From Lima to Yogyakarta
- Round
- Theory
- Points
- Not given
- Level
- Silver
- Taught in
- Section 1.5
Adapted from IOAA 2015 (Magelang), theory problem 15. We have restated it in our own words and kept the original data. The solution was last checked on 15 October 2026.
An aircraft flies from Lima in Peru (12°02′ S, 77°01′ W) to Yogyakarta in Indonesia (7°47′ S, 110°26′ E) along the shortest route over Earth's surface. Find the latitude of the most southerly point of the route.
-
In the triangle formed by the South Pole S, Lima L and Yogyakarta Y, Lima is
from S and Yogyakarta is . Going west from Lima, the difference in longitude is . This is less than 180°, so the shortest route heads west across the Pacific, and the angle at S is 172.55°. -
By (1.16),
so the route is
long, about 17 700 km. -
The cosine rule for the side SY gives the angle at Lima,
which is
. The angle at Yogyakarta, found the same way, is 20.57°. Both angles are less than 90°, so the point V of the route nearest to the South Pole lies between the two cities. -
At V the route crosses the meridian SV at right angles, so the triangle SLV has a right angle at V and the side SL opposite it. By (1.18),
, which is , so . -
V is 20.37° from the South Pole, at latitude
.
AnswerThe route reaches latitude 69°38′ S, far south of both cities.
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.