Auto Day night switch.

[gcb]

I am impressed by the way the I3 changes from Day to Night mode.

Dont think it uses a light sensor or it would switch when indoors surely.

It seems that as the I3 knows where it is exactly in the world that it also knows exactly what time is sundown on what day of the year.

As we aproach the winter months I3 seems to switch to night mode a couple of minutes earlier each day.

Maybe I am giving too much credit here and maybe the I3 uses a much simpler method to know if it day or night. Any ideas.

What ever it does, all I am saying is how ever much it has ballsed up routes, crashed whatever one thing it has always knew precisely when it was day or night.

[zogman]

some calenders/diarys have sunset and sunrise info so its not a big deal for a gps to change its screen colours..
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[gcb]

So do you recon the I3 stores sunset/Sun rise times for 24 or 25 time zones, or do you think that Garmin used a nice little algorithm to calculate the exact sunrise/sunset for the exact place you are standing.

I think the latter, its a nice little touch. Shame you cant display sunset time as you seem to be able to do on other Garmin GPS.

http://www.nicholsonprints.com/Articles/sun.htm

For anyone really interested, Its hard work for us humans armed only with a pecil and a calculator but with a little mathematical understanding and a spreadsheet you can write your own sunset, sunrise calculator

Theory:

Sunrise/Sunset Algorithm Example

Source:
Almanac for Computers, 1990
published by Nautical Almanac Office
United States Naval Observatory
Washington, DC 20392

Inputs:
day, month, year: date of sunrise/sunset
latitude, longitude: location for sunrise/sunset
zenith: Sun's zenith for sunrise/sunset
offical = 90 degrees 50'
civil = 96 degrees
nautical = 102 degrees
astronomical = 108 degrees

NOTE: longitude is positive for East and negative for West

Worked example (from book):
June 25, 1990: 25, 6, 1990
Wayne, NJ: 40.9, -74.3
Office zenith: 90 50' cos(zenith) = -0.01454


1. first calculate the day of the year

N1 = floor(275 * month / 9)
N2 = floor((month + 9) / 12)
N3 = (1 + floor((year - 4 * floor(year / 4) + 2) / 3))
N = N1 - (N2 * N3) + day - 30

Example:
N1 = 183
N2 = 1
N3 = 1 + floor((1990 - 4 * 497 + 2) / 3)
= 1 + floor((1990 - 1988 + 2) / 3)
= 1 + floor((1990 - 1988 + 2) / 3)
= 1 + floor(4 / 3)
= 2
N = 183 - 2 + 25 - 30 = 176

2. convert the longitude to hour value and calculate an approximate time

lngHour = longitude / 15

if rising time is desired:
t = N + ((6 - lngHour) / 24)
if setting time is desired:
t = N + ((18 - lngHour) / 24)

Example:
lngHour = -74.3 / 15 = -4.953
t = 176 + ((6 - -4.953) / 24)
= 176.456

3. calculate the Sun's mean anomaly

M = (0.9856 * t) - 3.289

Example:
M = (0.9856 * 176.456) - 3.289
= 170.626

4. calculate the Sun's true longitude
[Note throughout the arguments of the trig functions
(sin, tan) are in degrees. It will likely be necessary to
convert to radians. eg sin(170.626 deg) =sin(170.626*pi/180
radians)=0.16287]

L = M + (1.916 * sin(M)) + (0.020 * sin(2 * M)) + 282.634
NOTE: L potentially needs to be adjusted into the range [0,360) by adding/subtracting 360

Example:
L = 170.626 + (1.916 * sin(170.626)) + (0.020 * sin(2 * 170.626)) + 282.634
= 170.626 + (1.916 * 0.16287) + (0.020 * -0.32141) + 282.634
= 170.626 + 0.31206 + -0.0064282 + 282.634
= 453.566 - 360
= 93.566

5a. calculate the Sun's right ascension

RA = atan(0.91764 * tan(L))
NOTE: RA potentially needs to be adjusted into the range [0,360) by adding/subtracting 360

Example:
RA = atan(0.91764 * -16.046)
= atan(0.91764 * -16.046)
= atan(-14.722)
= -86.11412

5b. right ascension value needs to be in the same quadrant as L

Lquadrant = (floor( L/90)) * 90
RAquadrant = (floor(RA/90)) * 90
RA = RA + (Lquadrant - RAquadrant)

