Speed, Distance and Time
Primary 6 · Mathematics
Speed, Distance and Time
Lesson 2 of 5
15 min
Let's begin
At 12:00 noon in Kigali, Mugisha phones his aunt in Nairobi. She answers, "Good afternoon! It is already one o'clock here." Mugisha is puzzled. How can it be two different times at the same moment?
The Earth turns round once every 24 hours. While one side faces the Sun and has day, the other side has night. So when the Sun is highest over Kigali, it has already passed its highest point in places to the east, and it has not yet reached it in places to the west.
To keep clocks sensible, the world is divided into time zones. A full turn of the Earth is 360°, and 360° ÷ 24 hours = 15° of longitude for each hour. So each time zone is roughly 15° wide.
Every time zone is described by how far it is ahead of or behind UTC (Coordinated Universal Time), the time on the 0° line of longitude that passes through Greenwich in London.
| Place | Time zone |
|---|---|
| Accra (Ghana) | UTC+0 |
| Kinshasa (DR Congo) | UTC+1 |
| Kigali, Bujumbura, Goma | UTC+2 |
| Nairobi, Kampala, Dar es Salaam | UTC+3 |
| Dubai (United Arab Emirates) | UTC+4 |
| Beijing (China) | UTC+8 |
| Tokyo (Japan) | UTC+9 |
Rwanda uses Central Africa Time, UTC+2. Nairobi is UTC+3, one hour ahead of Kigali. That is why Mugisha's aunt was one hour ahead.
Step 1 find the difference between the two time zones.
Step 2: add it if the other place is further east (bigger number), subtract it if it is further west (smaller number).
Worked example 1
It is 10:00 in Kigali (UTC+2). What time is it in Dubai (UTC+4)?
Difference = 4 − 2 = 2 hours, and Dubai is ahead. 10:00 + 2 h = 12:00.
Worked example 2
It is 09:00 in Kigali. What time is it in Accra (UTC+0)?
Difference = 2 hours, and Accra is behind. 09:00 − 2 h = 07:00.
Worked example 3
(crossing midnight). It is 20:00 in Kigali. What time is it in Tokyo (UTC+9)?
Difference = 9 − 2 = 7 hours ahead. 20:00 + 7 h = 27:00, and 27 − 24 = 3, so it is 03:00 the next day in Tokyo.
Worked example 4
(a flight). A plane leaves Kigali at 08:00 Kigali time and flies for 1 h 30 min to Nairobi. What is the time in Nairobi when it lands?
Landing time in Kigali time: 08:00 + 1 h 30 min = 09:30.
Nairobi is 1 hour ahead: 09:30 + 1 h = 10:30 Nairobi time.
Using longitude. Two towns are 45° of longitude apart. 45 ÷ 15 = 3, so their clocks differ by 3 hours.
Tip
East is ahead, so add. West is behind, so subtract. The Sun rises in the east, so the east sees each new morning first.
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Quick check
It is 11:00 in Kigali (UTC+2). What time is it in Dubai (UTC+4)?
