The seer represents the sun,and mind the moon

Verse 4.19 

न तत्स्वाभासं दृश्यत्वात् ॥१९॥

na tat-svā-bhāsaṃ dṛśyatvāt ||19||

Mind is not self-luminous, being an object.

Mind being the seedling of the seer, its growth and luminosity depend upon the seed, the light of the seer. Its own light is as that of the moon, which is reflected light from the sun. 

The seer represents the sun,

and mind the moon. As a child feels strong and secure in the presence of its parents, mind, the child of the seer, draws its strength from the seer.

Mind, like the senses of perception, can normally see an object but not its own form. For an average person, the eyes pose as the seer when apprehending worldly objects. 

For an intellectual person, the eyes become the seen, and the mind the seer. For an enlightened person, mind and intelligence become objects for the consciousness. But for the wise seer, mind itself becomes the object perceived.

The seer can be subject and object at the same time; mind cannot.

It may therefore be inferred that mind has no light of its own. When the borrowed light of mind is drawn back to its source, the seer, or soul, glows brilliantly.

PATANJALI YOGA SUTRAS