 # Calculated sunrise/sunset times on an Arduino

Has anyone calculated sunrise/sunset times on an Arduino? I need sunrise/sunset calculations for my sprinkler and home lighting control. :) :) :)

Couldn't you just use a sensor to measure light (e.g. photoresistor)?

• Brian

How do you differentiate from a sunny day and a cloudy day with a photoresistor?

I need the exact sunrise & sunset times per latitude/longitude/GMT locations for automatic home automation control.
Since I already have a RTC (DS1307) and a SPI connected floating point processor (Micromega FPU V3.1) I will try to find a solution using the algorithm below.

Sunrise/Sunset Algorithm
Source:
Almanac for Computers, 1990
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
NOTE: the algorithm assumes the use of a calculator with the
trig functions in “degree” (rather than “radian”) mode. Most
programming languages assume radian arguments, requiring back
and forth convertions. The factor is 180/pi. So, for instance,
the equation RA = atan(0.91764 * tan(L)) would be coded as RA
= (180/pi)*atan(0.91764 * tan((pi/180)*L)) to give a degree
answer with a degree input for L.

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

1. 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)

1. calculate the Sun’s mean anomaly

M = (0.9856 * t) - 3.289

1. calculate the Sun’s true longitude

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

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

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

Lquadrant = (floor( L/90)) * 90

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

RA = RA / 15

1. calculate the Sun’s declination

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

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)

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

1. calculate local mean time of rising/setting

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

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

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

localT = UT + localOffset

Other notes:

I found an Application Note #38 (Calculating Sunrise/sunset times) at Micromega's corp website: http://www.micromegacorp.com/appnotes.html. There is even source code there for a Basic Stamp which has to be converted to the Arduino.

There is also an Arduino library made for this floating point processor. (FPU V 3.1)

The Arduino is a very good microcontroller but when it comes to floating point calculations the speed and the resources required will crushed it so additional muscle of a floating point processor is needed. 8-) 8-) 8-)