Why do Hindus greatly revere the sun?
Of all heavenly bodies, the sun is most important. Everyone needs its benevolent favours. At the same time, everyone is afraid of its malefic influence. It is therefore natural for many people to hold it in high reverence.
In the Suryopanishad, it is said that the gods, Gandharvas and sages reside in the rays of the sun. Irrespective of who you are, without reverence to Surya nothing can be attained. In the Skandpuran, it is said that eating without offering libation to Surya is like eating in sin. Without reverence to Surya one is not authorized to undertake any kind of auspicious work. It is important to offer special reverence to Surya on Sankranti (entering of the sun or a planet into a new sign of the zodiac) and Surya Shashthi (the sixth day of a fortnight). Otherwise, it is customary to offer prayers to Surya on all Sundays. Many observe a fast too.
Many Hindus make an offering to Surya every morning. For this, water is taken in a copper vessel and red sandalwood, rice, red flowers and kusha are mixed into it. Kneeling down outdoors, devotees face the sun and hold the vessel near the middle of the chest, offering the water slowly while chanting the Surya mantra or the Gayatri mantra. In all offerings devotion is key. Concentrating on the copper vessel and the sun, devotees visualize the round rim of the sun and then see the rays splitting to display a rainbow.