LMT information, geographics of Antarctica bases

Oscar van Vlijmen ovv at hetnet.nl
Tue Mar 22 16:18:22 UTC 2005

>> Some LMT (local mean time) values in the tz database seem to be incorrect.
>> Below are the values I believe to be correct.

> These LMT offsets agree with the current tz database.

>> Alaska    America/Yakutat    -9:14:55
> Are you sure about that one?

I found out what the problem was.
The tz northamerica file says:

Zone America/Yakutat     14:41:05 -    LMT    1867 Oct 18
             -9:18:55 -    LMT    1900 Aug 20 12:00

I took the _first_ LMT for granted and reported this could be an error.
But let's look at the file zone.tab:

US    +593249-1394338    America/Yakutat    Alaska Time - Alaska panhandle

The longitude -139:43:38 translated to LMT becomes: -09h 18m 54.53s
BTW, I made a typo: 14m instead of the correct 18m.

It is not always true that the _last_ LMT entry is correct in the sense of
true solar LMT.
Take for instance, from the africa file:

Zone    Africa/Sao_Tome     0:26:56 -    LMT    1884
            -0:36:32 -    LMT    1912    # Lisbon Mean Time

The first LMT time is the true solar LMT time.

First or second LMT entry: it is not always an international date line
effect. Take for instance (tz southamerica file):

Zone    America/Lima    -5:08:12 -    LMT    1890
            -5:08:36 -    LMT    1908 Jul 28 # Lima Mean Time?

The first LMT is the time, calculated from the longitude given in zone.tab.

Just by inspecting the Zone information in the tz database files it is not
always clear what the true solar LMT is.
The true LMT has to be calculated from the zone.tab location information.

It could be advised to document this somewhere, or to remove the ambiguities
in the LMT data.

The following zones have two LMT entries, as far as I detected, and I
recalculated the true solar LMT from the zone.tab data, the 180 degree
meridian being the natural border between + and -:

Zone    Africa/Sao_Tome     LMT = 00:26:56

Zone    Asia/Manila    LMT = 08:04:00

Zone    Pacific/Guam    LMT = 09:39:00
Zone Pacific/Saipan    LMT = 09:43:00
Zone Pacific/Pago_Pago     LMT = -11:22:48
Zone Pacific/Apia     LMT = -11:26:56

Zone    Europe/Lisbon    LMT = -00:36:32

Zone America/Juneau     LMT = -08:57:41
Zone America/Yakutat     LMT = -09:18:55
Zone America/Anchorage     LMT = -09:59:36
Zone America/Adak     LMT = -11:46:38

Zone    America/Lima    LMT = -05:08:12

Oscar van Vlijmen

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