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vinniepaz
08-22-2005, 12:35 AM
So some relatives of mine have a little summer house at Cape May Point, New Jersey. The defining characteristic of Cape May Point is a lighthouse. From the window of my bedroom at this house, I could see the lighthouse which was illuminated at night. But there was a problem. This is kind of hard to describe:

This lighthouse, as I assume all lighthouses, has a light that spins around 360 degress on its axis. Now, from my vantage point, I can see what I assume to be approximately 180 of these degrees. The light passes by my field of vision in approximately 4 seconds, from when I can first see it to when I can't at all. Here's the problem. The time it takes to get back around to where I can see it is at least three times the amount of time that I can see it in. I vaguely timed it in my head, and got 4 seconds to pass through my field of vision and 13 seconds to get back.

Why would this be?

Johnny L.A.
08-22-2005, 12:41 AM
Where is the house in relation to the lighthouse? Let's assume that the beam is shielded along 180°, and your house is 90° from the front. You would see ¼ of the rotation. If ¼ rotation takes four seconds, then ¾ rotation (which you don't see) would take 12 seconds -- three times as long.

Johnny L.A.
08-22-2005, 12:58 AM
I should point out that I don't know if lighthouses actually shield their beams.

08-22-2005, 01:08 AM
Now, from my vantage point, I can see what I assume to be approximately 180 of these degrees.Emphasis mine. This is probably a wrong assumption as lighthouse probably has a fresnel lens to focus the beacon. The time you see the light vs the time you don't makes it easy to calculate the width of the beam, in this case 4/(13 +4)*360 = about 85º.

Johnny L.A.
08-22-2005, 01:09 AM
This is probably a wrong assumption as lighthouse probably has a fresnel lens to focus the beacon.
:smack: I completely forgot about the fresnel lens!

panamajack
08-22-2005, 01:45 AM
If you can see the beam on other points inland, you could see how wide of an angle you're really catching.

On another note - The time between flashes/eclipses* of the light is different for lighthouses in a given region, as a backup navigational aid. This is known as the light's "signature".

*An "occulting light" is shining more often than not, and thus has eclipses rather than flashes.

08-22-2005, 03:35 AM
:smack: I completely forgot about the fresnel lens!
You MORON! :D

Scruloose
08-22-2005, 04:00 AM
This lighthouse, as I assume all lighthouses, has a light that spins around 360 degress on its axis. Now, from my vantage point, I can see what I assume to be approximately 180 of these degrees. The light passes by my field of vision in approximately 4 seconds, from when I can first see it to when I can't at all. Here's the problem. The time it takes to get back around to where I can see it is at least three times the amount of time that I can see it in. I vaguely timed it in my head, and got 4 seconds to pass through my field of vision and 13 seconds to get back.
I think Johnny L.A. and Padeye have it - you're only seeing a section of the light - not half of it. Looking at a chart (http://mapserver.maptech.com/homepage/index.cfm?lat=38.9182332783&lon=-74.9726103932&scale=80000&zoom=50&type=0&height=498&width=498&icon=0&searchscope=dom&CFID=2984904&CFTOKEN=73994028&scriptfile=http://mapserver.maptech.com/homepage/index.cfm&bpid=MAP0060030900%2C1%2C1%2C0&latlontype=DMS),the light is flashing every 15 seconds. Too me, that says the light is probably a single lamp rotating within a fresnel lantern. That beam will be greatly focused. Not all lighthouses work this way, however. Some lighthouses will be a non-rotating lamp and lense - no different than what you'd find on a buoy, only larger. Some lighthouses have a fixed lamp and there are paneled lenses which rotate around the lamp. An example. (http://www.lighthouse.cc/watchhill/VRB25.102_0220.jpg) These panels can be blanked, or colored as in the pic. There is a wide variety of ways lighthouses can be figured.
I should point out that I don't know if lighthouses actually shield their beams.Sure they do - all the time. Sometimes they will blank out one or more sections of light for whatever reason, and sometimes an arc of light will be a different color. The latter is often used to aid in navigating through hazards. For example, a small section of white light will illuminate a safe route through a harbor. If you see red, you're outside the safe zone. For example, see the light at the end of Spring Point. (http://mapserver.maptech.com/homepage/index.cfm?lat=43.6514707643&lon=-70.2278265876&scale=40000&zoom=50&type=0&icon=0&searchscope=dom&CFID=2984904&CFTOKEN=73994028&scriptfile=http://mapserver.maptech.com/homepage/index.cfm&latlontype=DMS) There are two large sections of red light facing South to Southwest, and Southeast. Between them is a small section of white light illuminating the safe route.

Johnny L.A.
08-22-2005, 08:52 AM
You MORON! :D
Well, in my defence I usually use my open lights. Haven't needed the 2K fresnels yet.

Billdo
08-22-2005, 01:44 PM
You're looking at the lighthouse from fairly close by.

If you're a bit out at sea, what you will see is just a sharp flash every 15 seconds as the beam is pointed directly at you. You won't see the rotating beam sweeping toward or away from you.

Since you were so near the light, you probably could see some of the reflection of the light within the top of the lighthouse (or the trace of the beam on the ground/in the sky) as the beam swept toward you. When the light was pointed directly toward you, it was probably a rather blinding focused flash. That flash is what is seen by those at a distance.

(By the way, when you are coming in to Cape May harbor, believe the section of the chart that says there is a danger area just to the south of the inlet breakwater. If you cut across it, you are likely to find yourself being shot at by Coast Guardsmen using their firing range.)

What Exit?
08-22-2005, 01:50 PM
Go visit it, it's a fun little outing. I went last year. It cheap and has a nice largely empty beach next to it. There is even a small science center between the Lighthouse & the beach.

To the rest, I think you've got the correct answer above.

Scruloose
08-22-2005, 03:59 PM
(By the way, when you are coming in to Cape May harbor, believe the section of the chart that says there is a danger area just to the south of the inlet breakwater. If you cut across it, you are likely to find yourself being shot at by Coast Guardsmen using their firing range.)
And not just any Coast Guardsmen - recruits! :eek: