Merging here means that the night slowly and gradually changes to day and vice versa. This phenomenon can only take place if the earth is spherical. If the earth was flat, there would have been a sudden change from night to day and from day to night.
The following verse also alludes to the spherical shape of the earth:
“Hecreated the heavens And the earth In true (proportions): He makes the Night Overlap the Day, and the Day Overlap the Night.” [Al-Qur’aan
The Arabic word used here is Kawwara meaning ‘to overlap’ or ‘to coil’– the way a turban is wound around the head. The overlapping or coiling of the day and night can only take place if the earth is spherical.
The earth is not exactly round like a ball, but geo-spherical i.e. it is flattened at the poles. The following verse contains a description of the earth’s shape:
“And the earth, moreover, Hath He made egg shaped.” 2 [Al-Qur’aan
The Arabic word for egg here is dahaha, which means an ostrich-egg. The
shape of an ostrich-egg resembles the geo-spherical shape of the earth. Thus the Qur’aan correctly describes the shape of the earth, though the prevalent notion when the Qur’aan was revealed was that the earth is flat.
The Qur’aan and Modern Science: Compatible or Incompatible?
by Dr.Zakir Naik