Mr. Sharma,
You seem to be learned in the ways and practice of our religion. I believe in Brahman and follow Advaita philosophy and to an extent Kabir-Panth.
What is the reason for Swami Deepam?
God does not need deepam, surely in this day of electric light you do not need deepam to see God.
I can understand the symbolism of deepam, or Kalpooram etc., but it is not for lighting.
So what is the purpose of keeping the deepam for 90 minutes?
I do light a lamp in the alter at home but it is electric candle.
Dear Shri Prasad,
I am taking your above post though what I write below is in general and for the OP as well, because you have given some well-structured questions. Hope you will not mind this.
1.
What is the reason for Swami Deepam?
As you know, we had no electric lights till very recently (say, 50 or 60 years ago) in most parts of India. Even the street lighting was done with kerosene or other (vegetable) oil lamps. Temples also had only oil lamps. In different parts of the country different types of vegetable oil were preferred.
At the house-hold level, when the darkness was about to engulf the world, soon after sunset, the custom was to light the lamps so that the normal routines of the house during the night (mainly the evening prayers, meals, some amount of family get-together and chatting/gossipping and then retiring to bed, etc.) could be carried out smoothly. Since the Hindu households generally revolved around a God, the custom was to light the lamp before the pooja first and then the rest. In traditional Kerala households even today a lighted lamp is first placed on the front verandah so as to get the full view of passers-by. The belief is that by doing this the Goddess of prosperity enters the lighted house and the goddess of poverty, suffering, etc., departs from that house. (This is an idea resonating in the śrī sūktaṃ as—क्षुत्पिपासामलां ज्येष्ठामलक्ष्मीं नाशयाम्यहम् । अभूतिमसमृद्धिंच सर्वां निर्णुद मे गृहात् । गन्धद्वारां दुराधर्षां नित्यपुष्टां करीषिणीम् ।ईश्वरीग्ं सर्वभूतानां तामिहोपह्वयेश्रियम् । kṣutpipāsāmalāṃ jyeṣṭhāmalakṣmīṃ nāśayāmyaham | abhūtimasamṛddhiṃca sarvāṃ nirṇuda me gṛhāt | gandhadvārāṃ durādharṣāṃ nityapuṣṭāṃ karīṣiṇīm |īśvarīgṃ sarvabhūtānāṃ tāmihopahvayeśriyam |).
The above sums up the reason for Swami Deepam and the importance attached to it in Tabra households.
2. God does not need deepam, surely in this day of electric light you do not need deepam to see God.
I can understand the symbolism of deepam, or Kalpooram etc., but it is not for lighting.
So what is the purpose of keeping the deepam for 90 minutes?
It is not God who wants us to light a deepam for Him but rather, we are the people who want to light the Swami Deepam first because, as I said above, most Hindu household chores used to be centred around the God-concept. Unlike Karpooram, which is also used for a sudden and more brilliant illumination of the deity's idol or the pooja at home, but not a long term lighting device, deepam was always for lighting and removal of darkness; but when the electricity came the original purpose got 'overpowered', so to say.
Now, coming to the crucial point of 90 minutes in the OP, I will say, with all humility, that the original poster seems to have been misled by whatever was the source of his knowledge.
A "muhurtham" can be approximately 48 minutes, not 90, and this is also on the assumption that day and night are equal and each has 15 muhurtas. The 30 muhurtas have been named - from brāhma to yumigadyuti. This is the astrological classification of Muhurta which is roughly defined as 18 X 30 X 30 twinklings of the eye or two ghadikas.
In astrology there is another sense for muhurtha which is the auspicious time length for performing auspicious functions like marriage, upanayana, seemanthonnayana, etc. Here, the process is to find out the suitable period after filtering out five constituents of the pañcāṃga, viz., tithi, vāra, nakṣatra, yoga, karaṇa. It may so happen that any one of these five constituents change and make the time inauspicious. So, the length of such a muhurtha will depend on the particular circumstances. Those days which have some of these good combinations for suitable length of time are listed in the usual pañcāṃgas.
I therefore feel that there is no logical background for the 90 minutes' limit. In actuality, the practices followed are very many. Some people insist that the swami deepam should "burn itself out" and should not be put out, while some others feel that it is a very bad omen if the swami deepam gets "put out" even by wind, the fan being switched on carelessly, etc. Therefore, it is better that no rule is laid down and each person is allowed to follow his/her preference.