Why does this time zone converter tell lies?

[ArcanaNoir]

I put in 20:00 GMT and asked for EST. It tells me 15:00 EST, which is wrong, according to people here.

http://www.timezoneconverter.com/cgi-bin/tzc.tzc

But, when I use this site: http://www.timeanddate.com/worldclo...=18&year=2011&hour=20&min=0&sec=0&p1=0&p2=410

I get the right information.

WhY??

 

[I like Serena]

Hey ArcanaNoir. :)

I suspect you're having problems with daylight saving.

Your first site says: "Daylight Saving Time is not in effect on this date/time in EST ".
But that is not right. Daylight Saving is in effect, which would be responsible for 1 hour of time difference.
Since Daylight Saving is defined separately for each region/state/country, you're better off looking up the time for a specific city.

 

[ArcanaNoir]

Oh. I thought DST was during the winter, since that's when you need to squeeze/save all the daylight you can. Although it's a pointless exercise since the sun comes up later and goes down earlier. It doesn't shift forward or backward. I guess it's especially hard to think about in Florida which has relatively equal days all year long. I can remember when I was a kid up north that during the summer the sun would stay up till nine and in the winter be gone by four. Here it's pretty much 7:30-7:30 all the time.

 

[I like Serena]

It's to save the daylight time in the morning when the days are extra long.
Without it your days should basically be 6:00-6:00 (depending on your location within a time zone).

In Florida it may not be particularly useful, but they'd still want Florida to use the same time as the rest of the States.

 

[klimatos]

Oh. I thought DST was during the winter, since that's when you need to squeeze/save all the daylight you can. Although it's a pointless exercise since the sun comes up later and goes down earlier. It doesn't shift forward or backward. I guess it's especially hard to think about in Florida which has relatively equal days all year long. I can remember when I was a kid up north that during the summer the sun would stay up till nine and in the winter be gone by four. Here it's pretty much 7:30-7:30 all the time.

No place in Florida has equal days and nights--not even on the two equinoxes. Pensacola averages 10.23 hours of daylight in December and 14.08 hours in June. Key West averages 10.81 hours per day in December and 13.64 in June. The times of sunrise and sunset vary with longitude, latitude, and civil time zones, but will never be exactly 7:30 am and 7:30 pm. I think there's something wrong with your clock.

 

[klimatos]

It's to save the daylight time in the morning when the days are extra long.
Without it your days should basically be 6:00-6:00 (depending on your location within a time zone).

Not true. No place in the world gets exactly twelve hours of daylight and twelve hours of dark every day of the year. The Equator comes closest, but even there the days are always a few minutes longer than the nights because of the diameter of the Sun's disc and the definition of sunrise and sunset.

The original purpose of DST was to give more daylight in the after-work hours for various activities and less daylight in the early morning hours when most people are asleep. Many countries that once used DST has since abandoned it, and it was never popular (or necessary) in the tropics.

 

[D H]

Many countries that once used DST has since abandoned it, and it was never popular (or necessary) in the tropics.

And at least one country abandoned winter time. In Russia, daylight savings time is now year-round.

 

[ArcanaNoir]

No place in Florida has equal days and nights--not even on the two equinoxes. Pensacola averages 10.23 hours of daylight in December and 14.08 hours in June. Key West averages 10.81 hours per day in December and 13.64 in June. The times of sunrise and sunset vary with longitude, latitude, and civil time zones, but will never be exactly 7:30 am and 7:30 pm. I think there's something wrong with your clock.

I only meant it varies very little throughout the year. Nothing like the five hour swing in Rhode Island.

 

[ArcanaNoir]

By the way, sunrise today is 7:17 and sunset is 7:30. I'd say I made a pretty good estimate.

 

[I like Serena]

By the way, sunrise today is 7:17 and sunset is 7:30. I'd say I made a pretty good estimate.

Yep!

Note that the sun is at its highest point where you are at about 13:25.
That is about 12:00 (@klimatos: not exactly, but close!) plus 1 hour daylight saving time and about 25 minutes because you're in the left part of your time zone.
There will be variations in the order of 10 minutes due to other factors.

 

[BobG]

Note that the sun is at its highest point where you are at about 13:25.

But the important question is: When is the longest day as measured from local noon (zenith) to local noon? At least to the nearest week, since the actual day changes from year to year.

 

[klimatos]

I only meant it varies very little throughout the year. Nothing like the five hour swing in Rhode Island.

If that's what you meant to say, you have no argument from me. The problem is that I have no telepathic skills. I can only respond to what you say, not what you mean.

 

[klimatos]

By the way, sunrise today is 7:17 and sunset is 7:30. I'd say I made a pretty good estimate.

Come on, Arcana! You lucked out and you know it! We are approaching the equinox, when all places on Earth have pretty close to twelve hours each of daylight. Try it again in December.

