Eratosthenes' Measurement Cities Explained
The question asks about the specific locations used by the ancient Greek scholar Eratosthenes to calculate the Earth's circumference. He cleverly used the difference in the angle of the Sun's rays at two different locations on the same day.
Understanding Eratosthenes' Method
Eratosthenes' measurement technique was based on a simple observation and geometric principles:
- He knew that on the summer solstice, at noon in Syene (modern Aswan, Egypt), the Sun was directly overhead. This meant that sunlight shone straight down into deep wells without casting any shadows.
- At the same time (noon on the summer solstice), in Alexandria, which was located directly north of Syene, the Sun was not directly overhead.
- Eratosthenes observed that vertical objects in Alexandria cast a shadow at this time.
- He measured the angle of this shadow in Alexandria. Using geometry, he deduced this angle represented the difference in latitude between the two cities relative to the Earth's center.
- Knowing the approximate distance between Alexandria and Syene, he could then scale this angle up to estimate the full circumference of the Earth.
The Key Cities Used
The two cities Eratosthenes used for his groundbreaking measurement were:
- Syene (modern Aswan)
- Alexandria
These two cities were chosen because they lay roughly on the same line of longitude, and Eratosthenes had information about the Sun's position relative to the vertical in both locations simultaneously.
Why Alexandria and Syene?
The choice of Alexandria and Syene was critical:
- Syene's location near the Tropic of Cancer meant the Sun could be directly overhead at noon on the summer solstice.
- Alexandria's position to the north allowed for a measurable shadow at the same time, providing the necessary angle difference.
This comparison allowed Eratosthenes to calculate the angle difference, which was approximately 7.2 degrees, or 1/50th of a full circle (360 degrees). By multiplying the distance between the cities by 50, he arrived at his estimate for the Earth's circumference.
Similar Questions
- Consider the following statements about the causes of ground surface inversion of temperature :
1. Ground surface inversion is also termed as radiation inversion.
2. The existence of negative lapse rate results in surface inversion.
Which of the statements given above is/are correct ? - Consider the following pairs with reference to world climate types and their characteristics :
(Climate) (Characteristic) I. Tropical wet
and dry: Winter dry season II. Mid-latitude
steppe: Mid-latitude semi-arid
or dryIII. Humid
sub-tropical: No dry season, warm
summerIV. Marine west
coast: No dry season, severely
cold summer
How many of the pairs given above is/are correctly matched ? - The pattern of planetary winds largely depends upon which of the following factors ?
1. Latitudinal variation of atmospheric heating
2. The distribution of continents and oceans
3. The rotation of earth
Which of the above is/are correct? - Identify the climate type based on the given below characteristics :
1. The annual precipitation is low and is around 15 cm.
2. The development of pronounced winter anticyclones and their weakening in summer sets in monsoon-like reversal of wind in this region.
3. Polewards, summer temperatures are low.
Select the answer from the options given below: - Consider the following statements regarding the rivers of India :
1. The Barak River, the head stream of the Meghna, rises in the hills of Meghalaya.
2. The Ghaggar River does not drain out to the sea and gets lost in the Rajasthan desert.
3. Puthimari and Manas are important tributaries of the Brahmaputra.
Which of the statements given above is/are correct? - Consider the following statements :
1. Ocean currents are dominated by huge surface gyres that are driven by the global surface wind pattern.
2. Equatorial currents move cold water westward and then poleward along the east coasts of continents.
With regard to the statements given above, which of the following is correct?
Important Questions from Climatology
In which of the following atmospheric layers ozone gas is concentrated?
Ozone layer depletion is a major phenomenon in :
By which layer of the atmosphere the ultra-violet rays are absorbed ?
______ is the uppermost layer of the atmosphere.
Which of the following is correct about insolation?
I. It influences the distribution of temperature.
II. It increases from the equator towards the poles.
