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Today in History: Asaph Hall discovers Phobos, Mars' larger moon

8/18/2026

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Summary

Story summary

American astronomer Asaph Hall identified Mars’s two moons in August 1877, first spotting the smaller Deimos and, six days later, the larger Phobos. He named them after the Greek gods of dread and fear, twin sons of Ares, and later calculated their irregular shapes, orbital paths and Mars’s mass.

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Story summary

American astronomer Asaph Hall identified Mars’s two moons in August 1877, first spotting the smaller Deimos and, six days later, the larger Phobos. He named them after the Greek gods of dread and fear, twin sons of Ares, and later calculated their irregular shapes, orbital paths and Mars’s mass.