|MadSci Network: Earth Sciences|
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 email@example.com
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