Photo of my face David Thomas Bernal

An LCD Sunset Timer in C#, Part 1

published jan 9 2008

An important tradition between a few people at my office is the daily viewing of the sunset. I’m not really sure how exactly, but at some point, it was discovered that from our office parking lot, we have a relatively unhindered view of the beautiful Arizona sunset. Combine this with programmers eager to quit coding for a while, and our lack of windows, and you’ve got a daily ritual. Well, back in December, I was rooting around in The Box, a box of random computer-y ephemera, when I discovered a Crystalfontz USB LCD leftover from a previous project, just sitting there looking sad and lonely. My coworkers quickly came up with the purpose for this thing: a countdown to sunset. We already had The Orb setup to grow redder as sunset approached, but found it sometimes was laggy and often lost its signal in our windowless office.

C# is my language of choice for desktop applications, so I immediately set out to find a way to use it to write the controller software for the LCD. At first, I looked to the Crystalfontz LCD software, but found the software to be poorly documented, and plugins difficult to write, requiring that a DLL be written, which would be loaded by a Windows service running the in the background. After some time fruitlessly spent searching for help on the Crystalfontz forums, I ended up finding a different program entirely: LCD Smartie. LCD Smartie has built-in support for writing plugins in .NET, and simply requires that plugin-authors create a class with the same name as their DLL filename, which implements up to 20 public methods named function1-function20.

Knowing that this would do exactly what I wanted, I moved on to calculating the time of sunset. Fortunately, I’d recently discovered one of those obscure functions that PHP is known for, date_sunset, and more importantly, djwice’s comment, containing links to information on the algorithm used by this function. I implemented the following sunset function using the algorithm outlined on this page.

public DateTime Sunset(DateTime date)
	int dayOfYear = date.DayOfYear;

	double tApprox = dayOfYear + ((18 - LON / 15) / 24);

	double meanAnomaly = (0.9856 * tApprox) - 3.289;

	double sunTrueLon = meanAnomaly + (1.916 * Sin(meanAnomaly)) + (0.020 * Sin(2 * meanAnomaly)) + 282.634;

	if (sunTrueLon > 360) while (sunTrueLon > 360) sunTrueLon -= 360;
	if (sunTrueLon < 0) while (sunTrueLon < 0) sunTrueLon += 360;

	double rightAscension = Atan(0.91764 * Tan(sunTrueLon));

	if (rightAscension > 360) while (rightAscension > 360) rightAscension -= 360;
	if (rightAscension < 0) while (rightAscension < 0) rightAscension += 360;

	rightAscension = rightAscension + 90 * (Math.Floor(sunTrueLon / 90) - Math.Floor(rightAscension / 90));

	rightAscension = rightAscension / 15;

	double sinDec = .39782 * Sin(sunTrueLon);
	double cosDec = Cos(Asin(sinDec));

	double cosHour = (Cos(ZENITH) - sinDec * Sin(LAT)) / (cosDec * Cos(LAT));

	double hour = Acos(cosHour) / 15;

	double time = hour + rightAscension - (0.06571 * tApprox) - 6.622;

	time = time - LON / 15;
	time -= 7;

	while (time < 0) time += 24;
	while (time > 24) time -= 24;

	DateTime sunset = new DateTime(date.Year, date.Month, date.Day, (int)time, (int)((time - (int)time) * 60), 0);

	return sunset;

Since geography is done using degrees, but math is done in radians, I decided to write my own trig functions that handled the conversion for me. I’m sure you all know how to do that ( :p ), but in case you don’t, here are cosine and arccosine, from which you should be able to extrapolate to figure out the rest.

private double Cos(double degrees)
	return Math.Cos(Math.PI / 180 * degrees);

private double Atan(double x)
	return 180 / Math.PI * Math.Atan(x);

Also note that I use the constants LAT (latitude), LON (longitude) and ZENITH. latitude and longitude are simply the ones corresponding to your location (lookup here), and the standardised value for the zenith is given in the above algorithm link to be 90° 50’, or 90.83333°. After some fiddling, I found that the above code in fact generated the same exact sunset time as the one printed in my local papers. With that done, I moved on to writing the plugin and tweaking the timer to display well on the LCD.

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