This paper presents a brief historical review of various attempts to estimate the age of the Earth, and reappraises the study of Patterson (1956) which revealed for the first time that the age of the Earth is 4550±70 Ma by measuring Pb isotope ratios of several meteorites and a marine sediment. The standard model for the planetary formation of early solar system is: formation of solid particles condensed from the cooling of hot nebular gas -> formation of planet-sized bodies by accretion of

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... e solid particles. The Moon is supposed to have formed from the accretion of the relicts produced by the collision of proto-Earth with Mars-sized body. It is not easy to pinpoint the age of the Earth, considering the series of events related to the formation of the Earth. So, I propose that the collision age as that of the Earth, since the present status of the Earth is thought to be the direct product of the collision. According to the previous studies, the collision age can be broadly constrained between the age (4567.30±0.16 Ma) of the earliest condensates (CAI, calcium-aluminum rich inclusion) of the nebula gas, i.e., the age of the solar system, and the oldest age (4,456±40 Ma) among rocks and minerals of the Earth and the Moon. We need more precise estimation of the collision age, since it is important in estimating time scale for the formation of planet-size body and in revealing thermal evolution of magma oceans of the Earth and the Moon presumably developed right after the collision.