NavList:

Hi John.

Welcome. Glad to hear you figured out the diagram. Since there may be others with questions, let's have a look at it...

I've included (below; attached) a portion of the diagram from the official Nautical Almanac from 1994. This edition of the NA has been available for download for many years, most likely due to a copyright violation in the early days of Google Books, but it has not been rectified, so I have assumed that HMNAO is not concerned about a single-year "sample". Here it is: Nautical Almanac 1994 (pdf). Diagrams like this have been around for a long time... Similar diagrams are often found in resources for astronomy, not just celestial navigation.

The key to understanding the diagram is the label at the top and bottom: Local Mean Time of Meridian Passage. What is "Local Mean Time", or LMT, as a shorthand, and what is "meridian passage"? First, LMT is close to local "Sun time". If it's near local noon where you are, and the Sun is at its highest altitude, due South or due North in azimuth, then the LMT is close to 12:00. It's not quite 12:00 because the Sun is not a great time-keeper. The Sun runs up to 15 minutes "fast" or "slow" compared to true time or the time kept by a clock "ticking" at a constant, steady rate, so actually the Sun's position gives us Local Apparent Time, which you could call LAT (hmmm... too easily confused with "Lat" for latitude?) or L.App.T. or just plain "Sundial Time". The difference between Sundial Time or L.App.T. and Local Mean Time or LMT is a correction called the "Equation of Time" (EqT) which takes care of those differences due to the Sun being "fast" or "slow". This is less than about 15 minutes on any date and nearly repeats its (double) oscillation pattern every year. Note that "equation" here is not much connected with the modern algebraic meaning of the word, and in earlier centuries, "equation" was closer in meaning to our word "correction".

We can see the Equation of Time in the planets diagram. The slightly wobbly solid line running straight down the middle of the diagram close to 12h Local Mean Time represents the Sun. That line deviates from 12h by about a quarter of an hour at maximum. In early November, as highlighted, this is on the "early side" of 1200, so it's telling us that the Sun is ahead of schedule in early November. If you look at the complete diagram, can you see the time of year when the Sun is "late" by almost the same amount of time, on the "late side" of 1200? Note: for reasons lost to the mists of time, the designers chose to draw this diagram with January at the bottom, December at the top, and the hours of time running from right to left (actually the latter choice has some value because it lets you look at the diagram as a direct representation of the sky, at least in the northern hemisphere).

Back to definitions of time scales...

LMT is Local Mean Time, and if you happen to live on the Prime Meridian, 0° of longitude, then your local time is the same as the time in Greenwich UK, so you're on Greenwich Mean Time or GMT. So GMT is the value of LMT on the Prime Meridian. The LMT at any place on the Earth is related to the (absolute, univeral) GMT, or UT, as it's better termed today, by the longitude itself. If you know the current time GMT / UT, you can get your LMT by expressing your longitude in time units and then adding that to GMT if you're in east longitude or subtracting in west longitude. To get longitude as time, take longitude in degrees and divide by 15. For example, my longitude is 71.5° (close enough for this purpose). Dividing by 15, I get 4.766... hours. Then take off the fractional part and multiply that by 60 to get minutes. So in this example, 0.7666... multiplied by 60 yields 46 minutes. Thus my longitude "in time units" is 4h46m, and that is also my LMT offset from GMT or UT. I'm in west longitude so I subtract that from UT. As I write this, the UT is 13:28 so my LMT is 4 hours 48 minutes earlier or 08:42. This is the best way to get the correct LMT for your location. Read a clock (or an app) for UT and then apply your longitude in time units.

In my example above, I found that my LMT was 08:42, but reading my clock the time is 09:30. What's wrong here?? First, the clock I am reading is two minutes fast (it's an analog wall clock, and I have to reset it every few months). Second, clocks here are on "daylight time" which means there's an extra hour added in. I should really be comparing with 08:28 then (two minutes off for clock error and removing that extra hour for DST). The difference is now only 14 minutes from 08:28 on the clock to my calculated LMT of 08:42. This remainder is the "zone time" offset. Often LMT is close to local "zone time" (without daylight time or equivalent) and differs by a modest amount like this, merely 14 minutes. I can live with that when planning sights for practice. But there are many exceptions. Zone time was designed to average out local longitude differences so at my location at 71.5° W longitude we keep time as if we live at 75° W longitude where the time offset to GMT/UT is exactly five hours. That was the plan, but many places around the world have substantial differences in LMT from local zone time, and you can't depend on this difference being small (even if you do remember to take out the daylight time offset). It's safer to use the system I described above: LMT is connected to GMT/UT by the longitude converted to time units (LMT = UT +/- lon/15°). Just forget about zone time in celestial navigation, if at all possible.

At any date on the planet diagram, we're looking at a basic simulation of the sky. It shows when each of the brighter planets crosses the local observer's meridian (true with almost no modification for any observer on the globe on that calendar date). Note that the planet Mercury is included. Mercury is otherwise omitted in the Nautical Almanac, and this diagram may be the only exception to that standard. It's useful to think of this local "meridian passage" as being "like" the Sun's "noon" for each planet. The local meridian is that line, or arc, on the sky running from the horizon exactly due South, through the local zenith (90° altitude exactly) and down to the horizon at due North. The Sun crosses or transits or "passes" the local meridian at local noon. By LMT the Sun's meridian passage or meridian transit occurs at 12:00 give or take 15 minutes or so (the Equation of Time, as above). The planets and other celestial bodies cross the meridian at other times of day.

If you place a straight-edge horizontally on the diagram at any date, you can read across and find the time of meridian passage for each of the planets. Let's try 5 Nov 1994. I've highlighted that date on the sample image below. We can see, first of all, that the Sun's meridian passage on this date is about 11:45 (remember, earlier is towards the right in this version of this diagram). That's consistent: the Equation of Time reaches a peak very close to this date of about 15 minutes early, implying that the Sun is fast, ahead of schedule for all its events of the day (not just meridian passage but also sunrise, sunset, etc.). Meanwhile, if we follow the curves for Jupiter and Venus, we find that they are in the shaded band within 45 minutes of the Sun, which means that they would be somewhat difficult to see on this date (not necessarily true, but that's what the diagram is suggesting). Then if we follow the curves for Saturn, Mercury, and Mars, we find that they transit (or cross or "pass") the meridian at about 19:30, 10:40, and 06:20 respectively. Well that's nice. But what's the point? We can't "see" the meridian transit of Mercury certainly so what good is it to know this? Well, here's where the diagram reveals that it's not as useful as it first appears. The diagram is telling us that Mercury is about 1h05m ahead of the Sun as it crosses the sky. So if we step out on deck 45 minutes before sunrise, we can expect that Mercury will be above the horizon towards the east because it would have risen something like twenty minutes earlier. But this is not exact at all, and the diagram could be used this way only in those cases where the Declination of the planet happens to be nearly the same as the Sun's. It also works without adjustment in low latitudes, near the equator. Otherwise, it's not really all that relevant to the needs of celestial navigation! The diagram is a crude approximation.

I saved those last lines for the final point of the last paragraph intentionally.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island, North America

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