Full Moon and Greenwich meridian passage

Roberto

Roberto

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Why is it that for days of full moon, the Almanac (at least the various electronic versions) does not indicate the time of the moon Greenwich Meridian Passage ?

These are the tables for April to September 2014, "FM" are full moon days and the software (I tried two) leaves a blank cell.

It would look like it is "around midnight", but for example second column, month of May 2014, on days 11, 12, 13... the Meridian passage time shows a difference of about 50-ish minutes every day, then 50min over *two* days one of which is full moon, then again about 50-55min delay a day on 15, 16, 17...

View attachment 36154

Should I want to use the HWF&C interval (which by definition is based on Full moon average High Water delay from Meridian Passage) on a full moon day, what Meridian passage time should I use ? Or, what meridian passage time for full moon do they use to compute HWF&C?


I suppose there is a very logical reason as in all astro matters... ?
 
Very interesting, Roberto. I haven't come across the Lunar Meridian passage as part of astro-nav.

I'm assuming that if you know the time of the moon's meridian passage, then you can calculate latitude. Is that correct?

One thought - the moon's meridian passage most often during darkness with no horizon visible, and so no sight possible. I'm only guessing.

Or are you more interested in tides? If so, tell us what you're thinking.
 
Hello Paul,

I was more thinking about tides, (I only have very occasional interest in astro navigation and I keep hearing something in the lines of "if you have a sextant, leave the moon alone" :D ...).

I am sure you are familiar with all this, so sorry if I am stating/repeating the obvious.

I was trying to see what kind of difference versus the official prediction one might expect with a tidal prediction made through the HWF&C method (High Water Full and Change).
The HWF&C is the average interval of time between the local meridian moon passage and local high water, at full moon and new moon.
If it is not full nor new moon, the time interval between HW and local moon meridian passage varies: in a couple of checks I made for my area I found the interval to have two maxima halfway between springs and neaps ***correction, between springs around Full moon and springs around New moon*** , in other words the HWF&C was a sort of minimum interval. I stress I just checked this on two places, probably there is no basis to extrapolate to other locations.

Alternate name would be lunitidal interval, though the UKHO say it should now be called MHWI Mean High Water Interval, (here, about half-way the page):
http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/Support/faq.aspx
They define it as the average period of time between the moon meridian passage and local high water, they have dropped the relation to Full/New moon so I guess the average is computed along all the days of several months.
I suppose the hwf&c interval is more precise at full/new moon, whereas the hwmi minimize the error all along the lunar month.


Anyway, in the absence of the time of the moon Gwch meridian passage for a full moon day, how do I apply either interval (hwf&c or mhwi) ?

Why there is no time indication for the moon meridian passage on full moon? or where does the moon go when full if it does not pass over Greenwich meridian ? :D

Sure it's all pure curiosity and of little practical use, just for the sake of understanding..


r



***just edited above, between the asterisks***
 
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Firstly, I think your tables are wrong. The apparent amount the moon moves forward each day doesn't change at full moon, it's still about 50 mins a day. I don't recognise the tables you give an extract from, but I think they are simply in error. I checked with this site http://aa.usno.navy.mil/data/docs/celnavtable.php and find:

Date GHA at midnight
16/10/13 27 38'
17/10/13 15 48'
18/10/13 03 58'
19/10/13 352 04'
20/10/13 340 06'

This brackets full moon and the interval is much the same each day (ie around 11 56' or 48 mins per day).

However as O.V. says, why would you want to know? Moon meridian passage time is useless for a sight I'd have thought as you still need a chronometer when taking moon sights as declination changes so fast. Hence no point taking a sight at meridian passage even if you can see the horizon well, just take a moon sight in the ordinary way (actually I think taking the sun at meridian passage is an anachronism, but not everyone agrees).

Secondly, if only for HWF&C you don't need the time of meridian passage - it's always midnight local time as full moon is defined to be when you, the Sun and the Moon are in line. Hence you only need the date as tides are the same local time time every HWF&C. The tide time calculated like this will be in error by only the sun-dial error, aka equation of time, which is only a few mins and so can largely be ignored for tidal purposes.

In between Springs, when HWF&C is most accurate, you are adding two sinusoids of unequal amplitudes, one from the sun and the other from the moon, with phases differing each day, which changes the phase of the vector sum and hence the time of HW.
 
