Arti is meant to be performed at sunrise and sunset. At sunrise the raja-tama predominant atmosphere present throughout the night is destroyed and the absolute fire element frequencies of Deities arrive in the universe. Hence, Arti is to be offered at sunrise to welcome them. The 'tarak Chaitanya' (saviour form of Chaitanya) transmitted during the arrival of the frequencies of Deities at sunrise is to be welcomed by the worshipper through the medium of the Arti, whereas at sunset, the Arti is performed to destroy the raja-tama frequencies and to invoke the Deities' 'marak Chaitanya' (destroyer form of Chaitanya). That is why Arti should be performed twice - at sunrise and at sunset.
What is the science in performing Arti at sunset?At sunset the proportion of the absolute fire element in the Sun's rays starts reducing and the predominance of the raja-tama particles in the atmosphere increases. The generation of raja-tama frequencies also increases. Taking advantage of this situation, the negative energies increase their movement in the environ-ment. To prevent distress from such a predominately raja-tama environment, it is essential to evoke the Deities through the frequencies of sound emitted through the Arti and bring these frequencies into the orbit of the universe. As a result the proportion of the frequencies of Deities enriched within the environment increases and the proportion of distressing vibrations decreases. This creates protective armour around the devotee's body. (Divine knowledge received through the medium of Ms Anjali Gadgil, 5.6.2005, 6.33 pm)
(Ref: Sanatan's Publication - 'How to Offer an Arti')