National Astronomical Observatory of Japan



Formation of the Moon


How was the Moon, Earth’s natural satellite, formed? This video is a visualization of lunar formation simulations based on the Giant Impact model. At the end of the formation of the Earth, 4.6 billion years ago, a protoplanet about the size of Mars crashed into the ‘proto-Earth.’ The power of this collision caused parts of the protoplanets to melt or evaporate, and this material formed a disk around the Earth. The idea that this disk became the material for forming the Moon is called the Giant Impact hypothesis.

From a Disk to a Moon

At locations close to the Earth, the Earth’s gravity causes material from the orbiting disk, from which the Moon forms, to fall to the Earth. On the other hand, farther away, mutual gravitational attraction among the lunar material causes it to collect, producing aggregates. Then several of the aggregates formed in this manner merge, and before long a single large Moon is formed. Based on these simulations, it is thought that it only took about 1 month for the Moon to form.

Giant Impact Model

Many theories about the birth of the Moon have been considered. But they couldn’t explain the characteristics of the Moon: (1) its mass is 1/100 that of the Earth, (2) it’s formed from rocks similar to Earth’s mantle, (3) its angular momentum (orbital energy) is large. Because the “Giant Impact hypothesis” visualized in this video can explain all of these characteristics, it is currently the most accepted model.

Video Data

First half: Giant Impact

Number of particles used6 x 104particles
Time scaleSeveral days
Spatial scale4-5 Earth-radii
ResearcherRobin M. Canup (Southwest Research Institute)

Latter half: Moon Accretion

Number of particles used1x105
Time scale1 month
Spatial scale4-5 Earth-radii
ResearcherTakaaki Takeda (NAOJ)


Simulation: Robin M. Canup, Takaaki Takeda
Visualization: Takaaki Takeda
Four-Dimensional Digital Universe Project, National Astronomical Observatory of Japan

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