I will try to outline how I learned it:
We se hara as bigger region ~ between pelvis and diaphragm: The belly. The tanden is a certain part of it which is located inside the hara.
The hara is more or less directly connected to the hips, the pelvis and chest. So they move "together": Hara, koshi, hips, pelvsis is one unit so to speak. You can move the hara by moving the hips, koshi, pelvis and maybe the legs. The tanden is freely moving inside the hara. So it can rotate. And it's movement is independent from pelvis, hips, koshi.
Little bit Like a coconat swimming in a fishbowl …
In our practice the outer movements of the body relate to the hara: Direction of the hips, stance, legs, moving forward, … The hara relates to the movements you can see.
The "inner movements" like guiding the kimochi, sending "strength" or "impulse" through the body without outer movement relate to the tanden.
Did this help (even if you don't agree with this distinction)?
I think most people are speaking of hara and tanden as synonym. That's why I asked for it.
thank you for your explanation, I understand the distinctions you make, even though they are a bit different from my own understanding/practice.
So much of our differences must be influenced by the terms our teachers use, when we first discover aikido, and their ability to get the concepts across and to their own ability to demonstrate exactly what they are talking about.
As my teacher studied with Tohei (amongst others), we have used the term 'one point' as reference to the centre. I'm not sure that this is the most helpful term for what it describes. For many years for me, it was a vauge area in the centre of the body a bit below the navel. This is what I learned, but my experience of it now is a bit more sophisticated. I used to think that with a connected body, the feet/legs/hips move the centre, which in turn, moves the hands (no shoulder/arm strength of course). For me now, the intent moves the centre, which in turn, moves the hands and feet to the place they need to be, as they can't be anywhere else.
I like your analogy of the coconut, as I agree that the tanden (dantien?) can rotate independently of the body. For me this is where the mental/energetic side of the practice really starts to get interesting. If I place my hands on the (mental) external surface of the dantien (I choose to experience my dantien as large enough to be outside of my hips/pelvis etc -), I can rest them there, without effort, if I choose to mentally expand, or contract my dantien, then my hands will naturally take up a new place in space, without any physical effort. If I mentally rotate my dantien, then my hands follow the circumference of the circle. Much fun can be had with this.
I find that the less I am interested in what my uke is trying to do to my body, and the more I am focussed on being completely whole and in myself and my own centre, the more effect I have on the uke, for less effort.
So I see the distinctions you are using and maybe we would both agree the subjective feeling in paired practice, just some differences in terminology.
I am finding all of my interest in aikido now, is related to the mind/intent and to the centre (whatever name we give it) and how we move it, how it moves, how it affects others, how we move their mind and centre. The body allows us to access this amazing playground, but only when we learn to use it in the right way - relaxation, posture, breathing, no shoulders etc. Although the various waza we do are taught as a body of techniques and exercises, they are nothing without the being driven by the centre. I think you'll agree?
p.s. sorry about the thread drift guys