length · International · Modern
SI base unit of length. Its definition fixes the speed of light in vacuum at exactly 299,792,458 m/s, with the second defined from the caesium-133 hyperfine frequency.
Converter
length · International · Modern
lunar distance
LDapprox.
- Record
- M278
- Definition quality
- Approximate
- Conversion mode
- multiplicative
- Calculator
- Approximately calculable
Label as representative/mean; never imply instantaneous Earth–Moon distance.
Definition
JPL defines one lunar distance (LD) for its data as 384,400 km, the Moon's mean semimajor-axis distance.
Reference value
One lunar distance is understood to be about 384,400,000 metres.
History & use
A practical astronomical comparison/distance unit tied to the mean Earth–Moon separation.
Modern use
Used by JPL/CNEOS for near-Earth-object distance reporting.
Did you know?
The real Earth–Moon distance varies; LD is a defined mean reference, not the Moon's instantaneous distance.
Sources & evidence
Source tier Tier A · confidence High
Related measurements
length · International · Modern
1 km = 1000 m by the SI prefix kilo-.
length · International · Modern
1 cm = 0.01 m by the SI prefix centi-.
length · International · Modern
1 mm = 0.001 m by the SI prefix milli-.
inch
inlength · UK / US · Modern / historical
1 international inch = exactly 0.0254 m.
foot
ftlength · UK / US · Modern / historical
1 international foot = exactly 0.3048 m.
yard
ydlength · UK / US · Modern / historical
1 international yard = exactly 0.9144 m.
mile
milength · UK / US · Modern / historical
1 international mile = exactly 1609.344 m = 1760 yd.