Narayana (Nara+Aayna) is the one who rests on water. But why is this Narayana, epithet of Lord Vishnu, used as suffix for Lord Surya, the Sun God? Is there any story behind this name?
The connection between Vishnu and the Sun is described in this chapter of the Shatapatha Brahmana of the Yajur Veda. The gods once accidentally cut off Vishnu's head with his own bow (this is all Vishnu's Lila, of course) and the head turns into the Sun:
They spake, 'Whoever of us, through austerity, fervour, faith, sacrifice, and oblations, shall first compass the end of the sacrifice, he shall be the most excellent of us, and shall then be in common to us all.' 'So be it,' they said.
Vishnu first attained it, and he became the most excellent of the gods; whence people say, 'Vishnu is the most excellent of the gods.'
Now he who is this Vishnu is the sacrifice; and he who is this sacrifice is yonder Âditya (the sun). ...
Taking his bow, together with three arrows, he stepped forth. He stood, resting his head on the end of the bow. Not daring to attack him, the gods sat themselves down all around him.
Then the ants said--these ants (vamrî), doubtless, were that (kind called) 'upadîkâ'--'What would ye give to him who should gnaw the bowstring?'--'We would give him the (constant) enjoyment of food, and he would find water even in the desert: so we would give him every enjoyment of food.'--'So be it,' they said.
Having gone nigh unto him, they gnawed his bowstring. When it was cut, the ends of the bow, springing asunder, cut off Vishnu's head.
It fell with (the sound) 'ghriṅ'; and on falling it became yonder sun. And the rest (of the body) lay stretched out (with the top part) towards the east. And inasmuch as it fell with (the sound) 'ghriṅ,' therefrom the Gharma (was called); and inasmuch as he was stretched out (pra-vrig,), therefrom the Pravargya (took its name).
Not to keep you in suspense, the gods attached a horse head to Vishnu's body, which is how he assumed his Hayagriva form. This was Vishnu's plan all along, to kill the horse-headed demon Hayagrivasura who had received a boon that he could only be killed by someone who was horse-headed. Here's what this chapter of the Devi Bhagavatam says:
The Devas became very glad and spoke this to Visvakarmâ :-- “Kindly do this Deva work and fix Visnu's head. He will become Hayagrîva and kill the indomitable Dânava.” Sûta said :-- Hearing these words, Visvakarmâ quickly cut off with his axe, the head of a horse, brought it before the Devas and fixed it on the headless body of Visnu. By the grace of Mahâmâyâ, Bhagavân became horse-faced or Hayagrîva. Then, a few days after, Bhagavân. Hayagrîva killed that proud Dânava, the Deva's enemy, by sheer force.
By the way, the method by which the head was attached is known as the Madhu Vidya, and it was taught to the Ashwini Kumaras by the sage Dadhichi; see my question here.
Bhagvatam SB 12.11 describes Lord Hari created Sun god through his beginningless material energy and is so indifferent from Lord Hari. And even Sri Narayan is always with him in all twelve months.
SB 12.11.29 — Sūta Gosvāmī said: The sun travels among all the planets and thus regulates their movements. It has been created by Lord Viṣṇu, the Supreme Soul of all embodied beings, through His beginningless material energy.
SB 12.11.30 — The sun-god, being nondifferent from Lord Hari, is the one soul of all the worlds and their original creator. He is the source of all the ritualistic activities prescribed in the Vedas and has been given many names by the Vedic sages.
SB 12.11.31 — Being the source of the material energy, the Personality of Godhead Lord Hari in His expansion as the sun-god is described in nine aspects, O Śaunaka: the time, the place, the endeavor, the performer, the instrument, the specific ritual, the scripture, the paraphernalia of worship and the result to be achieved.
SB 12.11.32 — The Supreme Personality of Godhead, manifesting His potency of time as the sun-god, travels about in each of the twelve months, beginning with Madhu, to regulate planetary motion within the universe. Traveling with the sun-god in each of the twelve months is a different set of six associates.
So, as Sun god is the material energy of Lord Narayana that's why he must be called as Surya Narayana.
For hindu months related to Sun god, I have written a question and answer here.
There are many meanings for the word Narayana. One of the meanings are the one who rests on Water. According to a verse in Narayana Suktam,
yacca kiñcijjagatsarvaṁ dṛśyate śrūyate'pi vā,
antarbahiśca tatsarvaṁ vyāpya nārāyaṇaḥ sthitaḥ.(Narayana suktham verse 5)
Whatever all this universe is, seen or heard of- pervading all this, from inside and outside alike, stands the supreme being (NArAyaNa).
Shreemannarayana is also considered as Sthithikaara among the divine trinity meaning who preserves the universe. Surya is the representation of Lord Vishnu in the material world. He is the father of this material universe because he controls time and provides sustenance for the world. Sun provides the light and energy to the world to survive. Without Sun we can not live even for a day. Lord Vishnu resides in Sun god and gives us light and energy. Sun god is also stated as the god who gives good health and te adhidevata of eyes. So people are suggested to worship Surya when they suffer with ill health. Surya NArAyaNa is the adhidevata of Gayatri mantra. Lord Rama worships Surya by a stotra called Aditya Hrudayam which was told by sage Agastya and he acquires victory in the war against Ravana.
As Vishnu takes the form of Surya to give energy and light to the world to provide sustenance, He is called as Suryanarayana. There is a temple in Andhra Pradesh in which Surya is worship as a form of Vishnu.
