If it is 10:00 a.m. at Greenwich Mean Time (GMT), then what will be the time at 90° E longitude?
4:00 p.m.
Calculating Time at 90° E Longitude from GMT
Understanding how to calculate time at different longitudes is a fundamental concept in geography. The Earth rotates 360 degrees in approximately 24 hours. This rotation causes different parts of the world to experience sunrise, noon, and sunset at different times.
We use longitudinal lines to define time zones. The Prime Meridian, located at 0° longitude in Greenwich, London, serves as the reference point for Greenwich Mean Time (GMT).
Relationship Between Longitude and Time
Here's the basic relationship between longitude and time:
- The Earth rotates 360 degrees in 24 hours.
- This means in 1 hour, the Earth rotates \( \frac{360}{24} = 15 \) degrees.
- Conversely, it takes 4 minutes for the Earth to rotate 1 degree of longitude (\( \frac{60 \text{ minutes}}{15 \text{ degrees}} = 4 \) minutes/degree).
- Longitudes to the East of the Prime Meridian (GMT) are ahead in time because they face the sun earlier.
- Longitudes to the West of the Prime Meridian (GMT) are behind in time.
Step-by-Step Time Calculation for 90° E
We are given that the time at Greenwich Mean Time (GMT) (0° longitude) is 10:00 a.m.
We need to find the time at 90° E longitude.
Step 1: Find the difference in longitude.
The difference in longitude is the absolute difference between the target longitude and the GMT longitude (0°).
\( \text{Longitude Difference} = |90^\circ \text{ E} - 0^\circ| = 90^\circ \)
Step 2: Calculate the time difference.
Since 15 degrees of longitude correspond to a 1-hour time difference, we can calculate the time difference for 90 degrees:
\( \text{Time Difference} = \frac{\text{Longitude Difference}}{15^\circ/\text{hour}} \)
\( \text{Time Difference} = \frac{90^\circ}{15^\circ/\text{hour}} = 6 \text{ hours} \)
Step 3: Determine if the time is ahead or behind.
The location is at 90° E longitude. Longitudes to the East of GMT are ahead in time.
So, the time at 90° E will be 6 hours ahead of GMT.
Step 4: Calculate the time at 90° E.
GMT time is 10:00 a.m.
Add the time difference of 6 hours:
\( 10:00 \text{ a.m.} + 6 \text{ hours} \)
Adding 6 hours to 10:00 a.m. gives us 4:00 p.m.
Therefore, the time at 90° E longitude when it is 10:00 a.m. at GMT is 4:00 p.m.
Summary of Calculation
| Reference Longitude | Time |
| 0° (GMT) | 10:00 a.m. |
| Target Longitude | 90° E |
| Longitude Difference | 90° |
| Time Difference (90° / 15°/hr) | 6 hours |
| Direction | East (Ahead) |
| Time at 90° E | 10:00 a.m. + 6 hours = 4:00 p.m. |
Revision Table: Time Zones and Longitude
| Concept | Key Point |
| Prime Meridian | 0° Longitude, reference for GMT. |
| Earth's Rotation | 360° in 24 hours. |
| Longitudinal Time Difference | 15° longitude = 1 hour time difference. |
| East Longitude | Ahead of GMT. |
| West Longitude | Behind GMT. |
Additional Information: International Date Line
While calculating time zones, it's also useful to know about the International Date Line. This is an imaginary line, mostly following the 180° longitude, that marks where one calendar day ends and the next begins. When you cross the International Date Line moving eastward, you subtract a day. When you cross it moving westward, you add a day. This line works in conjunction with the time zones defined by longitude relative to the Prime Meridian.
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