LD
lunar distance
A lunar distance is the mean centre-to-centre separation of the Earth and the Moon, 384,399 kilometres. It is the standard yardstick for reporting near-Earth asteroid encounters: a rock passing at 0.5 LD passed inside the Moon's orbit, which is the threshold at which the public notices.
Watch out: It is a mean, not a constant. The Moon's orbit is elliptical, running from about 356,500 km at perigee to 406,700 km at apogee, a spread of 13%, and it recedes about 3.8 cm per year. The value here is the conventional mean semi-major axis.
| 1 LD | 384,399,000 m |
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