Article of the month August

Islamic prayer lines

Point sundials in Islamic countries frequently have lines which indicate the time a prayer should be said.
The most important times are those for:
Zuhr
Asr-i evvel
Asr-i sāni

An example of a horizontal sundial from the south of Spain with these three prayer lines is shown below.
The hour lines on this sundial indicate antique hours.

The times for these prayers are prescribed as follows:
On a horizontal plane, a vertical gnomon of height g is set up.
On a particular day at noon, the length of the shadow of this gnomon is determined; say it is M.
To find the time of Zuhr, a quarter of the gnomon height g is added to M. When the length of the gnomon shadow reaches this value (M + ¼ g), the time for Zuhr has come.
This time should be indicated on the sundial.

For a considerable number of days these times are determined and indicated on the sundial. Afterwards, a smooth curve is drawn through all these points.

In a similar manner the times for Asr-i evvel and Asr-i sāni are determined from the shadow lengths M + g and M + 2g

In a table:

Prayer	Shadow length
Zuhr	M + 0.25g
Asr-i evvel	M + g
Asr-i sāni	M + 2g