yad āditya-gataḿ tejo
jagad bhāsayate ’khilam
yac candramasi yac cāgnau
tat tejo viddhi māmakam

Translation of Bhagavad Gita 15.12

The splendor of the sun, which dissipates the darkness of this whole world, comes from Me. And the splendor of the moon and the splendor of fire are also from Me.

Commentary by Sri A.C. Bhaktivedanta Swami Prabhupada of Gaudiya Sampradaya:

The unintelligent cannot understand how things are taking place. But one can begin to be established in knowledge by understanding what the Lord explains here. Everyone sees the sun, moon, fire and electricity. One should simply try to understand that the splendor of the sun, the splendor of the moon, and the splendor of electricity or fire are coming from the Supreme Personality of Godhead. In such a conception of life, the beginning of Krishna consciousness, lies a great deal of advancement for the conditioned soul in this material world. The living entities are essentially the parts and parcels of the Supreme Lord, and He is giving herewith the hint how they can come back to Godhead, back to home.

From this verse we can understand that the sun is illuminating the whole solar system. There are different universes and solar systems, and there are different suns, moons and planets also, but in each universe there is only one sun. As stated in Bhagavad-gita (10.21), the moon is one of the stars (nakshatranam aham sasi). Sunlight is due to the spiritual effulgence in the spiritual sky of the Supreme Lord. With the rise of the sun, the activities of human beings are set up. They set fire to prepare their foodstuff, they set fire to start the factories, etc. So many things are done with the help of fire. Therefore sunrise, fire and moonlight are so pleasing to the living entities.

Without their help no living entity can live. So if one can understand that the light and splendor of the sun, moon and fire are emanating from the Supreme Personality of Godhead, Krishna, then one’s Krishna consciousness will begin. By the moonshine, all the vegetables are nourished. The moonshine is so pleasing that people can easily understand that they are living by the mercy of the Supreme Personality of Godhead, Krishna. Without His mercy there cannot be sun, without His mercy there cannot be moon, and without His mercy there cannot be fire, and without the help of sun, moon and fire, no one can live. These are some thoughts to provoke Krishna consciousness in the conditioned soul.

Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

Whatever the jiva attains in his bound state, I, being the essence of the sun, moon and other objects, supply that. This is explained in three verses. The light situated in the sun, rising in the morning on Udaya Parvata, illuminates the world in order to cause action for the attaining present and future enjoyment. That light, and all light (akhilam) that is in the moon and fire, belong to me alone. What are known as the sun, moon and fire, are actually me. Know that these are my vibhuti.

Commentary by Sri Ramanuja of Sri Sampradaya:

15.12 That brilliance of the sun and other luminaries which illumines the whole universe — that brilliance belongs to Me. Know that this capacity of illumining is granted to them by Me who have been worshipped severally by them. Sri Krsna states that the power in the earth to support all those that reside on it belongs to Him alone:

Commentary by Sri Sridhara Swami of Rudra Sampradaya:

In the previous four verses the Supreme state of Lord Krishna has been described as well as confirmation that those who constantly endeavor can achieve it and will never again be subject to the enslavement of samsara the perpetual cycle of birth and death. Also anticipating any doubt of the transmigration of the atma or immortal soul, Lord Krishna has categorically and unequivocably identified it as eternal and distinctly different from the perishable physical body. Now in this verse and the next three the omnipotent nature of the Supreme Lord Krishna will be revealed beginning with the various kinds of natural light that exist in material existence. The factual understanding is that it is by the light of consciousness from the Supreme Lord that the sun, the moon, lightning and fire receive their luminousity and effulgence as they are solely illuminated by the Supreme Lord and thus a part of His potency.

Commentary by Sri Madhvacharya of Brahma Sampradaya:

Lord Krishna reveals more pertinent details and facts about His omnipotent attributes.

