Light-hours to Millimeters (lh to mm)

1 light-hour equals 1.079252849×10¹⁵ millimeters (rounded).

1 light-hour = 1.079252849×10¹⁵ millimeters

How to convert light-hours to millimeters

To convert light-hours to millimeters, multiply the number of light-hours by 1.079252849×10¹⁵. The factor is rounded to 10 significant digits. To go the other way, multiply millimeters by 9.265669311×10⁻¹⁶ (rounded).

millimeters = light-hours × 1.079252849×10¹⁵

Example: 10 light-hours = 1.079252849×10¹⁶ millimeters.

About these units

Light-hour (lh)

AstronomicalCurrent unit

The distance light travels in an hour (about 1.08 billion km). Pluto is about 4.8 light-hours from the Sun.

Reaching the outer planets takes light hours rather than minutes, and the light-hour is the matching unit. Equal to 3,600 light-seconds, it is exactly 1,079,252,848,800 meters, or about 1.08 billion kilometers. Like the other light-time units it is informal, defined entirely by the fixed speed of light and the length of the SI second rather than by any standards body, and it appears mainly in astronomy outreach and in descriptions of deep-space missions.

Neptune orbits at about 30 astronomical units, which with light taking roughly 499 seconds per unit works out to somewhat over four light-hours. Pluto, at an average of 34.72 astronomical units, is about 4.81 light-hours from the Sun. The Voyager 1 probe passed 23 light-hours from Earth and the Sun by 2025 and is expected to reach one light-day, 24 light-hours, around late 2026. Beyond that scale the light-day, light-week and light-year take over.

1 light-hour = 1,079,252,848,800 meters (exactly).

Sources: Wikipedia: Light-hour; Wikipedia: Light-minute; Wikipedia: Astronomical unit

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.