Sunset time off by almost 3 minutes for my area

[ahwman]

I compared the sunset time on my ISY to the known correct time for my area and it appears that my ISY is reporting 7:34:48 PM vs. 7:37 PM (correct time for Detroit, MI) as of this posting on 03/12/12. Any idea where the ISY pulls this data? I do have daylight savings time checked by the way...

[apostolakisl]

Perhaps you should look up your exact longitude/lattitude and enter that instead. A city like detroit might be 50 miles across and that could account for the difference. At that lattitude one minute is about 8 or 9 miles East/West.

[ahwman]

Great advice! Thanks so much. Always learning something new from you guys...

[apostolakisl]

I actually decided to get the real answer, 1 minute change in sunset/sunrise is about 12.9 miles east or west at 42 degrees lattitude.

[ahwman]

Makes for a great trivia question... Thanks again.

[andyf0]

If you have an iPhone you can download GPS Lite from Motion X for free. It will give you your exact GPS coordinates. I'm sure there are apps for other phones too.

[ahwman]

Funny you mention that. I have the paid version of that app and used it to pinpoint my location... Worked like a charm.

[aLf]

Fly-boy here. I have the exact coordinates for my house and ISY is still a bit off. I can pin-point lat/long to exact. No more than 5 minutes on any given day, just off. I have never understood why.

 

aLf

[andyf0]

Are you guys looking at the sky and watching the sunrise or sunset? How are you determining it's +/- 5 mins off and does it really matter?

 

Have you seen the calculations required to calculate sunrise / sunset times. I'd say 5 mins off was pretty good considering rounding issues. Here's one example:

 

http://en.wikipedia.org/wiki/Sunrise_equation

[apostolakisl]

Also don't forget that sunrise/sunset is based on perfectly flat land. It doesn't take much of a hill to change your times by a few minutes.

 

But I also have to agree with the above. What kind of work are you doing that you need to know the sunset to the exact minute?

[aLf]

My comments re: the times being off are not refering to the visual effect of it. I have several computer programs and also a manual way to figure it per lat/long and they are all withing a few seconds for a ceratin day. ISY has always varied with the others by up to five minutes for a particular morning or night. I have always wondered about the "internal" figuring done by ISY.

[andyf0]

Then how do you know it's the ISY that's off and not your other programs?

[aLf]

I'd have to think that three other programs including a manual formula would lend one to believe that three the same and one different (ISY)---well you do the math. Again, I'm not making a mountain out of this. In fact, I'm happy with it just being close. The fact is something is off.

[apostolakisl]

My guess:

 

ISY does not have dedicated trigonometric calculator built in. It therefore is lumping larger blocks of an earth grid together so that it doesn't eat up resources and since really who cares, a couple minutes this way or that.

[andyf0]

My guess:

 

The computer has a floating point math h/w chip and is able to handle long floating point numbers. The ISY does all it's math in software and makes some compromises as to how many decimal points it uses.