Astronomical Units to Millimeters (au to mm)

1 astronomical unit equals 149,597,870,700,000 millimeters exactly.

1 astronomical unit = 149,597,870,700,000 millimeters

How to convert astronomical units to millimeters

To convert astronomical units to millimeters, multiply the number of astronomical units by 149,597,870,700,000. This factor is exact. To go the other way, multiply millimeters by 6.684587122×10⁻¹⁵ (rounded).

millimeters = astronomical units × 149,597,870,700,000

Example: 10 astronomical units = 1.495978707×10¹⁵ millimeters.

About these units

Astronomical Unit (au)

AstronomicalCurrent unit

Exactly 149,597,870,700 meters, roughly the average distance from the Earth to the Sun. Used for distances within the solar system.

Astronomers began calling it the astronomical unit in the mid-nineteenth century, with the name first recorded in 1848, though the quantity itself had been chased since antiquity. Aristarchus put the Sun 18 to 20 times farther away than the Moon, when the true ratio is about 389. Cassini and Richer used the parallax of Mars in 1672, transits of Venus served for generations, and radar echoes from Venus and Mars in the early 1960s finally gave direct distances.

For most of the twentieth century the unit was tied to the Gaussian gravitational constant, which meant its value had to be remeasured as observations improved. In 2012 the IAU removed that dependence by fixing it at exactly 149,597,870,700 meters and recommending the symbol au. The figure used here is that exact value. It is close to the mean Earth-Sun separation, which varies from about 0.983 au at perihelion to about 1.017 au at aphelion. Light needs roughly 499 seconds to cross one au, and Neptune orbits at about 30 au.

1 astronomical unit = 149,597,870,700 meters (exactly).

Sources: Wikipedia: Astronomical unit; NIST SP 811 Appendix B.8: factors for units outside the SI; Wikipedia: Light-second

Millimeter (mm)

Metric (SI)Current unit

One thousandth of a meter. Used in engineering, manufacturing and rainfall measurement.

Carl Friedrich Gauss put the millimeter at the base of his 1832 proposal for absolute units, pairing it with the milligram and the second, although the centimeter-based CGS system later won out among scientists. The name combines the metric prefix milli-, from the Latin for one thousand, with the meter. Since 1983 a millimeter has been tied to the speed of light: it is the distance light covers in 1/299,792,458,000 of a second.

Millimeters are the language of fine mechanical work. Standard rulers are divided into them, digital calipers commonly read to 0.01 mm, and mechanical pencil leads come in 0.5 and 0.7 mm. Copy paper is about 0.1 mm thick, and a thin hair, at 0.02 to 0.04 mm across, is close to the smallest thing the naked eye can see. A wave with a frequency of 300 GHz has a wavelength of 1 mm. An inch is exactly 25.4 mm. One trap for readers in the United States is that mil there traditionally means a thousandth of an inch, not a millimeter.

1 millimeter = 0.001 meters (exactly).

Sources: Wikipedia: Millimetre; Wikipedia: Centimetre-gram-second system of units; NIST Guide to the SI, Appendix B.8: conversion factors

Related conversions

See all length units.