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.

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

About the light-hour

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.

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

Light-hour conversion chart

1 light-hour expressed in every other length unit:

More Astronomical units

• astronomical unit (length)
• light-second (length)
• light-minute (length)
• light-year (length)
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• megaparsec (length)
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• sidereal year (time)
• synodic month (time)
• sidereal day (time)
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