The angle at the celestial pole between the upper celestial meridians of Greenwich and any observer is equivalent to the Longitude of the observer. This follows from the fact that the Greenwich meridian, from which longitudes are measured, lies in the same plane as the Greenwich upper celestial meridian; and the observer’s terrestrial meridian lies in the same plane as the observer’s upper celestial meridian.

It will be clear from the foregoing that time and Longitude are closely related. Longitude is measured E. or W. from the Greenwich meridian, thus the difference between the time at the Greenwich meridian and time at any observers meridian at any instant, is a measure in time units, of the difference of Longitude between the Greenwich meridian and the observer’s meridian. This is, of course, equal to the Longitude of the observer. Longitude, therefore, provides the necessary connection between L.M.T. at any place and G.M.T.

This fact is revealed at once in the time difference, which exists between Greenwich and, for example, New York (see fig. 7-1).The Longitude of New York is roughly 75° W. The Mean Sun, travelling westward at 15° per hour, covers this angular distance in 5 hours. New York is thus 5h W. of Greenwich.

When the Mean Sun reaches the New York meridian its L.H.A. with reference to that meridian is Oh, and L.M.T. would be 12h, but to an observer at Greenwich, who measures the angle from the Greenwich meridian its L.H.A. is 5h (because that is the period which has elapsed since the Mean Sun crossed the Greenwich meridian), and L.M.T. at Greenwich (which is, of course, G.M.T.) would be 17h. In other words, the L.M.T. at New York is 5 hours behind G.M.T.

Similarly, when the Mean Sun is on the Greenwich meridian, its L.H.A. (or G.H.A.) there are Oh and G.M.T. is 12h, but to an observer at New York, its L.H.A. is (24h – 5h) or 19h and L.M.T. 07h because 5 hours must elapse before it reaches the New York meridian.

An intermediate time is considered in fig. 7-1, with the Mean Sun at M, the G.H.A. is the angle APM and the New York LHA is the angle BPM (measured westwards). In this figure, APM is about 3h and GMT, therefore, 15h. BPM, however, is about 22h and the LMT at New York would thus be 10h. The time of any event recorded by an observer at New York thus differs from the time recorded at Greenwich by the 5 hours which form the time equivalent of the difference of longitude of the two places.

What has been said of New York in relation to Greenwich holds good in principle for any other place on the Earth’s surface? To convert the L.M.T. on one meridian into G.M.T. it is, therefore, necessary to apply only the Longitude expressed in time.

It was shown above that at a place 5h. W. of Greenwich the L.M.T. is always 5 hours behind or slow on G.M.T. It is equally true to say that a place 5h E. of Greenwich is 5 hours ahead or fast on GMT. Thus the fact that L.M.T. on any meridian is fast or slow on the L.M.T. on a second meridian depends entirely on whether the first meridian lies E. or W. of the second. When the second meridian is the Greenwich meridian, this relation between the two times can be conveniently summarised by the same rhyme which was introduced in § 1-5, for finding the L.H.A., viz,

Long. W., Greenwich time best;            Long. E., Greenwich time least.

If for example the G.M.T. of an observation is 19h. 23m. 43s the L.M.T. of the same observation at places in 48°W 22½°E would be:


It is clearly impracticable for each place in the World to keep the time of its own meridian. Nor is it practicable for places all over the World to keep the same time. A compromise is therefore affected, and places in the same locality, usually adjusted to include an entire country so far as practicable, elect to keep the same time. That time is usually based on a meridian running through the centre of the area and differing in Longitude from the Greenwich meridian by a convenient number of hours, and is known as the standard time, of the area. The standard times kept by countries throughout the World are listed in the N.A. and in Admiralty List of Radio Signals, Vol. 11.

The standard times kept by the various countries of the World are really a modification of Zone time used by vessels at sea.  In the zone time system, the Earth’s surface is considered to be divided in a N.-S direction into time zones, which may be seen on a time-zone chart (See fig. 7-2). The time zones are regions bounded by meridians whose longitudes differ by 15° or one hour in time. Each time zone, of which there are twelve in the western and twelve in the eastern hemisphere, is designated by a zone number which is prefixed by a plus (+) sign for those in the western hemisphere and a minus sign (-) for those in the eastern hemisphere.

The zone time (Z.T.) in any given zone is always an integral number of hours difference from G.M.T., the number being the same as the zone number. If the zone number is () the G.M.T. at any instant is equal to the Z.T. plus a number of hours equal to the zone number. If the zone number is (-) the G.M.T. at any instant is equal to the Z.T. minus the number of hours equal to the zone number. Thus, if it is, says, 1020 Z.T. in zone (+4) it is 1420 G.M.T. If it is, say, 1650 Z.T. in the zone (-6) it is 1050 G.M.T. etc.

Zone (0) is bounded by the meridians of 7½° E. and 7½° W. and all ships within clocks are set to zone time, keep G.M.T.. (Zone (+1) extends from 7½° W to 22½° W and all ships keeping zone time in zone (+1) keep time, which is one hour behind G.M.T.. Zone (-1) extends from 7½° E to 22½° E and all ships keeping zone time in zone (-1) have their clocks set one hour ahead of G.M.T.

On crossing the boundary of a time zone the clock’s altered one hour so that when sailing westwards the clock is retarded and when sailing eastwards it is advanced.

