The occultation of a distant star by Triton, Neptune's largest moon, as observed on 5 October 2017 from Constancia, Portugal.
The large object in this view is Neptune, while the fainter source to its upper right is the star occulted by Triton. The star was first seen to dim, as Triton passed in front of it, then underwent the so-called central flash, appearing even brighter than before or after the event, before dimming again and eventually returning to its usual brightness.
The central flash, caused by focussing of the starlight by deep layers in the moon's atmosphere – about 10 km above surface, can be seen half way through the occultation under the right observing conditions.
Taking advantage of improved measurements of the star's position and a few hundred more stars, provided by Gaia, the Lucky Star team has coordinated a campaign to observe this occultation.
A collaboration of professional as well as amateur astronomers observed the phenomenon at more than a hundred stations across Europe, North Africa and North America. The improved prediction of the best locations to observe, based on the specially released, preliminary data from Gaia DR2, led to twenty-five successful detections of the central flash, from Spain and Portugal to the south of France and the north of Italy.
The astronomers are now busy analysing the data collected during this campaign to learn more about the atmosphere of Triton.
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