A forecast becomes apparently infallible after an observatory starts closing the routes its workers were not expected to use.
The Certain Horizon's early forecasts included several possible outcomes. Its operators used them to plan mirror cleaning, water deliveries, equipment repairs, and work around the dust weather. A forecast could be useful without being right about every person or every hour.
One surviving set of schedules follows workers travelling to the outer instruments. They had an exposed upper walk and a covered lower route. The upper walk was shorter. The lower route took longer but offered shelter when dust crossed the array.
The engine expected most crews to use the lower route during an afternoon dust front. Some workers still chose the upper walk. They were carrying light loads, had finished early, or judged that they could cross before the weather arrived. The original forecast allowed for those choices.
The superintendent wanted a single arrival schedule. Stores had to be opened, shutters moved, and receiving crews taken away from other work. Preparing for several possibilities required more staff. The superintendent selected the lower-route forecast and ordered the upper walk closed.
Every crew then arrived by the expected route. The result was entered as a successful prediction. The closure was filed with maintenance work rather than with the observations used to judge the forecast.
The following estimates became more confident. When conditions favoured the upper walk, a shutter fault still kept it closed. When a crew requested an exception, the request was refused because it conflicted with the approved schedule. A forecast originally made to help arrange the work became the reason other arrangements could not be considered.
The same practice spread beyond that route. Deliveries were held until their predicted arrival time. Work crews were moved to match expected occupancy. An unplanned journey was treated as a fault to be corrected before it affected the results. The engine received an increasingly orderly account of a place being forced to obey its estimates.
That apparatus still operates inside the Observatory of One Outcome. Outcome Surveyors track movement. Shutters and barriers close alternatives. The central engine displays the route it expects a party to take, then uses the observatory's machinery against movement outside it.
Its oldest tables show that the upper walk was sometimes faster and sometimes safer. Those possibilities have not become impossible through better knowledge of the future. A shutter stands across the entrance, and the control system will not authorize it to open.
Elsewhere in Nhal-Serat, crews still use forecasts with alternatives, conditions, and room for correction. The Certain Horizon's records must be read with its closures beside them. Without those, a reader sees an engine that correctly predicted every arrival and never learns that the workers had been denied another route.