The time zone which lies diametrically opposite to Zone (0) is divided into two parts by a line, which is known as the International Date Line. This line approximates to but is not exactly coincident with the 180° meridian. The part of this zone which lies between 172½° W. and the date line is designated zone (+12) and the between 172½° E and the date line is designated Zone (-12). When crossing the date line the zone number changes from (+12) to (-12) when sailing westwards. and from (-12) to (+12) when sailing eastwards. It follows, therefore that when crossing the date line, the date will have to be changed, advancing the date by a day when sailing westwards and retarding it a day when sailing eastwards. Thus for a vessel sailing westwards across the date line on, say, Tue 04th Jun, the following day will be Thu 06th Jun whereas if she were sailing eastwards across the date line on Tue 04th Jun, the following day would also be Tue 04th Jun.

The international date line does not coincide exactly with the 180º meridian but is modified in order that certain Pacific islands and other territory such as eastern Siberia which straddle the 180th meridian and which have a common administration, keep a common time and date.


In the N.A. and similar publications used all over the world, it is clearly uneconomic to tabulate information relating to any given instant in each time zone. This is because of the number of headings, which would be necessary for anyone to find the given instant under the heading of the particular time zone he happens to be keeping. Instead, the information is conveniently given under the appropriate G.M.T. heading, because any time zone can be easily converted into the corresponding G.M.T., which is thus the standard time for all tabulation purposes.

It will at once be apparent, however, that in time zones remote from Greenwich the difference between the Z.T. on the one hand and G.M.T. on the other may be enough to change the date, and since the N.A. is prepared in terms of date and time at Greenwich, it is important to be sure that the date at Greenwich, called the Greenwich Date, is correctly calculated.

From what has been said regarding the application of the zone description to zone time in order to obtain G.M.T., it should be seen that the Greenwich date may well differ from that on board a vessel at sea. For instance, for a vessel at sea in Longitude 62° W. (which is in Zone +4) at 2200 hrs Z.T. on Fri 16th May, the Greenwich Date and G.M.T. would be: –

The following examples will also show that the conversion of local time to G.M.T. may or may not change the Greenwich Date according to circumstances, and therefore, that great care must be taken to ensure that the N.A. is entered not only with the correct G.M.T. but the correct Greenwich Date: –

ZT 16:05 (+2) 05th Apr = 18:05 G.M.T. (GD. 05th Apr)            ZT 01:05 (-7) 06th Apr = 18:05 G.M.T. (GD. 05th Apr)

ZT 05:10 (-5) 09th Jul = 00:10 G.M.T. (GD. 09th Jul)                 ZT 16:10 (+8) 08th Jul = 00:10 G.M.T. (GD. 09th Jul)


Since the True Sun moves at a varying speed in the ecliptic and the Mean Sun moves at a constant speed in the celestial equator, their respective hour angles are seldom the same. At some periods in the year the Mean Sun is ahead or W. of the True Sun (as shown in fig. 7-3(a)), by which we mean that the Mean Sun’s hour angle at any instant exceeds that of the True Sun at the same instant.

At other periods, the Mean Sun is behind or E. of the True Sun (as shown in fig. 7-3(b)).

The angle at the celestial pole (or arc of the celestial equator) contained between the hour circles of the Mean and True Suns is called the Equation of Time (usually abbreviated to E). When the Mean Sun is ahead or W. of the True Sun the equation of time is said to be positive, and when the Mean Sun is behind or E. of the True Sun it is said to be negative, so as to conform with the definition that the equation of time is the excess of mean time over apparent time.

At about the 15th Apr, 14th Jun, 01st Sep and 24th Dec the equation of time becomes zero and changes its sign. Its extreme values are +14 minutes and –16 minutes. The value of the equation of time is tabulated in the daily pages of the N.A. for every 12 hours (at 0h. and 12h) on the meridian of Greenwich, but it is not normally necessary for the navigator to use this information because the G.H.A. of the Sun tabulated in the N.A. is, in fact, the G.H.A. of the True Sun tabulated against G.M.T., in other words, the equation of time has already been applied by the compilers of the Almanac from the formula G.H.A.T.S. = G.H.A.M.S. – equation of time.


To conclude this description of time measurement from the navigator’s point of view fig’s 7-4 & 7-5 serve to illustrate the relationship between time and angle. In both diagrams:

P is the N. celestial pole
Arc PO is the upper meridian of the observer.
PL is the loser meridian of the observer.
PM is the meridian of the Mean Sun
PT is the meridian of the True Sun

P ♈︎ is the meridian of the F.P. of Aries

Arc 0 ♈︎ = LST (Local Sidereal Time) 
LOM L.M.T. (Local Mean Time)
LOT LAT (Local Apparent Time)

♈︎ X S.H.A. of  
OM H.A.M.S. (Hour Angle of Mean Sun)
OT H.A.T.S. (Hour Angle of True Sun) 
MT Equation of Time (Negative in this case) 

Note carefully that in fig. 7-4, 7-5
L.M.T.  = H.A.M.S. + 12h
LAT  = H.A.T.S.  + 12h.
Equation of Time  =  L.M.T.    LAT (and is minus) But if the True and Mean Sun’s had been E. of the observer’s meridian, then:
L.M.T. would equal H.A.M.S.- 12h and  LAT would equal H.A.T.S. – 12h.
and if the Mean Sun had been W. of the TRUE SUN the equation of time would have  been POSITIVE.

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