Vega from two cities
- Round
- Theory
- Points
- 10
- Level
- Silver
- Taught in
- Section 1.5
Adapted from IOAA 2013 (Volos), theory problem 14. We have restated it in our own words and kept the original data. The solution was last checked on 15 October 2026.
Vega, at declination +38°47′, is crossing the meridian of Lisbon (longitude 0h 36m W, latitude 39°43′ N) at upper culmination. At that moment, what are its hour angle and its zenith distance as seen from Thessaloniki (longitude 1h 32m E, latitude 40°37′ N)?
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Vega's hour angle at Lisbon is 0. A place further east sees each star cross its meridian sooner, so at the same moment its hour angles are larger by the difference in longitude, as for sidereal time in Section 1.3. Thessaloniki is
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With
so
AnswerSeen from Thessaloniki, Vega's hour angle is 2h 08m and its zenith distance is 24°33′.
This problem also appears among the olympiad problems of Section 1.5, The astronomical triangle, which teaches the ideas it uses. Equation and figure numbers refer to the course.