The Observant Astronomer

The passing scene as observed by an observant Jew, who daylights as an astronomer.

Thursday, April 07, 2005

On Time Zones

Beside the disruption of changing the clocks twice a year, one of the reasons for objecting to Daylight Saving Time is that many of us are on DST year round anyway, so the change puts us on double DST in the summer.

To understand this, consider this time-zone map on the UK Met office site:

Time zone map

The faintly coloured vertical bands are the nominal time zones, 15 degrees wide, centered at 15 degree spacings starting with the Greenwich meridian. On the central meridian of each zone, the mean sun is due south at local noon. At the eastern boundary of each zone, local noon is at 11:30 AM zone time. At the western boundary, local noon is at 12:30 PM zone time.

The darkly coloured regions are the actual time zones. What is interesting is that most of them are systematically shifted to the west. For a striking example of this consider Spain. It ought to be in the GMT zone, like Britain due north of it. Yet it is in the zone GMT+1, so local mean noon in Madrid is at 1:15 PM. Asian Russian time zones are all systematically displaced 15 degrees to the west---year-round DST, except in the summer when they also go ahead one hour.

In North America the western boundaries of both the Eastern (GMT-5) and Central (GMT-6) time zones lie at roughly the central meridians for the Central and Mountain (GMT-7) time zones, so the western parts of these zones are already half-way to DST even when standard time is in effect. This westward bias is why Indiana, Arizona, and eastern Saskatchewan don't join in the annual jump to the west. They're already there.

Not every country deals with this the same way. China just puts the whole country on Beijing time. For the far west, that is triple DST. India and Iran, on the other hand, since they straddle the boundaries between two time zones, take the sensible option of shifting themselves a further half-hour from the nominal time zone in their western halves. This way they can keep noon at roughly 12:00 everywhere without requiring two time zones for the country.

If you must know who's going on or off DST when check out the links from here.

UPDATE: (15:30) CNN is reporting an AP report that the U.S. Congress is considering an amendment to energy legislation to extend Daylight Savings Time by two months, to run from the beginning of March to the end of November(!). Good luck to anyone in the western edges of the time zones who want to get in shacharis before work, especially in November. In Detroit, you won't be able to put on tefillin until ~7:30. I hope the kids enjoy the sunrise as they walk to school. How much energy can you really save if it is pitch black when you get up? Has this really been thought through, or is this a mindless reaction to rising oil prices?


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