[LEAPSECS] time zones and DST

[Tony Finch]

On Tue, 6 Jan 2009, Rob Seaman wrote:

>

> Your April Fool's post on risks may be the most coherent analysis I've

> read on the subject [of DST].


Thanks :-)


> Where I grew up in the U.S. mid-Atlantic states, the most obvious effect of

> DST was to extend the usable hours of daylight for Summer evenings.  (Perhaps

> some other narrative applies at higher or lower latitudes?)


Same everywhere as far as I can tell.


> Since we were off school, the morning issues were meaningless.  And

> workers go to work when their bosses tell them to.  The time they own

> for themselves and their families is after work.


Right. People generally prefer to use their evenings for recreation rather
than socializing in the mornings before work. Hence shifting work closer
to sunrise in the summer to get lighter evenings at the expense of the
less-used mornings. If you read David Prerau's history of DST it becomes
clear that it isn't possible to get bosses to agree on an earlier summer
timetable except by changing the clocks. http://www.seizethedaylight.com/

DST became popular in the 20th century because of the increase in
urbanization and the consequent increase in the time-related coupling of
our activities. This made it harder for individuals and organizations to
set their timetables according to their own preferences. The number of
organizations makes it impossible to get consensus on a co-ordinated
timetable change, so it has to be done by the government dictating when to
change the clocks.

This is why DST is a sensible solution to the problem of the mismatch
between natural human preferences and inflexible timetables based on mean
solar time.


> Recently, all discussions of DST are framed in turns of energy.


As you say, that is a red herring. It's similar in that respect to the
arguments made in Scotland (where winter days are not long enough for
daylight to cover both the morning and evening commute) about the
relationship between accident rates and choice of time zone. Whenever an
English politician suggests switching to CET to match the continent, the
Scots insist they will stay on GMT because they say children will get hurt
on the roads going to school. In truth, being on GMT means the accidents
happen in the evening instead, and the real reason for resisting change is
they prefer the sun to rise before work.

(The efficiency argument may have had some merit for war economies, but
DST would have been discarded in peacetime like other war measures were it
not popular for reasons other than efficiency.)


> If DST were really a mechanism for managing our natural daylight resource,

> rather than a naive attempt at PR regarding petroleum resources, it would be

> applied in the Winter when the daylight is in shortest supply.


Your phrasing there makes it sound like you think DST increases the supply
of daylight. Obviously it doesn't. It just improves the match between our
timetables and sunrise, reducing the amount of wasted light in the early
morning. There's no wasted light in winter mornings, so it doesn't make
sense to have DST then.

Having said that, there is a general tendency for time zones to move so
that they are centred further west than their nominal meridian. The
Central European and North American Central time zones are good examples.
This has the effect of making sunrise later in the winter than it would
otherwise be, in those areas near the edges of the time zones - that is, a
kind of winter DST, though often with even more DST in the summer.

Tony.
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