The sky above Chiayi
- Round
- Theory
- Points
- 6
- Level
- Bronze
- Taught in
- Section 2.5
Adapted from the 2020 Global e-Competition on Astronomy and Astrophysics (Estonia, online), held in place of the IOAA that year, theory problem 7, part (a). We have restated it in our own words and kept the original data. The solution was last checked on 17 October 2026.
An observer in Chiayi, Chinese Taipei (23.5° N, 120.4° E, UT+8) watched meteors at 21:00 local time on 25 September 2020. Find the local sidereal time of the observation. The Greenwich sidereal time at 0h UT on 1 January 2020 was 6h 40m 30s.
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21:00 at UT+8 is 13:00 UT. From 1 January to 25 September 2020 is
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By (2.11), in 268 days GMST grows by
which is 13h 19m 15s.
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The longitude 120.4° E is 8h 01m 36s, so LST = 21h 20m 51s.
AnswerThe local sidereal time was 21h 20m 51s. The official solution rounds the 268 days of growth down to 17h 36m 36s and gets 21h 20m 50s.
This problem also appears among the olympiad problems of Section 2.5, Sidereal time on any date, which teaches the ideas it uses. Equation and figure numbers refer to the course.