 

[klimatos]

But the important question is: When is the longest day as measured from local noon (zenith) to local noon? At least to the nearest week, since the actual day changes from year to year.

Um . . . . , I think you might want to rephrase that, Bob. From one zenith to the next zenith is always going to be pretty close to 24 hours. It might be a minute or two more around the time of the vernal equinox. And, except for a few horologists, I don't know of anyone who would consider that an important question.

Did you by chance mean sunrise to sunset? If so, then the South Pole wins (its elevation is higher than that of the North Pole, hence its day is longer).

 

[BobG]

Um . . . . , I think you might want to rephrase that, Bob. From one zenith to the next zenith is always going to be pretty close to 24 hours. It might be a minute or two more around the time of the vernal equinox. And, except for a few horologists, I don't know of anyone who would consider that an important question.

Did you by chance mean sunrise to sunset? If so, then the South Pole wins (its elevation is higher than that of the North Pole, hence its day is longer).

No, I meant from local noon to local noon. While the variation in a single apparent solar day will be around half a minute at most, the accumulation of all of those variations can cause the apparent solar time to vary from mean solar time by up to 18 minutes.

And it is an important question for those who use sundials.

 

[ArcanaNoir]

Come on, Arcana! You lucked out and you know it! We are approaching the equinox, when all places on Earth have pretty close to twelve hours each of daylight. Try it again in December.

It's not luck, I live here.

Yes the days will be shorter in December. Our forecasted sunrise and sunset on December 22 is 7:17 and 5:39. That makes the day only about two hours shorter in total. Not a huge difference.

And yes it is due to DST being in un-effect, but isn't it interesting that sunrise is at 7:17? It's pretty boring here.

 

[D H]

But the important question is: When is the longest day as measured from local noon (zenith) to local noon? At least to the nearest week, since the actual day changes from year to year.

~ Sept 26. That is when the equation of time (or cheating a bit, an approximation of the EoT) has the greatest positive slope.

 

[BobG]

~ Sept 26. That is when the equation of time (or cheating a bit, an approximation of the EoT) has the greatest positive slope.

That's the shortest. In some graphs of the equation of time, positive means a sundial will appear fast, which means you have to subtract when the value is positive. In other graphs, they display how the time has to be adjusted and a negative value means you need to subtract. If you're familiar with the Equation of Time and just assume the graph is showing what you think its showing, you can get misled due to the lack of standardization.

http://nfo.edu/solarday.htm
http://en.wikipedia.org/wiki/Equation_of_time

Equinoxes mean shorter solar days because the motion of the Sun relative to the equator is tilted by 23.4 degrees. Solstices mean longer days because the motion of the Sun is parallel to the equator, even if not on the equator.

Solar days are longer when the Earth is at perigee, because the Earth has to rotate further to get back to noon when the Earth is sweeping out larger angles in its orbit. Solar days are shorter when the Earth is at apogee and sweeping out smaller angles.

The tilt of the Earth's axis has the greater effect due to the Earth's orbit being almost circular (e = .017), hence the shortest days are close to the equinoxes in spite of apogee being the first week of July. But, the Earth is at perigee the first week of January, which is close to the Winter Solstice, so the longest day is between the Winter Solstice and the first week of January. Around Christmas is a decent estimate.

Even with the low eccentricity, it's enough that there's 184 days between the Spring equinox and the fall equinox, while there's only 181 days going from the fall equinox to the spring equinox.

 

[D H]

I used Wolfram Alpha with an approximation of the time derivative of the equation of time: http://www.wolframalpha.com/input/?...cos(2*pi*(x-81)/365-1.37357412))),+x>0,x<=365.

The local max at day 268.9 corresponds to September 26 (day #269).

 

[BobG]

I used Wolfram Alpha with an approximation of the time derivative of the equation of time: http://www.wolframalpha.com/input/?...cos(2*pi*(x-81)/365-1.37357412))),+x>0,x<=365.

The local max at day 268.9 corresponds to September 26 (day #269).

The equation of time gives you the difference between apparent solar time and mean solar time. So the derivative gives you the maximum rate that apparent solar time is moving ahead of mean solar time? And it would be moving ahead of mean solar time because the apparent solar day is shorter, thus restarting at zero while mean solar time is still counting the seconds left in the previous solar day?

 

[klimatos]

No, I meant from local noon to local noon. While the variation in a single apparent solar day will be around half a minute at most, the accumulation of all of those variations can cause the apparent solar time to vary from mean solar time by up to 18 minutes.

And it is an important question for those who use sundials.

I stand corrected, Bob. I had to Google "apparent solar time" to understand what you were saying.

Would I be correct in saying that the outcome of all this is that not all solar days are exactly twenty-four hours and that the northern hemisphere summer (equinox to equinox) is longer than the southern hemisphere one? If so, this has some interesting climatic implications.

The day is not wasted. I've learned something.