Commentary by Sri Keshava Kashmiri of Kumara Sampradaya:

Previously Lord Krishna has delineated the dual nature of the jiva or embodied being as either liberated or in bondage and the means to free oneself from the captivity of samsara or the perpetual cycle of birth and death. Now in this verse and the next three He reveals some of His divine glories. Here He illustrates that the sun, moon and fire are all part of His divine glory as their illumination manifests from Him and assist in the maintenance and preservation of creation. He explains them as a portion of His effulgence to show that they are but a portion of Him. The Supreme Lord being pleased by their competence and service in the material worlds has enhanced them with His effulgence acccording to their capacity.

Commentary by Sri Adi Shankaracharya of Advaita Sampradaya:

15.12 Yat, that which is; aditya-gatam, in the sun, which abides in the sun;-what is that-the tejah, light, brilliance, radiance; which bhasayate, illumines, reveals; akhilam, the whole, entire; jagat, world; yat, that ulluminating light which is; candramasi, in the moon; ca, and yat, which is; agnau, in fire, the carrier of oblations; viddhi, know; tat, that; tejah, light; to be mamakam, Mine. That light belong to Me who am Visnu. Or: The light that is Consciousness, which is in the sun, which is in the moon, and which is in fire, know that light to be Mine. That light belongs to Me who am Visnu. Objection; Is it not that the light that is Consciousness exists equally in the moving and the non-moving? Such being the case, why is this particular mention, ‘That light in the sun which…,’etc? Reply: This defect does not arise, because, owing to the abundance of the sattva quality, there can be an abundane [Ast. reads avistaratva (amplitude) in place of adhikya.-Tr.] (of Consciousness). Since in the sun etc. the sattva is very much in evidence, is greatly brilliant, therefore there is an abundance of the light (of Consciousness) in them alone. And so it (sun etc.) is specially mentioned. But it is not that it (Consciousness) is abundant only there. Indeed, as in the world, a face, though in the same position, is not reflected in wood, a wall, etc., but in a mirror etc. it is reflected according to the degree in which they are more and more transparent, so is it here. Further,

Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:

15.12-14 Yad aditya-etc. upto caturvidham. While clarifying the role [of the Lord] as the Creator, Sustainer and Destroyer – a role that has been indicated in the Tenth Chapter – by [the description of Him as identical with] the triple luminories like the sun etc., our venerable preceptor has rightly said : The present passage declares that the power, the pentad of elements has [both] collectively and individually, to sustain the world, is the Absolute power of the Bhagavat Himself. For instance : The sunlight has the power of illumining and sustaining, because the twin elements of the fire and the earth are one [with the Bhagavat]. This has been started by the twin hemistitches ‘[That light] which is found in the sun etc.’ and ‘And by penetrating the earth etc.’ The moon-light is illumining and nourishing because of its assoication of the elements of the earth, the water and the fire [with the Bhagavat]. That has been stated by the portion ‘[That light] which is in the moon’ and by the hemistitch ‘Being the sapful moon I nourish all plants’. The light of fire is, however, of the nature of illumining, parching, burning, sweating and ripening (or cooking). It is so because of the association of the elements, the earth, the water, the fire and the wind [with the Bhagavat]. The same has been said here by the portion, ‘[That light] which is in the fire’ and also by the [entire verse] ‘I, being the digestive fire etc.’ But the [element] ether is no doubt all pervaisve (inclusive), because it is of the form of room which is nothing but awareness. Therefore, having so far explained the nature [of the Absolute Self] as the object of knowledge, now, with a view to show the Self to be conscious of the freedom that remains as a background (or substratum) of that very nature of object of knowledge; to be of transcending nature; to be the Supreme Lord; to be independent in all knowledges; and to be the Creator of all; [the Lord] says –

Sanskrit Shloka Without Transliteration Marks:

yad aditya-gatam tejo
jagad bhasayate ’khilam
yac candramasi yac cagnau
tat tejo viddhi mamakam

Sanskrit to English Word for Word Meanings:

yat — that which; āditya-gatam — in the sunshine; tejaḥ — splendor; jagat — the whole world; bhāsayate — illuminates; akhilam — entirely; yat — that which; candramasi — in the moon; yat — that which; ca — also; agnau — in fire; tat — that; tejaḥ — splendor; viddhi — understand; māmakam — from Me.