History of Units of Measurement and Solar System Distances

Vocabulary flashcards covering historical measurement standards, modern redefinitions of the meter, and methodologies for determining distances within the solar system.

Inch (Original Definition)

The distance between knuckles on the finger.

Yard (Original Definition)

The span from the extended index finger to the nose of the British king.

Metric System

A system of measurement officially adopted in France in 1799 during the era of Napoleon, originally basing its unit of length on the dimensions of Earth.

Meter (1799 Definition)

Originally defined as one ten-millionth of the distance along Earth’s surface from the equator to the pole.

Standard Meter Bar (1889)

An intermediate physical standard consisting of a platinum-iridium metal bar set up in Paris, defined by international agreement in 1889 to be exactly one meter in length.

Meter (1960 Redefinition)

A redefinition of the meter equal to $1,650,763.73$ wavelengths of a particular atomic transition in krypton-86.

Meter (1983 Redefinition)

The distance traveled by light in a vacuum in $1/299,792,458\,\text{second}$.

Light-second

The distance light covers in one second, defined to be $299,792,458\,\text{meters}$.

Relative Distances (Solar System)

The ratios comparing how far from the Sun one planet is relative to another, established by Copernicus and Kepler without giving absolute measurements.

Absolute Distances (Solar System)

Direct physical distance measurements expressed in standard units of length such as meters, light-seconds, or kilometers.

Transit of Venus

An astronomical event where Venus crosses the face of the Sun; historically observed in 1761, 1769, 2004, and 2012 to help calculate absolute distances in the solar system.

Radar Distance Measurement

A technique using radio pulses bounced off solid objects to calculate exact distance based on roundtrip travel time at the speed of light.

Mars Antenna

A $70\,\text{meter}$-wide dish antenna in California's Mojave Desert, part of NASA's Deep Space Network, used as a radar telescope to transmit and receive radar signals to solar system bodies.

Astronomical Unit (AU)

The average distance from Earth to the Sun, defined as $149,597,870,700\,\text{meters}$, $499.004854\,\text{light-seconds}$, or approximately $8.3\,\text{light-minutes}$.