A thing to remember here is that these posts are strictly meant for the particular planet position. Like, today we are looking at Ketu in Sagittarius, so the description given here may change for you if your Ketu is although in the sign of Sagittarius but in conjunction with or aspected by other planets. -

Just like Rahu, Ketu also represents illusion as both are illusionary planets but the difference is that Ketu is also the one who removes the illusion and brings enlightenment. Besides this, Ketu represents isolation, separation, spirituality,mystical world, other dimension means things related to alien world, also represents Son, Dog, Intuition, research ability, curious nature, things which we have already achieved in past life and not much concerned to perform in this life.

Sagittarius is the 9th sign of zodiac belt, hence it represents many things related with the 9th house of horoscope like Religion, Knowledge, Wisdom, Higher Learnings, Gurus, Teachings from your Father etc. Besides it also represents Coaching, Universities, Professors and Preachers, Higher Wisdom, Written Law of the Land etc. Sagittarius has next set of 2 and half nakshatra which are Mula, Purva-Ashadha and Uttara-Ashadha. Sagittarius' lord is Jupiter.

Recently I have come across of the view that Rahu and Ketu don't have aspects on 5th or 9th house, for the simple reason that they are shadow planets and for aspects you need a sight not shadow. I am yet to verify it on at least 50 charts, so for now I will continue with traditional approach of 5th and 9th aspects of Ketu.

This one I would consider as one of the best planetary positions to have in horoscope, only if these people have an open mind to listen to others. Basically, when Ketu comes into Sagittarius, these people feel isolated from the religious beliefs and criticize them. They won't be fearful of even criticizing others' beliefs. They would find it hard to believe in holy books as they would say that it is still written by an individual and published by a publishing house. How am I supposed to believe that these are word of God? In even better cases, they will reject the idea of God. That's what Ketu does here, it isolates the person from religious beliefs and higher philosophies of the world.

I am fine with this if these people are ready to embrace the habit of research then. If you have so many questions, then responsibility of finding answers is also upon you. You can't just raise some questions and run away. You have to research about it. If someone is trying to reply you, you have to listen his logic, that's how you are going to find the answers. Normally, I found that people keep these doubts to run away rather than to find the answers. Like, being in this profession now, I daily get 1 or 2 people who say that they don't believe in Astrology. Excellent, Great!!!! But my query to them is how much you experienced Astrology? How much time you dedicated or are willing to dedicate to understand how Astrology works? If I send you 50 Articles on Astrology along with charts of people and records of how their life has moved with Dashas, then how much time you are willing to dedicate in understanding those articles and charts. That's why I said this is the best planetary position, if person is willing to research and have an open mind to listen to the views of others which may be helpful in finding the answers. But if questions are just ways to escape from truth then it can be equally bad position.

Again, impact of Ketu's effect in Sagittarius will be controlled by the various nakshatra Ketu might be in. Like, if Ketu is in Moola nakshatra, then it is more powerful as it is its own nakshatra. If Ketu is in Purva-Phalguni nakshatra, then Ketu is in nakshatra of Venus and we need to check Venus position. If Ketu is in Uttara-Phalguni nakshatra, then its lord Sun will impact Ketu.

Sign lord wise, Ketu is in Jupiter's sign, so as I said Jupiter will have the highest say on working of Ketu.