BAAO — 2018 Astronomy & Astrophysics Challenge
Question 8
On the evening of 27th July 2018, there was a total lunar eclipse (where the Moon passes into the Earth's shadow). As viewed from Oxford, totality began whilst the Moon was below the horizon. Interestingly, moonrise was at 8:55pm, yet sunset was only at 9:01pm, so for 6 minutes both the fully eclipsed Moon and setting Sun were visible above the horizon. This very rare event is known as a selenelion. What is the explanation behind this seemingly impossible observation?
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Source
Source: British Astronomy and Astrophysics Olympiad (BAAO), Astronomy & Astrophysics Challenge 2018. Question text, answer-choice text, and answer were transcribed from the official BAAO paper and solutions PDFs; the explanation and metadata are original.