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    Re: Online Nautical Almanac
    From: Frank Reed
    Date: 2011 Feb 13, 08:53 -0800

    Hello Matt,
    You wrote:
    "I'm in the far South suburbs - Monee on I-57, 10 sm S. of I-80."

    Greetings from Wrigleyville.

    There are a couple of spots not too far from you where you could try LAN sights over a real horizon. Cedar Lake due east of you in Indiana is marginal, and there appears to be a boat launching ramp right at the north end of the lake, perfect for sights. Probably the best place is Waukegan Pier. That's about an hour and three-quarters from you if the traffic is moderate. If you look at this location on a map, you will find that most of the pier has a wide open view to the south, but there's land some distance south as the lake shoreline gradually curves around. Notice that the distance due south to the nearest land is almost seven nautical miles. Since the distance to the horizon is approximately equal to the square root of your height of eye in feet (actually a little greater than that), if you're sitting/standing on the pier with a height of eye above the lake water of some 9 to 16 feet, the horizon is only about three or four miles away. The horizon is, therefore, in front of the land due south of you. It can be a little hard to picture this. The curvature of the Earth means that the lake is like a hill in front of you and the top of this "hill" is in front of the land beyond. So even if you can see trees and buildings far in the distance, the water line you see before that distant landscape is a true horizon, every bit as good as a wide open ocean horizon. In addition, the end of the pier there has clear views for over 120 degrees to the east and northeast so you can practice Sun sights all morning long. I don't know if the pier is accessible after dark but perhaps in fishing season and then you can try stars, too...

    You also wrote:
    "I expect that I'll just practice with what I can. I'm probably not going to convince my wife I need afew hundred bucks worth of mercury so soon after I convinced her that I needed an Astra III!"

    You don't need Mercury, though it's the best. You can use water or better yet heavy oil.

    You concluded:
    "Here's what I'm looking for an online almanac for. So far, the only thing I know how to do is figure LAN. I've found some calculator type almanacs and I have a Long Term Almanac through Starpath, but none have Mer Pass of the sun - at least I haven't been able to find it. "

    You can figure out the time of meridian passage a number of ways. There are online tools that will do this for you, but let's do it the right way. You do need one table: the "equation of time". Go to Wikipedia or analemma.com and you can find data good enough for your purposes. This will tell you how early or late the Sun is to the meridian at noon on any day of the year compared to a good clock. Just a couple of days ago was what I call "Sun Slow Day". It's the minimum of the equation of time for the year making it the day when the Sun is laziest and doesn't get to the meridian (LAN) until about 14 minutes and 13 seconds after mean noon. So, if that was all there was to it, the Sun would arrive on the meridian right around 12:14:13. But there are two catches with this calculation. First, you need to ask yourself whether we're on Daylight Time or not and, if so, subtract an hour from your watch. We won't be on Daylight Time until Spring so you don't have to worry about that for now. Second, you have to correct for the fact that we set our watches to "Standard Times" which are really only accurate as "mean time" at the center of the time zones. The center of the Central Time zone in the US is 90 degrees West longitude. In Monee, your longitude is about 2.25 degrees east of that. The Sun takes 24 hours to do a 360 degree lap, so that's 15 degrees in an hour or ONE degree in four minutes (memorize that!). That means that the Sun will reach LAN nine minutes earlier (2.25 degrees * 4 minutes/degree) at your location than implied by your watch time. So for February 11, LAN occurred in Monee right around 12:05:13. The "equation of time" is now heading towards zero so it's a bit earlier each day as the Sun "catches up" with mean time.

    But why bother with all of this? A navigator at sea wouldn't know when LAN was going to occur unless he or she already knew the vessel's position. Instead of calculating the time of LAN, you should just observe it. Take sights every few minutes and wait for the maximum. That's how it's done.

    In answer to your question, here's the equation of time and the declination of the Sun for 0h UT for the next couple of weekends:
    2/19: 13m52s 11d 28.0' S
    2/20: 13m47s 11d 06.7' S
    2/26: 13m00s 08d 55.1' S
    2/27: 12m50s 08d 32.7' S

    -FER


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