What is the Twilight Angle?
If the earth did not have an atmosphere, the sky would become dark immediately after sunset. The earth's atmosphere causes scattering of sunlight so that light reaches the observer before sunrise and after sunset. This scattered light is called twilight. After sunset, as the depression of the sun increases the sky gets darker and darker until no scattered light reaches the observer. Conversely, in the morning light starts to appear in the sky even before sunrise. The morning twilight is called dawn whilst the evening twilight is known as dusk.
In astronomy the twilight period is divided into civil, nautical and astronomical twilight corresponding to solar depressions of 6, 12 and 18 degrees respectively.
Civil twilight
roughly equivalent to lighting up time. The brightest stars are visible and at sea the horizon is clearly visible.
Nautical twilight
the horizon at sea ceases to be clearly visible and it is impossible to determine altitudes with reference to the horizon.
Astronomical twilight
when it is truly dark and no perceptible twilight remains.
Twilight and calculation of fajr and isha
Fajr starts with the morning twilight (dawn) whilst isha starts at the end of the evening twilight (dusk). There is some debate as to which twilight angle should be used in the calculation of fajr/isha.
In modern times, astronomical twilight (18 degrees) has come to be widely used for the determination of isha and fajr times.
As the average intensity curve of evening twilight indicates, the flux decreases to a minimum level, and thus it would seem appropriate that even for Islamic purposes, this should indicate a reasonable starting value for the end of 'astro-lslamic twilight' (AIT). Indeed, 18 degrees depression was a commonly used value for fajr and isha in the medieval period, when it must have been based on careful observations. Nevertheless, slight variations from this value- between 16 degrees and 20 degrees- were also used during the medieval period. According to King, 20 degrees and 16 degrees were the parameters used by Ibn Yunus for morning and evening AIT respectively, whereas 19 degrees and 17 degrees were the parameters used by various Egyptian astronomers.
Nasr also refers to 19 degrees being used in the Islamic world for the fajr and isha times. King has confirmed that although Muslim astronomers widely used 18 degrees/l8 degrees symmetrical values or a slight variation to 19 degrees/17 degrees (morning/evening)-and in a few (earlier) cases even 20 degrees/16 degrees values were adopted-no record has been found of the use of a value as small as 15 degrees. This is of specific interest to us because a few years ago, values of 15 degrees/15 degrees for both AlTs were proposed by Bagvi on the authority of Maulana Rashid Ahmad Ludianvi of Karachi, who is reported to have made some observations personally. Indeed, I myself followed Bagvi in preparing the first South Australian Islamic Time Table, although I did consider the need to examine the matter very carefully and introduced a distinct term for it. Subsequently, this 15 degrees proposition has come under criticism and is apparently not proved by others' observations made in recent years in that particular area (Karachi). Concern on this matter is obviously understandable, because an erroneous delay in the beginning of fajr would have serious implications for the beginning of the Fast.
One of the useful studies arguing against this 15 degrees depression for AIT is that of Latiff, who claims to have conducted some observational work. Unfortunately, Latiff has argued for a fixed 18 degrees/18 degrees case for all over the globe-no less and no more. This, it seems, may not be the true situation either as there seems to be some room for geographical variability and perhaps 18 degrees serves as a good upper limit only. The value for morning AIT has widely been used as 18 degrees, although a practice of using 20 degrees prevails in Egypt and Indonesia/Malaysia (under Egyptian influence, apparently owing to the concerned persons having been trained there). Possibly, 20 degrees is taken as a safeguard because of the sanctification of the Fast. On the other hand, the use of smaller values than 18 degrees for India and Yemen has been reported to me in personal communications by Abdul Hafiz Maniar of Surat, India, and Dr. M.M. Qurashi of Islamabad, Pakistan, respectively.
However, as we have noted elsewhere, in general, at 18 degrees depression, no detectable trace of twilight flux will be found, meaning that 18 degrees represents an upper limit.