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Simplest way to find our longitude with the help of shadows. (Without GPS and Sextant)

By K S Chadha, Jun 15 2016 07:06AM

Place of observation : NEW DELHI, INDIA


1) At solar noon (merpass) sun sits highest above the horizon for that day.

2) This means the sun casts the shortest shadow at that time.

Steps and Procedures

1. Place a whit sheet on the cardboard. The cardboard needs to be secured and levelled. (See Image)

2. To collect data note and record time, then mark end of the shadow of the nail. (See Image)

3. Prefer to start at 1000 hrs zone time.

4. Take first few recordings with 20-30 minutes apart and then reduce the time period to 5-10 minutes. Finish your observations at 1400 hrs zone time. (See Image)

5. Determine time for the shortest shadow. In this case it is 1220 hrs zone time. (See Image)

6. Now we have to relate our local noon time to time in Greenwich.

7. We know time zone in India is (UTC +5:30), so the solar noon occurred in NEW DELHI, INDIA was at 0650hrs GMT.

8. We know that merpass approximately takes place at 1200hrs LMT. That means earth has to rotate for about 12h – 0650h = 5hrs 10min in order for sun to reach Greenwich.

9. At rotation speed of about 15°/hr, the earth has to rotate for 5h 10min (5.167 hrs) to reach Greenwich.

10. Total rotation by earth in 5.167 hrs = 5.167 * 15 = 77.51°

11. Thus calculated longitude is 077.51° E (E because our local time is ahead of GMT).

12. GPS Longitude = 077.21°E

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