Lesson Plan Longitude calculation

Longitude and Time

By Tempa Tshering

Lesson Objectives:​
By the end of the lesson, students will be able to:
1. Define the relationship between longitude and time.
2. Calculate local time using longitude.
3. Calculate longitude using time differences.
4. Differentiate between local time, standard time, and GMT/UTC.
5. Apply the formula: 15° = 1 hour or 1° = 4 minutes

​Introduction (5 minutes)

​Begin with a question: "If it’s 12:00 noon in Bhutan, what time is it in London?"
Recap: Earth rotates 360° in 24 hours, so 15° = 1 hour.
Link the rotation of the Earth to the measurement of time.

​Development (25 minutes)

​A. Explaining the Concept
Every 15° of longitude = 1 hour of time difference Earth rotates from west to east:
Places east of Greenwich are ahead of GMT
Places west of Greenwich are behind GMT

Formula for Calculation
Time = Longitude ÷ 15
Use when calculating time difference from longitude
Longitude = Time Difference × 15
Use when longitude is unknown, but time is given

​Worked Examples
Example 1: If it is 12:00 noon at the Prime Meridian (0°), what will be the time at 75°E?
→ 75 ÷ 15 = 5 hours
→ 12:00 + 5 hours = 5:00 PM

Example 2: A place has local time 3 hours behind GMT. What is its longitude?
→ 3 × 15 = 45°
→ Since it’s behind, 45°W

​Class Activity (5 minutes)
Distribute worksheets with simple questions like:
1. Calculate the local time at 90°E when GMT is 6:00 AM.
2. A place is 2 hours ahead of GMT. What is its longitude?
3. Bhutan is at 90°E. What is its standard time if GMT is 4:00 PM?

Discussion (5 minutes)
Ask: “Why is standard time necessary?”
“Why does Bhutan follow the time of 90°E though not all places lie exactly on that longitude?”

​Conclusion (3 minutes)
Recap key points:
15° = 1 hour,
1° = 4 minutes Time is calculated eastward = gain time, westward = lose time Reiterate formulas for both time and longitude

Homework
1. Calculate the longitude of a place where the local time is 2 hours 30 minutes behind IST.
2. If it is 10:00 AM in New York (75°W), what is the time in Tokyo (135°E)?