We get the name 'Surya-Narayana' in the Devi -Bhagavatam. Lord Narayana divided Himself in twelve parts for twelve months and this seems the source of the name "Surya-Narayana":
Nārāyaṇa, roaming about, has divided the Trayī Ātmā into twelve parts for the perfect happiness of all and for Karma Śuddhis (the purification of Karma, acts). The sages furnished with Jñān and Vijñāna have thus argued on the point, following the path as laid out in the Vedas. The Sūrya Nārāyaṇa, moving on in the six seasons, spring, etc., has established, cold, heat, etc., as the Dharma of the seasons, duly for the fructification of the Karmas of the individual beings.
Vishnu and Surya have been related very closely in our scriptures.
The "Hiranmaya Purusha" seen in the Surya-Mandala (Chandogya 1/66) is well- known.
The name of Vishnu is derived popularly as :
yasmad vihvam idam sarvam tasya saktyaa mahaatmanah/ tasmad eva uchchyate vishnur vishadhatuh praveshanad.
English Translation : From Whom everything of the world is created is called Vishnu.
One subtle meaning of Savita (Sun) is also 'Prasavita' or the Origin of everything.
In Vishnusahasranaama there are names like Arka, Bhaanu, Aditya etc which are names of Surya.
Vishnu or Narayana is always meditated as sitting in the Sun :
"Dheyah sada Savitri-mandala-madhyavarti narayanah sarasijasannivishtah" (Ref: Omkarnath-Rachanavali, Vol 7, page 93).
On the otherhand, the Surya-Arghya -mantra of Sandhyavandana is:
Om namo vivasvate brahman vaasvate vishnu-tejase suchaye savitre....(Ref: Vaidiki Tantriki Sandhya -Ramaranjan Kavyavyaakaranatirtha, page 23.), mentioning Surya as the Aura of Vishnu.
Surya is the Son of Kashyapa. Kashyapa's wife is Aditi. Vamana -Avataara of Vishnu is also a son of Aditi.
The Rishyadi-NyAsa of the Aditya-Hridaya Stotra reads as
Asya sri-aditya-hridaya-stotrasya srikrishna-rishir anushtup-chhandah sri-surya-nArAyano devata..sarvapapakshaya-siddhi-artham jape viniyogah.(Bhavishyottara-PurAna).
Surya is called Surya Narayana in the Surya Upanishad a text which origin is found in the Atharva Veda.
shhatsvaraaruudhena biijena shhadangam raktaambujasansthitam saptaashvarathinam hiranyavarnam chaturbhujam padmadvayaabhayavaradahastam kaalachakrapranetaaram shriisuuryanaaraayanam ya evam veda sa vai braahmanah
The six limbs consist of the seed with six vowels added. He indeed is a Brahmana who thus knows the golden Surya Narayana seated on the chariot with seven horses, impeller of the time-wheel, having four arms bearing two lotuses, the others bestowing promise of refuge and boon, set in the red lotus.
To be honest it is quite hard to explain why the Lord named Himself so. If we look at the Maha Narayana Upanishad the following is being said
- The universe arose from Visvakarman through water, earth, fire and other elements. He excelled Aditya, Indra and other gods. The sun called Tvasta rises in the morning embodying His brilliance. In the beginning of creation the mortal world enveloped in gloom received its divine brilliance from the sun shining in the glory of Paramatman.
- I know this Great Person who is beyond ignorance and darkness and whose splendour is comparable to that of the sun. Knowing Him thus in this life itself, one transcends death. There is no other path leading to the attainment of liberation.
- The sun who is the Lord of creatures moves about in the space between heaven and earth causing day and night. Although He is unborn, being the Self of all, He manifests Himself as the manifold universe. Wise men realize the source of the universe, the all-pervading Paramatman. Prajapati, the first patriarchs, sought the position, which Marichi and other sages attained.
- Salutation to the resplendent Sun-God who is the son of Para-Brahman, who shines for the benefit of gods, who is invoked as the beneficent leader of the gods, and who was born as the eldest among the gods.
- When the gods instituted the Knowledge of Brahman they declared thus teaching about the Supreme reality. - That sage who knows the Supreme as described before will have sovereignty over gods, for he has become the Inmost Self of all.
- O Sun, Hri and Lakshmi are Thy consorts, Thyself being Brahma, Vishnu and Siva. Day and night are Thy two sides. Asterisms in the sky are Thine own form. The Ashvins are Thy mouth. Being such, grant me whatever I desire, spiritual illumination, happiness here and other objects of desire.]
To understand why Surya is called Surya Narayana we can even look further in the Narayana Suktam. The Narayana Suktam says
nārāyaṇaḥ paraṁ brahma tattvaṁ nārāyaṇaḥ paraḥ, nārāyaṇaḥ paro jyotirātmā nārāyaṇaḥ paraḥ.
Narayana is the Supreme Brahman. Narayana is the Supreme Reality. Narayana is the Supreme Light. Narayana is the Supreme Self.
The Surya Upanishad tells us that Surya is the Self of all in the following sentence
suurya aatmaa jagatastasthushhashcha suuryaadvai khalvimaani bhuutaani jaayante suuryaadyagyah parjanyoannamaatmaa namasta aaditya
The Sun is the Self of the world, moving as well as un-moving. From Surya indeed are these creatures born, so also the Yajna (Sacrifice), Parjanya (Rains), food and spirit.
So to be honest if we look at the scriptures, I don't think there is any difference between Narayana and Surya Narayana. The name Surya Narayana may imply His form as the Sun and at the same time be the Supreme God Himself.