Time zone – YM


Before reading this page grab a copy of the training almanac (and a real one if possible) so that you can follow what is going on.

Why is this important?

Before you do a tidal calculation (either tidal height or tidal stream) you need to know which time zone the data from the almanac is in and which time zone you are working in.

Time zones of the world

To make life easier we could have one time zone that covers the whole earth however in general countries, cities and villages around the world have in the past set there clocks so that 12 O’clock midday is when the sun is highest in the sky. This leads to every place with even the slightest difference in it longitude having a different time.

To solve this problem the world today is divided into wide bands or “zones” which stretch from pole to pole and share the same time. For example the whole of the UK fits into one zone and the same time is recorded by all of us in the UK. Broadly speaking the zones are 15o wide be it is a little more complex than this, see this article on Wikipeadia for more details on time zone [time zones…]

The sun rises in the east and countries to the east of Greenwich set their time earlier, and to the west the time is later. However on the RYA courses you will be please to hear you only have to get you head around times zones that are equivalent to UK time (Universal Time or UT) and French time (one hour ahead), the only complication being the summer times in both time zones.

How do I know which time zone I am working in?

At the top of the tide table for every port in the almanac you will find the time zone.

For ports in the Northern Peninsula (equivalent to UK) it will say:

For Summer Time add ONE hour in non-shaded areas

This is time zone 0000 is conventionally known as UT or UTC (Coordinated Universal Time). Zone zero is also known as ‘Zulu’ time in many applications in the Met Office, Coastguard and Services.

In the fictitious RYA charts, Northern Peninsula Daylight Saving Time (NPDST) is Summer Time in that area, and is one hour ahead of UT.

For ports in the Southern Peninsula (equivalent to France) it will say:

  TIME ZONE -0100
For Summer Time add ONE hour in non-shaded areas

The -0100 means you need to subtract one hour to get back to UT or you must add 1 hour to UT to obtain local time.

In the fictitious RYA charts, Southern Peninsula Daylight Saving Time (SPDST) is Summer Time in that area, and is two hours ahead of UT.



Wikipeadia: Time zones
Everything you need to know about time zones.

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Now that you understand how to find the correct time you can move on to calculating tidal heights or tidal streams.


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Reading the content of these pages is not a substitute for completing a RYA Shorebased course or similar.