بسم الله الرحمن الرحيم
In the name of Allah most Gracious most Merciful
By the sun and its brightness.
wash-Shams wa duha ha.
The sunnah is for Allah to swear/do an oath by anything He wishes, and for us humans to swear only by Allah.
Allah swears by the sun (ash-shams), and its duha – soothing morning light.
Duha – light of the sun.
– or Allah is swearing by the entire day (because the day is a light from the Sun, and its absence is darkness of the night.)
Duha: When the sun comes completely out and manifest without being scorching or glaring, but instead it is a clear/soothing glow and easy to see with the eye. This is the duha ha – the glow of the sun.
I swear by the sun and its ability to give the soothing morning light.
The duha is exclusively for the sun, and this duha (glow) is not used for any other object of light (like a torch etc.)