( I know there not moving faster @ closer distance ).
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post #2 of 18 Old 01-07-2011
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Perception is everything.

The questions that you're trying to answer are:

Is this a small, dimly lit vessel crossing me close by or
Is this a large, brightly lit vessel crossing me far away?

They might look very similar.

Things you can do to help determine this are to observe how high off of the water the lights are. That might indicate a larger vessel if they're high off the water.

Then you observe how fast the lights are moving past fixed points on land (or fixed lights if you can't make out a land feature). Also, if you suspect that the vessel might be close aboard, listen for engine and wake noises (though you might not hear any.)

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post #3 of 18 Old 01-07-2011
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By "light's moving faster", you might mean changing their relative bearing to you. In other words, you are turning your neck as you watch the lights.

I'd only add that the change in bearing to the light can tell you something, but not enough without BubbleHeads' answers.

A faster bearing change will happen if the boat is
closer
faster
not heading to intercept you (If he's going to hit you there will be no bearing change.)
Also, the perceived height angle (azimuth) could change fast, but by then it's probaly too late.

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Last edited by Bene505; 01-07-2011 at 10:13 AM.
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post #4 of 18 Old 01-07-2011 Thread Starter
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Let me try to word this another way. If I see a sail boats port light cross in front of me over the horizon and it is sailing at 5 knts about a mile away would it take the same amount of time to cross 5 degrees of vision as if the same boat was 2 ml away still doing 5 knts.
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post #5 of 18 Old 01-07-2011
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Originally Posted by lapworth View Post
Let me try to word this another way. If I see a sail boats port light cross in front of me over the horizon and it is sailing at 5 knts about a mile away would it take the same amount of time to cross 5 degrees of vision as if the same boat was 2 ml away still doing 5 knts.
No, it would take about half the time.
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post #6 of 18 Old 01-07-2011
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The lights you see that are "moving" aren't the ones that are the most dangerous to you.

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post #7 of 18 Old 01-07-2011 Thread Starter
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No, it would take about half the time.
Then I should be able to make a device that could estimate distance of moving objects. Maybe something similar to the way you use a sextant on a bridge or light house.
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post #8 of 18 Old 01-07-2011
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Then I should be able to make a device that could estimate distance of moving objects. Maybe something similar to the way you use a sextant on a bridge or light house.
That would work, as long as you know the speed and angle the object is moving at. Of course, you could ask them via radio :-)
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post #9 of 18 Old 01-07-2011 Thread Starter
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That would work, as long as you know the speed and angle the object is moving at. Of course, you could ask them via radio :-)
I could use it other ways. If I know it is in the channel and I know how far away I am then I would be able to calculate there speed.
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post #10 of 18 Old 01-07-2011
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I could use it other ways. If I know it is in the channel and I know how far away I am then I would be able to calculate there speed.
Very true! If they're in the channel you can make reasonable estimates of both their distance and their course. Isn't mathematics wonderful?
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