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October 29, 2011 8:11 AM   Subscribe

I'm looking for a map that shows the variance between the standard time and 'local time' within the timezones around the world, or at least within the US.

I understand that timezones are approximately 15 degrees wide. There must be some longitude within each timezone where the 'local noon' time coincides with the standard time, and ideally that longitude is near the middle of the timezone band. East or west of that meridian there is a variance that gets greater (+ or -) as you approach the time zone boundary. I seek a map that shows that longitude and that variance.

I do not want to conflate this with discussions on the impending change from daylight savings time, tho that has been the spark for this question.
posted by TDIpod to Science & Nature (11 answers total) 3 users marked this as a favorite
 
Adam Cadre made one some years back.
posted by Nomyte at 8:38 AM on October 29, 2011


Nomyte, it appears your link doesn't work because Adam Cadre doesn't like hotlinking.
posted by madcaptenor at 8:45 AM on October 29, 2011


Ah hah! Clearing the cache lets me see the kitties. In a masterstroke of design, the link to the map is buried way at the bottom of his site's Miscellany page.
posted by Nomyte at 8:51 AM on October 29, 2011


You have to go to this page of his and continue from there. Search for "Daylight Saving Time".
posted by dfan at 8:51 AM on October 29, 2011


Wow, there's a lot more red than green on that map.
posted by madcaptenor at 8:55 AM on October 29, 2011


I'm afraid there is mistaken assumption in your question. Local apparent noon depends not only on one's longitude, but also on the date. Adam Cadre's map is interesting, but presumably he had to generate it for a particular date.

Local apparent noon depends on the date because an apparent solar day can be up to 20 seconds shorter or 30 seconds longer than a mean solar day. Because many of these long or short days occur in succession, the difference builds up so that mean time is greater than apparent time near February and mean time is less than apparent time near November (swinging a total of about 30 minutes over the course of a year). This means every time zone has an approximately 8° wide band in the middle where local apparent noon coincides with mean standard noon at some point during the year (ignoring daylight savings time).

You can use a solar noon calculator (like this one) to determine the exact time of solar noon each day at your location. If you're in the band you'll see when in the year solar noon will match standard noon at your location.

P.S., madcaptenor, that's because of the date chosen when generating that map.
posted by RichardP at 9:05 AM on October 29, 2011


RichardP: you're right that the date matters. But i'm assuming the date is some date when mean noon and apparent noon coincide. So at some dates the map would be less red than it is but at other dates it would be even more red.
posted by madcaptenor at 9:09 AM on October 29, 2011


I guess another caveat is in order, given that Cadre is a hobbyist and not a geographer. For instance, the map doesn't seem to represent the fact that many places run on non-standard time (i.e., time zones that are not in whole-integer increments). In a number of places (map) you also get two-hour jumps when you cross a time zone. Cadre's map, as provided, shows deviance across a time zone (at some chosen date), not its relationship to solar noon.
posted by Nomyte at 9:12 AM on October 29, 2011


Nomyte, which non-standard jumps are you saying Cadre doesn't show? For example, Nepal (5:45) looks to be slightly ahead of India (5:30) in his map, the jump between Western Australia (8:00) and Central Australia (9:30) appears to be larger then the Central Australia (9:30) to Eastern Australia (10:00), Iran (3:30) is visible as an area that has different time than any of its neighbors, and so on.

Something looks a little weird about Africa to me, though; it looks like your map and his map disagree in Namibia, Libya, and Sudan at least, although time zones do change. Maybe one of the maps needs an update.
posted by madcaptenor at 9:28 AM on October 29, 2011


I was looking at Borneo and Malaysia, but other websites suggest that Borneo does lie in two different time zones now.
posted by Nomyte at 9:38 AM on October 29, 2011


Thank you Nomyte- that is perfect! Next time (hah) I'll google 'deviance' instead of 'variance'. The rest of the conversation is very interesting too. I understand the slight differences in the solar day, but I'm not an astronomer or a sailor with a sextant, so Cadre's map is close enough.
posted by TDIpod at 11:01 AM on October 29, 2011


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