Surya aka Sun is the moola graha, the source graha that created itself and everything else in the solar system. It is a star, a luminary that enlightens our sky, creates the day/night cycle and keeps all the other grahas in orbit. Its light and heat are essential for the solar system to thrive. It has many special features such as the most unique and powerful among grahas, the largest, brightest, most massive and the hottest. It creates time, provides vitality to the planets, gives light and warmth to sustain life on earth. It is committed to its duty, acting as a leader and ruling all the grahas but at the same time it has the power to burn everything to ashes if it comes closer. Surya is the oneness (that can devour anything) and purity (that can atomise anything).
Lord Surya is the Sun God, the king of all the grahas and upagrahas (sub planets) residing in the Bhuvarloka (above the plane of Bhuloka, the earth). Lord Surya is a deva (demi-god) who is under the control of Indra, the king of the devas (residing in Swarka loka). There are many different lokas (realms) above and below the earth (according to vedas). Every loka above earth has the same kings, queens, people etc. but in a better way and every loka below earth has also the same things but in a worse way. Its like a grading system. I'll write more on this later. This is the image of King Surya, the beautiful shining, bright ruler with seven horses.
Every graha represents many tangible and intangible things that are called significations. Based on the above mentioned facts you can think of several intangible significations for Surya such as time, duty, commitment, work, career, power, light, strength, vitality, valour, leadership roles, government, organisation, authority, ruling power. and tangible significations can be one’s self (atma), father, mentor, guru, boss, king, leader, head of an organisation. These significations will be used to understand Surya in a horoscope. Surya is simply the most attractive and important one, wherever the Sun is that's where the light is and the crowd is. Anything that is rich, artistic, luxurious and flamboyant is Surya. So it is the ego that drives you, that gives you the identity. If i asked you who you're, your first answer will be you're a doctor, writer or something that you're doing. What you're doing for a living brings you the identity, that which is giving you the identity becomes your Sun.
When a person has a strong Surya you may notice that their presence can lighten up the space, they may always attract the people around them easily. This is one of the main and easily visible significations of Sun. Apart from this you can also see many other significations from what i said above. For ex. the person can be dominant, tall or stout, self made, having a big family, helping others etc.
The interpretations become interesting and easier when you go through intricacies. What you can do is simply think about the item’s quality and attach it to Surya’s quality, if it fits then it's also one of the significations.
Surya is masculine graha and matures at the age of 21. It's when the Surya within you gets its full strength (like becoming an adult) so you can be able to perform things signified by Surya with ease (like work, rule, becoming a leader). Also the numbers 1 and 15 are more important.
1. Sun gets exalted (strong) in Aries and debilitated (weak) in Libra
2. Sun rules the sign Leo
3. Gets directional strength in the South (10th house)
4. Favourable direction is East (1st house)
6. Sattvic graha
7. Associated colours are orange, saffron and yellow (secondary colour is white associated with sattva)
8. Most independent graha of all
9. Ruling deity is Agni
10. Surya represents agni (fire) and the body part is eye (esp. right)
I can also mention the food items, places, professions and more. The list is endless to be honest, instead i'll tell you the ways to find out the associated items on your own. Anything stands out from the ordinary in an outstanding way (based on its qualities) is associated with Sun. For ex. a giant skyscraper in a city is the representation of Sun, gold among metals, mango among fruits etc. It should attract everyone because that's what Surya does.
Exaltation and Debilitation of Surya
Exaltation means full strength, full power, full happiness. Surya attains exaltation in the sign of Aries because Aries provides him a platform to do so. For ex. you may have certain wants and needs and when its met you will feel strong, powerful and happy. Likewise Surya has certain requirements and Aries helps fulfil it.
To understand this better you have to imagine Surya as a living entity. Think about the requirements of Surya.
"He represents number one, he wants to be the first, he wants to think of himself before everything, he wants to be at the centre, he wants to be self made, he has emotions but in the outside world he wants to be practical, he wants to be quick, he has a big appetite, he wants everyone under him well taken care of but doesn't want to involve directly, he loves party and fun but afterwards he wants to get back to his duty, he doesn't like talking much, he wants to be physically and mentally strong, loves arts and culture, likes everything in its place, he wants to know his enemies. There are more but you get the point. All these are fulfilled by Aries. But how? Just look at the zodiac belt. From Aries every sign is in its own place, so Surya can perform at his best here."
Surya rules Leo but not even Leo has the capacity to provide everything. That's the beauty of astrology. It's intriguingly artistic.
On the other hand Libra is the Debilitation sign of Surya. Debilitation means no strength, no power and no happiness (hence its also called "fall position"). Libra is directly opposite to Aries in the zodiac belt so its easy to understand the opposing nature. Its an upside down world for Surya (remember he likes everything in its place), here he cant think of himself first, he has to listen to others for every opinion to be made and it'll take a lot of time so he can't be quick too, he is forced to talk a lot here and waste time and energy on random people because Sagittarius is 3rd from Libra so he'll talk about big stuffs like astrology and philosophy to an 8 year old kid, he has a big appetite but he can't eat well here because Scorpio (2nd from Libra) suddenly takes it away (house of death), he's forced to be emotional in the external world (Capricorn in the 4th), he will be confused about his enemies (Pisces in the 6th) and more.
So this is how Libra deprives Sun of its requirements.
Note: I've written this exaltation/debilitation concept after years of observational research and contemplation.
A folklore to understand Surya
God Vishnu, one of the trinities from vedic scriptures took 9 avatars to restore dharma and to bring massive changes on earth everytime an apocalyptic situation occurred. Every avatar is associated with every planet in astrology. Lord Ram, the 7th avatar is associated with Surya. Lord Ram was a dharmic persona according to Ramayana (the epic of Ram) and had the power to change anyone to sattvic guna.
There is one short story about Ram that can give a hint of what Surya really is (keep in mind that this story is not on Ramayana but a folklore from Tamilnadu, South India). Once Ravanasura was looking very sad after abducting Sita Ma (wife of Ram) to Lanka.
The conversation between raavan and his minister follows:
Minister: Oh powerful king ravana what happened to you?
Ravana: I couldn't go near Sita no matter what i tried.
Minister: "Oh my king, no need to worry as you have the power of disguise, you can change yourself as Ram and go to meet her".
Ravana: "Oh dumb minister, i have ten heads so i've tried it already but didn't work as well".
Ravana: Yes, after changing myself as Ram i couldn't think of anyone but my wife Mandodari.
That's the power of Lord Ram. Anyone who ever tried to face him lost themselves to his powerful gaze. They became dharmic and a follower of Lord Ram. The same way our Sun engulfs everything that comes close to it. The engulfed particle gets atomised (purified) and becomes part of the Sun (follower).