Q1: I live in London and I found Fajr and Isha times provided by your program are not the same as the times given by our local Mosque in London. Why there is a difference and what are the correct times?
A1: The times of Fajr and Isha are effected by the chosen depression angles (Fajr, Imsak and Isha angles). Some Mosques are using the value of -18 degrees for all angles while SalatTimes has default of -16, -13.5 and -16 degrees respectively. If you look into old textbooks you will find out that the conditions for True Dawn are not fulfilled when using -18 degrees (i.e. it is too dark). I believe the times provided by SalatTimes are the correct times but still you can get the same times as your Mosque by changing the values of Fajr, Imsak and Isha angles to the same value as the one used by your local Mosque.
Q2: What is the difference between Zawal and Duhr times? Is Duhr time really the time of Duhr prayer or we have to add few minutes to obtain the time of Duhr prayer?
A2: Duhr time is the time when the sun passes local meridian completely. Noontime is the time of midday((Nisf-Annahar). The difference between the two times is at most 1.5 minutes. In Arabic, Zawal time indicates the time when the sun passes the middle of the day. Thus, Duhr and Zawal time should be the same. However, some people believe that Zawal time is the time when the sun starts passing the local meridian (i.e. at midday). Therefore, they give the time of Noon as Zawal time. This definition is not correct and should be avoided. In addition, as the case with other prayer times, certain people are adding few minutes to Duhr time to compensate for something (e.g. to not pray with the Sun worshippers). Anyhow, as I failed to find the source for such compensation in Hadith and Sunna, I do not recommend adding any extra minutes.
Q3: I would like to know what formula are you using for Qibla direction and why are you converting geographic latitude to geocentric? According to the paper written by Kamal Abdali no such conversion is needed.
A3: SalatTimes uses the following formula to calculate Qibla direction:
mod(atan2(sin(lonb-lona)*cos(latb),cos(lata)*sin(latb)- sin(lata)*cos(latb)*cos(lonb-lona)),2*pi)
The formula is mathematically equivalent with the one mentioned by Abdali and others. Both formulae are valid only on perfect sphere. If the Earth was a perfect sphere then geocentric latitude would be the same as geographic. However, since this is not the case there is a difference between the two. The difference is quiet small and has a maximum (11'32".73) when geographic latitude is +-45° 05' 46".36. Since the difference is small it is often not mentioned. However, if one needs to be precise, geographic latitude should be converted to geocentric.
Q4: I have a question about Moon data. Why you did not implement any sighting criterion when you already calculated all-important parameters?
A4: The visibility of the Moon is not definitive or conclusive (as the case with the instant of New Moon). There are several criteria available to predict the visibility of the Moon. However, the crescent of Ramadan, 1418H, was the definite answer that non-of them is reliable enough. Most of these criteria indicated the crescent will be visible only in certain parts of the Pacific (close to Hawaii islands) and in best case in certain parts of USA. But reports indicated the crescent was seen in places as far as Pakistan! I did not implement any sighting criterion because there is no reliable one yet.
Q5: I like your program but have problem to set the configuration data correctly. Specially, I do not know to what values I should set for Fajr, Imsak and Isha angles. You mention that default values are good enough to be used but I see that the provided times are much different from what the rest of my people is using. I live in United Arab Emirates. Please help?
A5: Most Muslim countries (including UAE) are using the value of -18 degrees for Fajr and Isha depression angles. Imsak time is defined to be few (5-10) minutes before Fajr time. The question I would ask is why they use -18? or even better I would ask, do we see the True Dawn when the sun reaches -18 in Muslim countries? I do not think so and anyone can easily proof this (i.e. there is no sign at all for the Fajr or True Dawn when the sun is at -18). However, I am not a Faqih and can not make a conclusion in such subject. To be on the safe side ask your Faqih about this. If the answer is follow the rest of the people then just change the default setting. I have already sent this question to many scholars but so far non-replied to me. When I receive any reply, I will include it in this Faq.
Q6: I think I found a bug in your program. The Qibla direction from magnetic north is different from the value provided by our Qibla compass?!
A6: I am afraid the fault is in your Qibla compass rather than in SalatTimes! Many people do not realize that the position of earth magnetic pools is changing all the time. Qibla compasses produced several years back would give higher error margin when used during resent years. This is due the fact earth magnetic pools are not in the same place as they were when these compasses were produced. That's why many navigation instruments have to be continuously adjusted to the changes in the position of earth magnetic pools. Thus, I would encourage you to buy new compass and use it instead of your old Qibla compass.
Q7: I have access to a program that calculates rise and set times for the Sun. This program is just like yours claiming an accuracy of 1 second. However, results from this program do not agree with the results from SalatTimes and the error can reach up to 1 minute! Would you mind telling me which program is the correct one?
A7: Perhaps both!
Programs written for astronomical application does not follow the principles used in religious applications. Many astronomers used to define the sun set as when the center of the sun is touching the horizon. This definition is an accordance with set times for other planets and stars. In addition, they use fixed value for refraction, which is applicable for this exact position of the sun (i.e. when the center touches the horizon). In Islam, the Sun is set when it is completely below the horizon. This means, the upper limp of the sun is just passed below the horizon. This is the definition used in SalatTimes and perhaps this is the reason for difference.
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