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How Prayer Times Are Calculated
Prayer times come from the position of the sun, which means they can be computed exactly for any place and date without looking anything up.
How are Islamic prayer times worked out?
Each of the five prayers is defined by a position of the sun, so the times can be calculated from your latitude, longitude and the date. Dhuhr is solar noon. Asr is when an object's shadow reaches a set multiple of its own length. Maghrib is sunset. Fajr and Isha are when the sun sits a set number of degrees below the horizon — and that angle is the only part that differs between calculation methods.
The five times, in astronomical terms
Fajr begins at true dawn, when light first appears along the horizon — expressed as the sun being roughly 15° to 20° below it. Sunrise and sunset both use 0.833° rather than 0°, which accounts for atmospheric refraction and the width of the sun's disc. Dhuhr is the moment the sun crosses the meridian. Asr depends on shadow length rather than an angle. Isha is when the last light leaves the sky, again a set angle below the horizon.
Why Asr has two times
Asr is defined by shadow length, and two conventions exist. In the majority view, Asr begins when an object's shadow equals its own length plus its length at noon. In the Hanafi view it begins when the shadow reaches twice that. This is why prayer-time apps ask which one you follow, and why the two can differ by an hour or more in summer.
Why sources disagree
Almost all disagreement comes from the Fajr and Isha angles. A method using 18° for Fajr will give an earlier time than one using 15°. Different authorities settled on different angles, and each is followed in particular regions. A few minutes of variation between sources is normal; a large difference usually means a different method rather than an error.
High latitudes
Above roughly 48° the sun may not drop far enough below the horizon in summer for Fajr or Isha to occur at all. Rules are used in place of the astronomy: dividing the night in half, taking one seventh of it, or scaling by the method's own angle. This is where sources disagree most, because they are applying different conventions to a case the astronomy does not answer.
Common questions
Do I need an internet connection for accurate prayer times?
No. The calculation needs only your coordinates, the date and a method — all of which a device already has. Times fetched from a server use the same arithmetic.
Why do my times differ from my local mosque by a few minutes?
Most likely a different calculation method, or a deliberate adjustment your mosque applies. Follow your mosque: praying with the congregation around you matters more than matching a calculation.
Does altitude change prayer times?
Slightly. Being higher up means seeing the sun a little longer, which moves sunrise earlier and sunset later by a minute or two in most cities.
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