The astronomical unit started as a guess. Aristarchus of Samos tried in the 3rd century BC and got a number 20 times too small. Kepler tried. Cassini tried — he got within 7%, using parallax from Paris to Cayenne during a Mars opposition. In 1672, Jean Richer sailed to French Guiana with a telescope. Giovanni Cassini stayed in Paris with another. They measured Mars against the background stars at the same moment. The difference gave them the distance to Mars in Earth radii. From that, using Kepler's third law, they worked out the AU. They were within 7% of the true value. Not bad for the 17th century.
For three centuries after Cassini, astronomers refined the AU with each new transit of Venus. The 1761 and 1769 transits sent expeditions to Siberia, Tahiti, Hudson Bay, and the Indian Ocean. Captain Cook's first voyage was a Venus transit mission. His secret orders — opened only after the transit — told him to find Terra Australis. The transit data, combined across a hundred observing stations, narrowed the AU to within half a percent. By the 20th century, radar ranging to Venus and laser ranging to the Moon had pushed the precision past the point where anyone could question it.
Then came the problem. The AU was defined as the semi-major axis of a massless particle orbiting the Sun with a Gaussian year of exactly 365.2568983 days. That definition embeds the mass of the Sun. The Sun loses about 4 million tons of mass per second to fusion — negligible for practical purposes, but not for a unit definition. Worse, general relativity means the AU depends on the coordinate system. In 2012, the IAU gave up on the orbital definition and said: the AU is now exactly 149,597,870,700 meters. The meter is defined by the speed of light. The speed of light is exact. The AU is now frozen. Multiply with confidence.
Multiply AU by 149,597,870.7 to get kilometers. Jupiter orbits at 5.2 AU — 778 million km. Neptune at 30 AU — 4.5 billion km. The Voyager 1 spacecraft, launched in 1977, is now at about 163 AU and still transmitting. At that distance, the Sun is just another bright star, and the round-trip signal delay is 45 hours.
Solar System in AU and km
| Body | Distance (AU) | Distance (km) | |
|---|---|---|---|
| Mercury | 0.39 | 57,909,000 | Closest planet to the Sun |
| Venus | 0.72 | 108,208,000 | Earth's nearest planetary neighbor |
| Earth | 1.00 | 149,597,871 | The yardstick itself |
| Mars | 1.52 | 227,939,000 | At opposition, 0.52 AU from Earth |
| Jupiter | 5.20 | 778,570,000 | Galilean moons orbit at ~0.003 AU |
| Saturn | 9.58 | 1,433,530,000 | Rings span ~0.0015 AU across |
| Uranus | 19.2 | 2,872,460,000 | Discovered 1781, doubled known Solar System |
| Neptune | 30.0 | 4,495,060,000 | Predicted by math before observation |
| Pluto | 39.5 | 5,906,380,000 | New Horizons: 9.5 years, 4.8 billion km |
| Voyager 1 | ~163 | ~24,400,000,000 | Most distant human-made object |
Frequently Asked Questions
Why 149,597,870.7 km and not a round number?
Because the 2012 IAU redefinition preserved the old measured value as closely as possible. The goal wasn't a round number — it was continuity. Every paper, textbook, and planetary ephemeris written before 2012 used approximately this value. Changing it by more than a meter would have invalidated decades of published work. So the IAU set it to the value that best matched the existing best estimate: 149,597,870,700 meters, exactly.
How long does it take light to travel 1 AU?
499.0 seconds, or 8 minutes and 19 seconds. Light travels at exactly 299,792,458 m/s, and 1 AU is 149,597,870,700 meters. Divide: 149,597,870,700 ÷ 299,792,458 = 499.0 seconds. When NASA talks to a rover on Mars, the one-way light time varies from 4 minutes to 24 minutes depending on where the planets are. Even at closest approach, every command takes 4 minutes to arrive. Driving a Mars rover is a 4-minute-lag video game.
Do astronomers actually use AU or do they use kilometers?
AU for distances inside the Solar System. Kilometers for spacecraft navigation. Parsecs and light-years for everything beyond. The AU is practical because orbital mechanics runs on it: Kepler's third law relates orbital period to semi-major axis in AU. If you work in kilometers, the proportionality constant gets messy. If you work in AU, the relationship is clean. Most planetary ephemeris files — the tables that predict planetary positions — are computed in AU and converted to kilometers only at the final step.
Engineering Context
Interplanetary mission design lives in AU-to-km conversions. A trajectory is computed in dimensionless AU coordinates, then mapped to kilometers for the propulsion team, who need thrust durations and delta-v budgets in metric units. When the Mars Science Laboratory approached Mars in 2012, its entry corridor was 10 km wide — an error of 0.00000007 AU. At 5.8 km/s, crossing that corridor took 0.002 seconds. The conversion from AU to km for targeting was checked by two independent teams. If you get it wrong, you fly past the planet and the mission is over. Length Conversion Guide.
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