Example:
Lquadrant = (floor(93.566/90)) * 90
= 90
RAquadrant = (floor(-86.11412/90)) * 90
= -90
RA = -86.11412 + (90 - -90)
= -86.11412 + 180
= 93.886

5c. right ascension value needs to be converted into hours

RA = RA / 15

Example:
RA = 93.886 / 15
= 6.259

6. calculate the Sun's declination

sinDec = 0.39782 * sin(L)
cosDec = cos(asin(sinDec))

Example:
sinDec = 0.39782 * sin(93.566)
= 0.39782 * 0.99806
= 0.39705
cosDec = cos(asin(0.39705))
= cos(asin(0.39705))
= cos(23.394)
= 0.91780

7a. calculate the Sun's local hour angle

cosH = (cos(zenith) - (sinDec * sin(latitude))) / (cosDec * cos(latitude))

if (cosH > 1)
the sun never rises on this location (on the specified date)
if (cosH < -1)
the sun never sets on this location (on the specified date)

Example:
cosH = (-0.01454 - (0.39705 * sin(40.9))) / (0.91780 * cos(40.9))
= (-0.01454 - (0.39705 * 0.65474)) / (0.91780 * 0.75585)
= (-0.01454 - 0.25996) / 0.69372
= -0.2745 / 0.69372
= -0.39570

7b. finish calculating H and convert into hours

if if rising time is desired:
H = 360 - acos(cosH)
if setting time is desired:
H = acos(cosH)

H = H / 15

Example:
H = 360 - acos(-0.39570)
= 360 - 113.310 [ note result of acos converted to degrees]
= 246.690
H = 246.690 / 15
= 16.446

8. calculate local mean time of rising/setting

T = H + RA - (0.06571 * t) - 6.622

Example:
T = 16.446 + 6.259 - (0.06571 * 176.456) - 6.622
= 16.446 + 6.259 - 11.595 - 6.622
= 4.488

9. adjust back to UTC

UT = T - lngHour
NOTE: UT potentially needs to be adjusted into the range [0,24) by adding/subtracting 24

Example:
UT = 4.488 - -4.953
= 9.441
= 9h 26m

10. convert UT value to local time zone of latitude/longitude

localT = UT + localOffset

Example:
localT = 9h 26m + -4
= 5h 26m
= 5:26 am EDT

[ps73uk]

[gcb]

When you use floor in XL you have to put ,1

eg

N3 = (1 + floor((year - 4 * floor(year / 4) + 2) / 3))

In XL

Where Year is in C1

=(1 + FLOOR(((C1 - 4 * FLOOR((C1 / 4),1) + 2) / 3),1))

But Im afraid that looks even worse.

Still if you were to just plod through in XL checking against the example using the same date and location it should work then you can change the date to anything you like.

[littlelostboy]

Any maths professors out there like to add ther bit

[zogman]

following gcb's formulea ,i've just built a nuclear reactor...
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[gcb]

Look :

If I put this in a spread sheet where you just enter the date and the longitude,latitude. It should tell you the sun down sunrise times for that place. You dont have to understand the reasoning behind it all.

Anyone find this at all interesting? All this clever stuff is in any Garmin GPS, its just your I3 doesnt tell you it.

[portman]

Another little program to download

http://www.sunrisesunset.com/sun.html

[tomthompson]

gcb I am dumbstruck by the complicated calculations and applaud your knowledge.
Me I look out the window and think Ah daylight and then later think Oh its dark


_________________
Asus Zenfone 2 twin sim, Garmin Zumo 550, 660 and pocketgps speed camera database

I keep on learning but they invent new things faster ,How do I keep up ?

[RODTROTTER]

thats the way i understand it too pmsl :D

[MaFt]

[gcb]

Yes your all right , its complicated and theres no point in understanding all that really , but point I was making is your Garmin I3 , does all this and all it uses it for is to know when to switch between day and night settings for the display.

Thing is theres so much these GPS units do even the cheap ones like the I3. The main diference between diferent GPS through out the price ranges seem to be just down to restrictions in the software, whats enabled, accessible and what isn't.

[timkemp]

impressive though how as the sun sets in your mirror, and the glare finally disappears, the unit changes at exactly the same moment.

[mcintyre]

I've been impressed with that too.

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