KJSEA 2025 Maths Q32 — Longitude and Time
Published
The Question
“Town P lies on longitude 50 degrees west and town Q lies on longitude 40 degrees east. The local time at town Q is 4:00 in the afternoon. Determine the local time at town P.”
Find the longitude difference
The two towns sit on opposite sides of the Greenwich meridian, one west and one east. When places are on opposite sides of the zero line, you add their longitudes together to get the angular gap between them. So we add 50 degrees and 40 degrees to find how far apart P and Q are.
If both towns were on the same side (both east or both west), you would subtract instead of add.
Convert the angle to a time difference
The Earth turns a full 360 degrees in 24 hours, which works out to 1 degree of longitude every 4 minutes of time. Multiply the 90 degree gap by 4 minutes to get the time difference between the two towns, then change the minutes into hours.
Decide whether to add or subtract
Places further east see the sun earlier, so their local time is ahead. Town P is to the west of town Q, so P is behind Q in time. That means we subtract the 6 hour difference from the time given at Q.
Final Result
The local time at town P is 10:00 in the morning (10:00 a.m.).
Why this method works
Longitude measures how far around the Earth a place is, and the Earth spins steadily through 360 degrees every 24 hours. Dividing 24 hours by 360 degrees shows that each degree corresponds to exactly 4 minutes, which is why the angular gap can be turned straight into a time gap. Because the sun rises in the east, eastern places reach any given time first, so a western town like P always lags behind an eastern town like Q by that same time gap.
Working backwards, 10:00 a.m. at P plus the 6 hour difference gives 4:00 p.m. at Q, matching the value in the question.