Category: Zmanim

I was recently contacted by Jan Terje Johansen, a developer at Datek Wireless AS in Norway, with an interesting bug report. Datek uses the Zmanim API (the AstronomicalCalendar base class) to allow their clients (the power company and stadiums) to remotely (via a web interface) control streetlights and stadium lights throughout Norway using a wireless lighting control system that they developed. The zmanim code is used to allow setting the lighting times based on an offset of sunrise/sunset. For the technically curious, they are controlled primarily through GPRS, with SMS as fallback. A version under development uses ZigBee. The bug encountered was that for Tromsoe (Tromsø), Norway, as well as other areas within the Arctic Circle that experience the midnight sun, from May 13th to May 17th (the date of last rise of the season in Tromsoe), the zmanim API produced correct sunrise/set times, but the date component (The API returns all times as Java Dates, something that might change with v2.0 of the API that will target JDK 7 to take advantage of JSR-310 Date and Time API) was a day off. For non-automated systems, the date component is not important, but in their case it would cause the lights to go on/off on the wrong day. Jan provided a suggested patch that worked well. The actual fix I used was slightly different because I took advantage of the time spent on fixing the bug to refactor and simplify the code. This change as well as a few other changes are part of the Zmanim 1.1 beta release that will likely be released as a final release in a few days. Jan mentioned that:

“IMHO your API is easily the best and most accurate Open Source sunset/sunrise API out there”. He continued: “Officially (according to the Norwegian Meteorology Institute), the midnight sunset/sunrise is from May 20 to July 22 in Tromsoe, ie. the complete sun is above the horizon 24 hours. Parts of sun is visible 24 hours a day in Tromsoe from May 18th to July 25th. This is the same as in your calculations.” … “I have tested your calculations against other official midnight sunrise/sunset (part of sun) dates in Northern-Norway (North Cape, Hammerfest, Longyearbyen (78,049762N -15,458252E)) and they are spot on.”

In response to my question regarding his testing of the NOAACalculator versus the USNOCalculator he had an interesting and very practical answer

“We prefer to use USNO calculator as it is more in tune with the sunrise/sunset times printed in most newspapers. You see, our experience is that most users don’t look at the sun to determine sunrise/sunset but read the times in the newspaper. If our times don’t correspond to the printed ones, something is wrong with our system in their mind.”

The Zmanim API was developed from the ground up as an API which allows for easy plugging in of different algorithms. The Zmanim API ships with 3 “Calculator” implementations. Two calculators implement the US Naval Observatory’s algorithm, the SunTimesCalculator and the ZmanimCalculator (no longer available). Both produce identical zmanim using slightly different code and are included for comparison. There is also the JSuntimeCalculator (no longer available – see below information on the NOAACalculator), an implementation of the NOAA algorithm by Jonathan Stott. I was recently contacted by Eliezer Bulka who wanted to know why sunrise/sunset times generated by the NOAA algorithm were about 2 minutes off of the sunrise/sunset times generated by the NOAA JavaScript implementation that is the source of the JSuntimeCalculator. To compare apples and apples required modification of the NOAA JavaScript to allow entry of decimal latitude/longitude and changing the output to display seconds. No change was made to the algorithm itself. I then ported the JavaScript directly to Java. This involved nothing more than slight syntax changes between the languages. Once this was done, I noticed that the sun rise/set output from the Java port exactly matched the output of the NOAA JavaScript. Analysis of Jonathan’s code showed (or at least my interpretation of it did) that there were two areas that caused the difference. Once is that he used a slightly different method of computing the Julian date, a key part of the algorithm. His change includes the time of day as part of the calculation. The net result of this change is that solar time generated using his algorithm varies based on the time of day the calculation is run, something that is incorrect. This means that there can be a discrepancy of up to one calendar day. If the user calculated sunrise at 11:59 PM, sunrise would be calculated for the following day even if the user attempted to calculate it for today. In addition, the other calculations do not match the output of the matching NOAA code. I have deprecated the JSuntimeCalculator and in its place added the NOAACalculator that was the result of the direct port of the NOAA code, shoehorned into the Zmanim API Calculator interface. I ran some tests to compare the maximum and minimum discrepancy between the 2 implementations, and calculations for Lakewood, NJ, latitude 40.0828, longitude -74.2094 show a discrepancy of between a minute and 34 seconds to a minute and 37 seconds for sunrise and sunset across an entire year of sunrise and sunset calculations. I also compared the USNO algorithm to the new NOAA implementation and ended up with a maximum deviation of less than 30 seconds, something that had been about 1.5 minutes apart previously. While I do believe that the Julian date calculation is a bug, I do not know that this is a case as far as the rest of the calculation, but it is clear that it does not match the NOAA implementation that is was based on, and I recommend that you download the latest version that has the new NOAACalculator that fixes this issue. In addition to this fix, an additional patch will be released later this week that will address issues with calculations in the arctic circle. Stay posted for the next post.