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Why can the antennas of the Very Large Array be moved along Y-shaped tracks?

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Radio telescope Lovell Telescope, a fully steerable radio telescope at Jodrell Bank, Macclesfield, Cheshire, England.

The antennas of the Very Large Array (VLA) are mounted on transporters that move along Y-shaped railroad tracks to allow astronomers to vary the array's resolution.12

By changing the positions of the 27 parabolic dishes along these tracks, the length of the array's arms can be adjusted between 600 metres (2,000 feet) and 21 km (13 miles). This flexibility is essential because the angular resolution of the system depends on the configuration of the antennas and the wavelength being observed. When the dishes are spread out to their maximum extent, the array achieves a resolving power equivalent to a single radio telescope 36 km (22 miles) in diameter.12