MadSci Network: Earth Sciences

Re: What are the Midnight and Twilight Zones in the ocean?

Date: Tue Feb 13 04:06:20 2001
Posted By: John Metcalfe, Staff, Computing and Information Services, Texas A&M University at Galveston
Area of science: Earth Sciences
ID: 981305488.Es

Dear Jason,
Your question is a little confusing for me, but I'll try to answer it for 
you.  If it doesn't sound right, please write me back and I'll try again.  

Answering about twilight is a bit easier to explain so I'll start with 
that.  According to "Bowditch: American Practical Navigator, 9th edition, 
page 303, section 1910"  Morning twilight ends at sunrise, and evening 
twilight begins at sunset... Now from here there are three different 
measurements for the begining and end of twilight, respectively (or the 
darker limit of twilight).  For civil twilight (the one you're most likly 
to see on the news or in the paper) it's when the center of the sun is 6 
degrees below the celestial horizon.  For nautical twilight (the one we 
use when we're waiting for the sky and horizon to be light enough to do 
celestial navigation) it's when the center of the sun is 12 degrees below 
the celestial horizon.  The last twilight is astronomical twilight, when 
full night sets in.  For astronomical twilight, the sun is 18 degrees 
below the celestial horizon.  

Okay, now you're probably wondering what this celestial horizon thing 
is... The celestial horizon is basically a circle (great circle) for which 
you are the axis.  That circle comes out of the center of the earth 
straight down from where ever you are and goes out into space forever.

Okay, now for the midnight question... This one depends alot on what you 
mean by midnight.  If you're talking about the definition of it, it's when 
the sun is actually 180 degrees around the earth from your position.  Now, 
on a ship, we have time zones just like here on land.  However, they 
aren't as rigid as they are on land.  In fact you could sail from here in 
Texas, where I'm from, to, let's just say Ireland, and never change your 
clocks until you tied up to the dock.  Now I wouldn't recommend this, as 
it would be pretty rough on the crew, but it could be done... and midnight 
would still be the same as here in Texas for the entire time at sea.  
Except you'd have a nice sunny midnight for a large part of the voyage.  

If you still have any questions about all this, please drop me an e-mail 
and I'll give it another try.

John Metcalfe
3rd Mate, Unlimited
United States Merchant Marine

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