The measurement of time is calculated from an internationally agreed zero point, Greenwich, in the United Kingdom, giving us the term "Greenwich Mean Time". It is more frequently referred to now as Universal Time. Every 15 degrees of longitude east or west of Greenwich equals one hour earlier or later than GMT, and is the means by which Local Time is calculated from GMT or UT. One degree of longitude equals about four minutes of time.
For a detailed understanding of the relationship between the measurement of time and lines of longitude, we recommend Dava Sobell's best-selling book Longitude, which can be bought from on-line bookstores such as Amazon and Booktopia, etc.
These maps show the major lines of longitude on which Local Time is calculated. These are marked "SAME". The predicted best fishing times shown in the app are the same for any place on or close to the SAME lines. Drawn east or west of these SAME lines are similarly coloured lesser lines which indicate that you should add (if west) or subtract (if east) 20 minutes if the place where you plan to fish is near these lesser lines. You could make even smaller time corrections within these lines but we doubt they will add significantly to your bite rate. We always advise being set up and fishing a minimum of 15 minutes before the predicted times. Variations can and do occur.
When Local Time is set by geopolitical boundaries instead of local longitude, further correction of the tables will be required.
AUSTRALIA, NEW ZEALAND, PAPUA NEW GUINEA
If the Moon passes over Rockhampton, Australia, at 9.22am Local Time in Rockhampton (about 150o East longitude), it will pass over Nanking in China (120o East longitude) at exactly the same Local Time in Nanking of 9.22am.
Where time zones coincide with the lines of longitude shown on the map, the predicted times for that line are shown as “Same”. The lines of longitude are shown at increments of 5o. For every line of longitude east of your nearest time zone line, subtract 20 minutes (-20). For every line of longitude west of that time zone line, add 20 minutes (+20).
Adjustment can easily be made mentally for correction of variations either side of a line of longitude (or time line).
Brisbane is about two thirds of the distance between 150o East and 155o East. If 5o equals 20 minutes, two-thirds of 20 minutes is about 14 minutes. Subtract 14 minutes from the predicted times to gain the corrected predicted time for Brisbane.
Sydney, at 151o East longitude, is only one degree east of the 150o line, which equals a four-minute variation. You could subtract four minutes, but it probably won't make much difference to your catch rate.
Melbourne is almost 5o west of the nearest “Same” line (150o East): add 20 minutes.
Perth is about four fifths west of its nearest “Same” line (120o East): add 16 minutes.
Central Time for South Australia and the Northern Territory does not coincide with the normal longitudinal adjustment for local time variations based on Greenwich. Add or subtract the times shown on the map, which has been adjusted according to Central Time to provide the corrected Local Time for these areas.
For example, although Le Havre is next to the “SAME” line from which Western European Time (W.E.T.) is taken (0o longitude), its Local Time is based on Central European Time (C.E.T.), shown on this map of Europe in yellow, which is an hour earlier. Anyone whose watch is set to C.E.T. and is fishing along the line passing through Le Havre would need to add an hour to these predicted times.
Anglers at Algiers on C.E.T. would need to add about 50 minutes while those dipping into the Danube at Budapest would need to subtract about 15 minutes.
Gibraltar on C.E.T. would need to add about 80 minutes whereas Tangier across the strait would add only 20 minutes as its Local Time is set to Western European Time.
Odessa, on Eastern European Time (E.E.T.) would be the “SAME” while Athens could add about 25 minutes.
Moscow takes its time from 45o East Longitude, which means anglers there would have to add 30 minutes, and those at Sevastopol in the Crimea would add about 45 minutes. Belarus has switched its Local Time to Moscow time, which means the folk in Minsk would have to add 70 minutes to their Local Time.
Yet Kiev to the south-east is on Eastern European Time like Odessa and would not have to adjust.
Although Buenos Aires in Argentina is next to the “SAME” line from which Central South American Time is taken (60o West Longitude), its Local Time is based on 45o West Longitude which is the “SAME” line which passes just west of Rio De Janeiro in Brazil. That means that anyone fishing near Buenos Aires and using Local Time should add 60 minutes to the predicted times.
Manaus, on the Amazon, also is in Brazil but it takes its Local Time from 60o West Longitude and anglers there would not need to adjust.
Although Cape Town in South Africa is closer to 15o East Longitude on which Central European Time is based, it has the same Local Time as the rest of South Africa, which takes its Local Time from 30o East Longitude, the same as Eastern Europe. Anyone fishing near Cape Town would need to add about 45 minutes to the predicted times, and those at Port Elizabeth would need to add 20 minutes.
Anyone fishing on the Tanzanian side of Lake Tanganyika would have to add an hour to their Local Time which is based on 45o East Longitude (the “SAME” line passing up through Madagascar).
Someone fishing on the western side of the lake using Zaire’s Local Time time at, say, Kalemie, would not have to adjust unless they were particular about adding five minutes.
For Beira in Mozambique, subtract about 20 minutes.
Lowestoft, on the North Sea coast in Suffolk, is almost two degrees east (earlier) of the GMT SAME line and you could subtract eight minutes to correct the Almanac’s predicted best fishing times. But because we always advise that you should be set up and fishing at least 15 minutes before the predicted times occur, and because we know that that’s what you will do, there wouldn’t be much point in applying the eight minutes.
However, at Falmouth in Cornwall, you would add the 20-minute correction of the lesser line almost passing through the town as shown on our map.
Uig on the Isle of Skye is west of the +20 line by about one and a half degrees and you could add a further six minutes but, again, there’s not much point if you are already setting up 15 minutes or earlier.
Clifden, in the far west of Ireland in County Galway, is pretty much astride 10o West longitude and 40 minutes should be added to the predicted times.
UNITED STATES OF AMERICA
An extreme example of time differences when geopolitical time zones take precedence over longitude are El Paso in Texas and New Orleans in Louisiana. If the Moon passes over El Paso at 9.30a.m. Local Time, the Moon will already have passed over New Orleans almost 65 minutes earlier, even though the clocks in both places show 9.30a.m. That is because the clocks in El Paso are aligned to the rest of Texas which is on Central Time, as is New Orleans in Louisiana.
Because El Paso is so far west but still within Central Time, you have to count three lines (3 x 20 = 60 minutes) across from the "SAME" line passing through New Orleans, then add its distance west of the third line, giving a total of 65 minutes to be added to the times given in the tables. Nashville in Tennessee also is on Central Time but it is east of the Central Time SAME line. It is about two-thirds of the distance to the Central Time’s –20 line, which means you should allow for the predictions to start happening about 10 minutes earlier than shown in the tables. Be set up and fishing at least 25 minutes earlier.
The folk in Pueblo, which is east of El Paso but on Mountain Time, won't have to do any maths. They can pretty much use the tables as they appear.
Boise, on the Boise River in Idaho, also is on Mountain Time but is west of Las Vegas which is on Pacific Time. The folk in Boise should add about 45 minutes to the times in the predictions. The folk in Las Vegas would do well to forget about any fish and concentrate on their chips.
DAYLIGHT SAVING TIME
Anglers fishing in regions which apply Daylight Saving Time, also known in some places as Summer Time, must correct for the adjustment by adding the difference, usually one hour, to the predicted times and on top of any geopolitical time differences.