Finding the Local Standard Time (LST) from the Local Mean Time (LMT)
by Martin J. Powell
The times given in the Rise/transit/set table should on the whole work fairly accurately for most locations which are situated along (or close to) the latitudes listed. However, errors of up to 30 minutes or more could result because of your longitudinal position within your own Time Zone. If you are located on - or very close to - the central longitude of your Time Zone (i.e. the time zone meridian), no correction to the rise, transit and set times will be necessary.
If you are unsure of your Time Zone, a map & look-up table of World Time Zones can be found here. Your latitude and longitude can be found from a world atlas, or by visiting the Heavens Above website (enter the name of your nearest town or city in the 'Search' box.
However, for every degree of longitude that you are positioned away from the central longitude, celestial bodies will rise, transit and set four minutes earlier or later, depending upon whether you are located East or West of the central longitude.
The following method can be used to determine the time correction required for all times derived from the table (note that this correction only needs to be calculated once for any given location on the globe, after which the same time correction will be applied to all rise, transit and set times extracted from the table).
Determine the central longitude (time zone meridian) of your Time Zone by calculating:
Your Time Zone (hours) x 15º
[Your Time Zone will be the number of Standard hours difference from Greenwich Mean Time (i.e. ignoring Daylight Savings [Summer Time] adjustments); this includes countries operating half-hourly Time Zone corrections. Your Time Zone will either be ahead of, or behind, Greenwich (London) time. For example, the Time Zone for Athens (Greece) is 2 hours ahead (i.e. Eastern Europe Time) and that for New York (USA) is 5 hours behind (i.e. Eastern Standard Time). Hence the central longitude of the Time Zones in these cases will be (2 hours x 15º) = 30º E for Athens and (5 hours x 15º) = 75º W for New York.]
Having determined the central longitude of your Time Zone, apply the following rule:
If you are positioned to the EAST of your Time Zone's central longitude:
SUBTRACT FOUR MINUTES from the rise, transit and set times for each degree of longitude that you are positioned East of your Time Zone's central longitude.
If you are positioned to the WEST of your Time Zone's central longitude:
ADD FOUR MINUTES to the rise, transit and set times for each degree of longitude that you are positioned West of your Time Zone's central longitude.
Once calculated, this time correction will apply throughout the period shown in the table, including that during which DST is in operation.
Time corrections to apply to the Rise/transit/set table for selected cities across the world (listed in alphabetical order by city). Hence observers in Toronto, Canada should ADD 18 minutes to the times listed in the rise/transit/set table, and observers in Budapest, Hungary should SUBTRACT 16 minutes from the times in the table.
A few examples are given here to illustrate how to apply the correction:
Example 1: A Westerly longitude
The geographical longitude of Las Vegas, Nevada, USA is 115º West and its Time Zone (Pacific Standard Time) is 8 hours behind London time. What time correction should be applied to the rise, transit and set times in the table?
> The central longitude of Las Vegas' Time Zone is (8 hours x 15º) = 120º West, giving a longitudinal difference of (120º - 115º) = 5º. The time correction to apply will be (5º x 4 minutes) = 20 minutes. Since Las Vegas (115º W) lies to the East of its Time Zone's central longitude (120º W), the time correction will be subtracted from the times in the Rise/transit/set table to obtain the local times of events at Las Vegas (ie. the rising, transit and setting times will occur 20 mins earlier than the times given in the table).
Example 2: An Easterly longitude
The longitude of Calcutta, India is 88º East and its Time Zone (Indian Standard Time) is 5½ hours ahead of London time. What time correction should be applied to the rise, transit and set times in the table?
> The central longitude of Calcutta's Time Zone is (5.5 hours x 15º) = 82º.5 East, which gives a longitudinal difference of (88º - 82º.5) = 5º.5. The time correction to apply will be (5º.5 x 4 minutes) = 22 minutes. Because Calcutta (88º E) lies to the East of its Time Zone's central longitude (82º.5 E), the time correction should be deducted from the times in the Rise/transit/set table to obtain the local times of events in Calcutta (ie. the rising, transiting and setting times will occur 22 mins earlier than those given in the table).
Example 3: Non-Correlated Time Zones
Some countries, for political, social or economic reasons, do not use a Time Zone which is strictly correlated with their geographical longitude; instead, they have adopted an adjacent Time Zone. This can result in transit time discrepancies amounting to well over an hour. The following example demonstrates the large time corrections that are required in these circumstances:
The longitude of Reykjavik, Iceland, is 22º West and its Time Zone is the same as that of London, ie. 0 hours. What time correction should be applied to the rise, transit and set times in the table?
> The central longitude of Reykjavik's Time Zone (Greenwich Mean Time) is (0 hours x 15º) = 0º. The longitudinal difference is then (22º - 0º) = 22º, which multiplied by 4 gives a time correction of 88 minutes (ie. 1 hour 28 minutes). Since Reykjavik lies to the West of its Time Zone's central longitude, the time correction will be added to the times in the Rise/transit/set table to obtain the local times of events in Reykjavik (ie. the rising, transiting and setting times will occur 88 mins later than those given in the table).
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Copyright Martin J Powell August 2007