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jdc said:
Secondly, if only for HWF&C you don't need the time of meridian passage - it's always midnight local time as full moon is defined to be when the Sun and Moon are opposite (for full, and the same side if new). Hence you only need the date as tides are the same local time time every HWF&C. The tide time calculated like this will be in error by only the sun-dial error, aka equation of time, which is only a few mins and so can largely be ignored for tidal purposes.
Click to expand...


Sorry I should have expressed better on my first message, yes I was just thinking about tidal calculations.
Thanks for your reply, it's a bit late now, I'll try to visualize what you mean tomorrow morning :)


I just looked at an official (printed) almanach: actually, the moon meridian passage time at full moon is the same as the following day. The almanac is from 1994 but for Moon Meridian passage it gives:
May 24th (one day before FM) 2348h,
May 25th (full moon) 2450h (not sure why they write 2450?)
May 26th (one day after Full moon) again 0050h
May 27th, 0152h..

Similar behaviour around all the other full moon days of the year.

:confused:
 
Roberto said:
Sorry I should have expressed better on my first message, yes I was just thinking about tidal calculations.
Thanks for your reply, it's a bit late now, I'll try to visualize what you mean tomorrow morning :)


I just looked at an official (printed) almanach: actually, the moon meridian passage time at full moon is the same as the following day. The almanac is from 1994 but for Moon Meridian passage it gives:
May 24th (one day before FM) 2348h,
May 25th (full moon) 2450h (not sure why they write 2450?)
May 26th (one day after Full moon) again 0050h
May 27th, 0152h..

Similar behaviour around all the other full moon days of the year.

:confused:
Click to expand...

The reason that the almanack says 24:50 is the clue to the answer. There is no meridian passage during the 24 hours at full moon, so the entry is blank. I was wrong when I said the almanack was in error, it's just a confusing way of tabulating it. Ie, in your example, the 25th of May doesn't have a meridian passage, it's actually 50 mins after midnight, so on the 26th.
 
jdc said:
There is no meridian passage during the 24 hours at full moon,
Click to expand...


That's an interesting one, I'll take a lamp, an orange and a pingpong ball (I don't play golf) to try and visualize it, thanks.

It's weird though they (UKHO etc) had built this system (hwf&c, mhwi etc) with intervals computed from a specific moment that does not actually exist.
 
Roberto said:
That's an interesting one, I'll take a lamp, an orange and a pingpong ball (I don't play golf) to try and visualize it, thanks.

It's weird though they (UKHO etc) had built this system (hwf&c, mhwi etc) with intervals computed from a specific moment that does not actually exist.
Click to expand...

UKHO don't use HWF&C or MHWI, they use harmonics, which even if you only have data for M2 and S2 give you more and better data:

For a port with:
M2: g of 354° and H of 2.02m
S2: g of 051° and H of 0.58m

Mean Spring range = 2(H of M2 + H of S2) = 2(2.02 + 0.58) = 5.20m
Mean Neap range = 2(H of M2 - H of S2) = 2(2.02 - .58) = 2.88m
Mean tide range = 2 * H of M2 = 2 * 2.02 = 4.04m
Time of day for spring tide = g of S2 / 30 = 51/30 = 1.7h = 01:42 or 13:42 (these are UTC for the UK, of course)
Spring lag = (g of s3 - g of M2 (+360 if negative) ) / 24.96 = (51 - 354 + 360) / 24.96 = 57 / 24.96 = 2¼ days (springs are 2¼ days after new and full moon)

Tide at this port today (last full moon 2½ days ago) : 13:43 (UTC) and a range of 5.54m.

There are various internet sources for harmonic data - or there's always the UKHO tide tables.

John
 
John Barry said:
UKHO don't use HWF&C or MHWI, they use harmonics,
Click to expand...

Yes of course, the purpose was to have a look at the discrepancies between historical methods like HWF&C, which was the only method taught for example in this early 800' "Epitome of Navigation.." and today's harmonic constituents derived values.

View attachment 36200

Still, today there are places -for example some inland Brazilian charts, admittedly for remote places- where the National Hydro Office does not provide harmonic derived tide times but only a value for HWF&C. :)

regards
r
 
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