Surveying the earth Originals Instruments Commentary

Jean Picard geodesy

Surveying the earth

• Eratosthenes figured
• the size of the earth
• by comparing the lengths of shadows
• and knowing the distance between
• Aswan and Alexandria in stadia.
• Eratosthenes could have been off
• by as much as sixteen
• or as little as two percent,
• depending on how long
• you think his stadia were.
• Jean Picard improved
• surveying instruments,
• and surveyed the distance
• of one degree of latitude
• along the Paris Meridian.
• The polar radius
• that Picard calculated
• was under a half of a percent less
• than the currently accepted value.

Originals

• Picard wasn’t the first or the last to measure a meridian arc.
• Francesco Maurolico described this method in 1543,
• and it is thought to have been suggested by Eratosthenes.
• Gemma Frisius invented triangulation in 1533.
• Willebrord Snellius (known as Snell) developed the math
• for measuring by triangulation from city to city in 1617,
• which he did in an attempt to follow Maurolico’s method,
• although his result not as accurate as Picard’s.
• William Gascoigne had invented the micrometer
• and the telescopic sight with crosshairs in 1638.
• All Jean Picard did was put it all together.
• Picard measured his baseline with wooden rods,
• attached Gascoigne’s telescope and micrometer to a quadrant
• to measure his angles, used Snell’s math
• with John Napier’s logarithms in his calculations
• and achieved enough accuracy to be considered
• the first modern measurement of the polar radius.

Instruments

• If Archimedes had a lever long enough
• and a fulcrum on which to place it
• then he could have moved the world.
• If I had a reason strong enough,
• a judgment clear enough,
• a meaning near enough
• then I could help people
• put aside their differences
• and feel compassion for each other.
• We live on a small fragile world,
• in which greed has measurable harm
• and generosity measurable benefit.