09 July 2010

What Time is Noon?

Last time, we learned that we need to measure both the exact time of Solar Noon, and the exact height of the Sun over the horizon at Solar Noon. But when is Solar Noon? What time should we scamper out on deck to take our measurements?

If you've never studied celestial navigation before, you might think the answer is obvious. Just look at your ship's clock. 12:00 pm. Lunch time. Easy peasy.

If your ship's clock is a sundial, you'd be correct, because sundials run on Solar Time. And 12:00 pm solar time is exactly when we need to take our sight.

Unfortunately, your clock is probably not a sundial, but one of those new-fangled ticking ones with minute and hour hands, or maybe even a digital clock. And even the fanciest Chelsea ship's clock can't keep accurate solar time.

Why can't normal clocks keep solar time? The answer is incredibly interesting --  Bowditch devotes an entire chapter to it. Unfortunately, the explanation requires lots of math and mumbo-jumbo, so it will have to wait.

However, a picture can save me a thousand fairly technical words...

Suppose you wanted to know when it's Solar Noon in Jacksonville, FL, where the Blue Moon is now waiting patiently for me. The image below shows exactly where the Sun is over the earth, on July 9th, 2010 at 12:00 pm Eastern Daylight Time. Click on the image to see a larger version.

Position of Sun (yellow star) at 12:00 PM in Jacksonville, FL

You can see that the Sun is not due south of Jacksonville, but still dawdling out over the Leeward Islands (well, who can blame him?)

So we can't just look at our ship's clock to predict Solar Noon. On some days of the year, solar noon occurs before 12:00. On other days, it's after. So inconvenient. What can we do?

One solution is to start early. Go out on deck at, say, 10:30 am and start measuring the Sun's height with your sextant. If you measure every few minutes, you'll see the Sun climb higher and higher. At solar noon, it will start to go down.

But suppose you have better things to do with your time. Can we estimate the time of solar noon?

Estimating the Time of Solar Noon

Yes, you can. In fact, with a Nautical Almanac and a bit of math, we can nail it down to the minute. We'll get to that someday (maybe?), but for our Hello World! problem, we'll use a simpler, more concrete method to estimate Solar Noon.

First, set your navigation clock to Greenwich Mean Time. Using GMT eliminates a whole bunch of math and confusion. Explaining why is too complicated for our discussion, but trust me on this. Even when you're an expert, using GMT can save you tons of trouble.

The easiest way to get GMT is to go right to the source, the Royal Observatory website.

GMT from the source
photo wikimedia commons

Next, start thinking in GMT, at least when you're doing navigation. Don't be converting GMT to local time. That way lies confusion, error, and ship wreck.

Put your GMT navigation clock where you can see it, and think GMT. Do it now, before you read any further, so you can read the rest of this blog while thinking in GMT time.

Even a crummy windup alarm clock set to GMT is better than nothing. You want to start thinking in GMT, rather than converting constantly to local time.

Got it? Okay, now look at an ordinary Time Zone map, like the one below. Click on the map for a larger image.
Click on this map for a larger image

You probably know that Greenwich, England is at Longitude 0°. Not coincidentally, it is in the center of the 'Z' time zone (the source of the word 'Zulu Time', used in all those Tom Clancy novels.)

The Sun moves across this map from right to left. It travels at a speed of 1 time zone per hour.

At approximately 12:00 GMT, the Sun will cross Longitude 0°, and the instant it does, it will be solar noon in Greenwich and at every other location on Longitude 0°. ('Approximately', because the actual time can vary by about 15 minutes, but we will ignore this complication for now.)

One half hour later, at 12:30 GMT, it will be solar noon in Casablanca, because the Sun moves west at 1 time zone/hour and you can see from the map that Casablanca is about half a time zone west of Greenwich.

"I told you... never mention solar noon again."

Get it? So if it is solar noon in Greenwich at 12:00 GMT, when will it be solar noon in Jacksonville, FL? See if you can figure it out from the map above. Just measure the approximate distance in time zones and add 1 hour per time zone, plus any needed fractions.

If you answered about 17:30 GMT, you are correct. Jacksonville is about 5.5 time zones west of Greenwich, so it will be solar noon in Jacksonville about 5 hours and 30 minutes after solar noon in Greenwich, or about 17:30 GMT

Try again. How about my current location, in Huntington, NY?

Yup. Huntington is almost exactly 5 time zones west of Greenwich, so it will be solar noon here today at around 17:00 GMT (in fact, it's at 16:58:49 GMT)

Too easy? How about Sydney, Australia, about 14 time zones west of Greenwich? When will it be solar noon over Sydney Harbor Bridge?

Sydney Harbor Bridge, under construction, 1925
photo wikimedia commons

If you add 14 hours to 12:00 GMT, you get 02:00 GMT the following day, or July 10th. See how handy the International Date Line is? This date change will matter to us, later.

So you now have a quick and easy way to estimate the approximate time of solar noon, any place on the planet. All you need is your navigation clock, carefully set to GMT, a time zone map, and your approximate location (called your Dead Reckoning (DR) location, in celestial navigation lingo.)

The time you calculate will be approximate, but that's all we need to get started, because we're going to be on deck well before Solar Noon to start taking sights. And after your first noon sight, you will know the exact time of solar noon at your location. The next day, it will occur at the same time, plus or minus a few minutes, depending on how fast you are sailing.

So, now that we know when to scamper out on deck, what do we do when we get there? That's what we'll talk about, next time!

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>>> Next Episode: The Sights

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  1. Please keep em coming - navigation in small doses suits my brain better than digesting a whole book!

  2. Hello from the UK,
    Been following your blog since the beginning as I too am building a boat. Will now have to get out the sextant I inherited from my uncle.
    Are you sure you're an American? You read Arthur Ransome, follow the world cup and do the Sunday Times crossword!
    Best wishes, Edward

  3. Edward: Born and bred... though I do like a nice bacon sandwich on a cold, misty morning...

  4. One can set their timepiece to Greenwhich time easily enough, on mine, a Casio Atomic Watch, I merely press a buttom once, and there I have it, Greenwhich Time.

    Otherwise, this time of the year in the U.S., Eastern Time Zone, simply ADD 5 hours to your watch time. If I have misstated anything above, let me know.


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