Distance Units

What is a
lunar distance?

One lunar distance (LD) is 384,400 km - the average gap between Earth and the Moon. Asteroid close-approach reports use this unit because it gives immediate scale to distances that raw kilometre figures obscure.

What counts as a close approach? →

One scale, from orbit to the Moon and beyond

Every distance in asteroid tracking lives somewhere on this line. Each step to the right is ten times further away, which is why a pass at 1 LD and a pass at 19 LD are such different events.

1 LD equals

384,400 km

Average Earth-Moon distance

Moon's actual range

356,500–406,700 km

Perigee to apogee

Most tracked approaches

2–20 LD

Typical range for logged close passes

The definition

A lunar distance (LD) is defined as the average distance between Earth and the Moon: 384,400 km. The Moon does not orbit in a perfect circle. Its path is elliptical, which means the Earth-Moon gap changes continuously over the course of each month. At perigee - the closest point in its orbit - the Moon sits roughly 356,500 km from Earth. At apogee - the farthest - it reaches about 406,700 km. The 384,400 km figure is the mean, adopted as the standard reference value for the unit.

As a unit of measurement, LD predates modern asteroid tracking. Navigators and astronomers used it for centuries to describe distances within the inner solar system. Today its primary use in public-facing astronomy is to communicate how close an asteroid or comet passes to Earth during a flyby.

Why astronomers use LD for close approaches

Raw kilometre figures lose meaning at millions of kilometres. "3.4 million kilometres" gives the reader almost nothing to anchor to. "8.8 lunar distances" immediately conveys that the object passed nearly nine times farther away than the Moon - a gap most people can conceptualise in relation to something they know exists.

The unit is well-matched to the range where most asteroid close approaches occur. Objects passing between 1 and 20 LD are genuinely close in astronomical terms. Below 1 LD is exceptional. Above 20 LD, the astronomical unit (AU, approximately 150 million km) becomes more practical. LD occupies exactly the middle ground where asteroid tracking most often operates.

This tracker displays miss distances in LD as the primary unit for this reason. A glance at the dashboard tells you whether an approach was closer than the Moon, ten times the Moon's distance, or somewhere between. The kilometre figure is shown alongside for precision, but LD carries the intuition.

LD in context

Object / threshold	Lunar distances	Kilometres	Note
Geostationary satellites	0.093 LD	~35,786 km	From Earth's centre
Apophis 2029 flyby	~0.099 LD	~38,000 km	From Earth's centre; inside geostationary belt
The Moon	1 LD	384,400 km	Mean distance; varies ±7%
Typical close approach	2–15 LD	769,000–5.8 million km	Most logged NEO flybys fall here
PHA monitoring threshold	~19.5 LD	~7.5 million km	0.05 AU - outer orbital proximity boundary
1 astronomical unit	~389 LD	~150 million km	Average Earth-Sun distance

Common conversions

The three units used to describe near-Earth distances each have their natural range. Lunar distance suits asteroid flybys. The astronomical unit (AU, approximately 150 million km) suits planetary distances and orbital parameters. Kilometres suit precision work at human scales.

1 LD

= 384,400 km

1 LD

= 0.00257 AU

1 AU

= ~389 LD

0.05 AU (PHA threshold)

= ~19.5 LD = ~7.5 million km

Putting Apophis 2029 in perspective

Geostationary satellites orbit at approximately 35,786 km from Earth's centre - just 0.093 LD. The Apophis 2029 flyby will pass at roughly 38,000 km, slightly farther out than that. Both are well beyond Earth's atmosphere, which extends only a few hundred kilometres. The Apophis pass is genuinely close for an asteroid of its size, but "close" in this context still means tens of thousands of kilometres of clear space.

How this tracker uses LD

All miss distances on this tracker's dashboard and individual approach pages are displayed primarily in lunar distances, with raw kilometre values alongside. The choice is deliberate: at 2 LD you can immediately see that the asteroid passed at twice the Moon's distance. At 0.5 LD, it came closer than the Moon. At 15 LD, it was well outside even the PHA monitoring zone.

This mental model scales naturally from the closest approaches - those within 1 LD that attract the most attention - to routine passes at 10 to 20 LD that are still worth logging but carry no risk whatsoever.

What is a close approach?

How NASA defines and tracks asteroid close passes.

How close do asteroids pass?

The distribution of miss distances and what is typical.

Record miss distances

The closest confirmed asteroid flybys ever recorded.

NASA's asteroid tracker

How NASA's NeoWs API feeds data to trackers like this one.

Common questions

What is a lunar distance in km?

One lunar distance (LD) equals 384,400 km - the average distance between Earth and the Moon. This is a mean value because the Moon's orbit is elliptical, not circular. The actual Earth-Moon distance varies from approximately 356,500 km at perigee (closest point) to about 406,700 km at apogee (farthest point). The 384,400 km figure is the standard reference value used in astronomy.

Why do astronomers use lunar distances?

For close-approach distances in the range of hundreds of thousands to tens of millions of kilometres, the lunar distance gives immediate intuitive scale. "5 lunar distances" is easier to place than "1.92 million kilometres." The unit is proportionally appropriate for asteroid flybys: close approaches typically fall in the 1 to 20 LD range, making LD a natural fit. For much larger distances - between planets, for instance - the astronomical unit (AU) is used instead.

Is the Moon always 384,400 km away?

No. The Moon follows an elliptical orbit, so its distance from Earth changes continuously throughout each month. At perigee it reaches approximately 356,500 km - sometimes called a "supermoon" if this coincides with a full moon. At apogee it sits roughly 406,700 km away. The 384,400 km value is the mean orbital distance and is used as the standard definition for the lunar distance unit.

How does lunar distance compare to an astronomical unit?

1 AU (astronomical unit, approximately 150 million km) equals approximately 389 lunar distances. Working the other way, 1 LD is approximately 0.00257 AU. The PHA monitoring threshold of 0.05 AU converts to about 19.5 LD, or roughly 7.5 million km.

What is a close approach at 1 LD?

A 1 LD pass means the asteroid comes as close to Earth as the Moon itself - approximately 384,400 km from Earth's centre. This is genuinely close in astronomical terms and relatively rare for catalogued approaches. Objects that pass within 1 LD receive particular attention. Most tracked close approaches occur at several lunar distances or more. Apophis in 2029 will pass at roughly 0.099 LD - about 38,000 km from Earth's centre, which is well inside 1 LD and closer than geostationary satellites.

What does 0.05 AU in lunar distances mean?

0.05 AU converts to approximately 19.5 LD, or about 7.5 million km. This is the orbital proximity threshold used to classify potentially hazardous asteroids (PHAs): any asteroid with an orbit that passes within 0.05 AU of Earth's orbit, and a diameter of 140 metres or more, qualifies as a PHA. Geostationary satellites orbit at about 0.093 LD. The Moon sits at 1 LD. The PHA threshold is nearly 20 times farther out than the Moon.