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It’s Later Than You Think

February 22nd, 2008

In the past I have never complained all that much about Daylight Saving Time, but I have often wondered why some people complain more about the time change than others. Thinking about it, I do like having more daylight in the evening in the summer time, but it is the actual time change that bothers people and some more than others.

It seems that it depends a lot on where you live.

On Sunday, March 9 2008 we in most of North America will be switching over from Standard time to Daylight Saving Time (DST). This is the second year of a grand experiment where we turn our clocks forward three weeks earlier than we had in the past and then turn our clocks back a week later than normal. At least I thought that I understood the reasoning for delaying of the end of DST in the fall, but I can’t seem to understand why we are now starting DST three weeks earlier than before and also almost two weeks before the start of spring.

The whole purpose of DST is to take an hour of sunshine from morning and extend the amount of sunlight in the evening. In other words it makes for darker mornings and longer brighter evenings – at least that is the idea. In summer time this does make a difference as the period of daylight in the Northern Hemisphere increases as we progress from Spring to Summer, but in late Winter it not only does not make much of difference due to fact that the period of sunlight is still less than 12 hours, it has a negative impact on what our mornings look like. “Look” as is in not be able to see because it is still dark out as we are starting to go about our day.

Look at the sunrise/sunset times for four US cities. This is for Sunday, March 9, 2008 – after we have switched our clocks forward to begin Daylight Saving Time…

City Sunrise Sunset Length of Day
Pittsburgh 7:40 AM 7:21 PM 11h 40m 18s
Denver 7:20 AM 7:01 PM 11h 41m 06s
Los Angeles 7:11 AM 6:57 PM 11h 45m 39s
Houston 7:38 AM 7:26 PM 11h 48m 37s

What’s wrong with this picture? First look at Pittsburgh, the most northern of the four cities, on Sunday March 9 it be will darn near 8 o’clock in the morning before the sun comes up.

At this time of the year, still winter in the northern hemisphere, the difference in the length of the day in the four cities is only about 8 minutes, with Houston, the most southern city of the four, having the longest period of sunlight. But wait a minute! How is it that the sunrises for Houston and Pittsburgh are only 2 minutes apart? One would think that the since Houston is much further south than Pittsburgh and its day in early March is 8 minutes longer that sunrise would occur much earlier. After all, Los Angeles, which is more north than Houston, has a sunrise occurring nearly 30 minutes earlier than Pittsburgh and Houston. Are these times incorrect? Nope, I got them from the United States Naval Observatory. (see http://aa.usno.navy.mil/data/docs/RS_OneDay.php )

Here’s the explanation… Not only is each city on a different latitude – the order from north to south being Pittsburgh, Denver, Los Angeles, Houston -, each is located in different places in relation to the time zone in which they reside – Pittsburgh (Eastern), Houston (Central), Denver (Mountain), Los Angeles (Pacific).

In the continental US there are four time zones:  Pacific, Mountain, Central, and Eastern with each time zone being roughly about 15° wide. Each time zone has a  central meridian for which the “Local Apparent Time” for any location along that meridian is designated as the standard time for the entire time zone – such that when it is true noon along the central meridian of a time zone it is also noon for any place 7 ½° east or west of the central meridian.

Local Apparent Time (LAT or Local Mean Time) is the time according to a mean or average (read imaginary) sun that moves evenly across the sky.

What this means is that if you live east of the meridian of your time zone and your clock is set to the standard time for the time zone your clock is actually set for a time that is earlier than the local mean time for your location and if you are west of the meridian for your time zone your clock is set for a time that is later than local mean time for where you live. For example the central meridian for Central Standard Time is 90° west longitude which is a line that runs north and south just west of Memphis, Tennessee. Houston is 5° west of Memphis and as a result the longitudinal correction for LAT at  Houston is about twenty minutes earlier than Memphis (-21.48 mins). So that when it is noon in Memphis it is actually 11:39 AM  in Houston, but because everyone in that time zone has “standardized” by setting their clocks to same time as it is along the central meridian for that time zone we say that we are on “standard time” and not local time.

The meridian for the US Eastern time zone is 75° west which is a line running north and south about 2° west of New York City. Pittsburgh is located at ~80° west longitude so Pittsburgh’s longitude correction offset is 20 minutes earlier than its time zone’s central meridian – the same as Houston.

So further west of you are of the central meridian for your time zone, your clock is set for a time that is much later than the Local Apparent Time for your area. On March 9, the folks in Indianapolis, Indiana – where they do now observe Daylight Saving Time – will not see the sunrise until 8:05 AM!

On the other hand, Denver is situated on the central meridian for Mountain time and in the case of Los Angeles, it is actually east of the meridian for Pacific time – by 8 minutes – hence the earlier sunrise occurring at that city.

Now consider those same four cities at the beginning of summer (June 24, 2008)…

City Sunrise Sunset Length of Day
Pittsburgh 5:51 AM 8:54 PM 15h 02m 28s
Denver 5:33 AM 8:32 PM 14h 58m 43s
Los Angeles 5:43 AM 8:09 PM 14h 25m 26s
Houston 6:22 AM 8:26 PM 14h 03m 15s

This is where the real impact of DST comes in to play as days are much longer in the summer time than they are in the winter. The more north you go in summer the longer the period sunlight gets. If you go all the way to the north pole the sun appears to never set in the middle summer.

But look at the difference between Houston and Pittsburgh. Them Yankees get a whole extra hour of sunlight – The day is not longer there because of Daylight Saving Time, of course, but because the city is further north and they are on DST, they get to be out playing ball to ‘till after 9 o’clock while the kids down in Houston have to start coming in around 8:30.

Now what happens when we delay the end of DST by a week in the Fall? Originally I had thought that one of the driving forces behind delaying the end of DST to the first week of November was to allow people voting on Election day to have more “time” in the evening to get to the polls – by more “time”, I mean more daylight in the evening. But that does not seem to be case, because this year (2008) – a Presidential Election Year – DST will end on Sunday November 2 and Election Day is on the following Tuesday November 4.

So, one has to ask: What is the point of changing the our clocks three weeks early and delaying the end of DST by only one week? Shouldn’t it be the other way around?

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