Question: How was Stonehenge arranged at the center of the inner ring?
About the article "Stonehenge", section "Stonehenge 3 IV (2280 BC to 1930 BC)".

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 3 IV (2280 BC to 1930 BC).
This phase saw further rearrangement of the bluestones.
They were arranged in a circle between the two rings of sarsens and in an oval at the centre of the inner ring.
Some archaeologists argue that some of these bluestones were from a second group brought from Wales.
All the stones formed well-spaced uprights without any of the linking lintels inferred in Stonehenge 3 III.
The Altar Stone may have been moved within the oval at this time and re-erected vertically.
Although this would seem the most impressive phase of work, Stonehenge 3 IV was rather shabbily built compared to its immediate predecessors, as the newly re-installed bluestones were not well-founded and began to fall over.
However, only minor changes were made after this phase.
Stonehenge Aotearoa, was built by members of The Phoenix Astronomy Society and is located near Carterton, New Zealand is a modern adaptation of the Stonehenge ruins on the Salisbury Plain of England.
The henge contains 24 pillars and is in diameter and about high.
The pillars are capped with lintels, completing the circle, and a -tall obelisk marks the center of the henge.
The modern henge was designed to demonstrate how ancient peoples used such constructions to understand astronomy.
Since Stonehenge Aotearoa is at a different latitude and longitude than the original Stonehenge, it is not an exact replica—some measurements had to be changed to preserve certain astronomical properties.
Its design was inspired not only by the original Stonehenge, but also by navigational and astronomical techniques of Polynesia and of the New World.
Stonehenge Aotearoa also differs from its Salisbury cousin in construction; the pillars, lintels and central obelisk are not hewn stone, but are hollow structures with concrete molding forming their exterior.
It was intended to generate revenue for The Phoenix Astronomy Society to raise money for the Matariki Research Observatory. 
Stonehenge Avenue is an ancient avenue marked as a UNESCO World Heritage Site.
Discovered in the 18th century, it measures nearly 3 kilometers, connecting Stonehenge with the River Avon.
It was built during the Stonehenge 3 period of 2600 to 1700 BCE.
The avenue is aligned with the sunrise of the summer solstice, suggesting a time of most frequent use.
At the end of the Avenue, a similar ring of bluestones, Bluestonehenge, was discovered in 2009.
Mike Parker Pearson of the Stonehenge Riverside Project believes that the Avenue was inspired by, and built over top of a natural glacial formation with an existing astronomical alignment. 
The new henge discovered at Stonehenge appears to be a Neolithic henge and timber circle 900m from Stonehenge itself.
It was discovered in July 2010, two weeks into a project looking at the archaeology around the Stonehenge World Heritage Site in Wiltshire, United Kingdom.
The project, scheduled to run until 2013, is looking at how the surrounding landscape would have looked when Stonehenge was first built. 
Throughout the 20th century, Stonehenge began to be revived as a place of religious significance, this time by adherents of Neopagan and New Age beliefs, particularly the Neo-druids.
The historian Ronald Hutton would later remark that "it was a great, and potentially uncomfortable, irony that modern Druids had arrived at Stonehenge just as archaeologists were evicting the ancient Druids from it."
The first such Neo-druidic group to make use of the megalithic monument was the Ancient Order of Druids, who performed a mass initiation ceremony there in August 1905, in which they admitted 259 new members into their organisation.
This assembly was largely ridiculed in the press, who mocked the fact that the Neo-druids were dressed up in costumes consisting of white robes and fake beards.
Between 1972 and 1984, Stonehenge was the site of the Stonehenge Free Festival.
After the Battle of the Beanfield in 1985, this use of the site was stopped for several years and ritual use of Stonehenge is now heavily restricted.
Some Druids have arranged an assembling of monuments styled on Stonehenge in other parts of the world.