Plutarch, De facie in orbe lunae 923C–D: the moon kept from falling as a stone in a sling

Companion source

Source: LacusCurtius, The Face in the Moon, section A, reproducing the Cherniss and Helmbold translation (Loeb Classical Library, Moralia XII, 1957); consulted and verified 2026-09-18. Metadata: Plutarch, De facie quae in orbe lunae apparet, c. 100 CE. The argument is put in the mouth of Lamprias against Pharnaces the Stoic. Rights: the Loeb translation is not public domain, so no excerpt file is stored here. One sentence is quoted below for scholarly comment, and the Greek text is not reproduced.

What it supplies

The one ancient author who argues with the sling, and the passage Newton’s own classical preface leans on. At 923C, against the objection that the moon must fall:

Yet the moon is saved from falling by its very motion and the rapidity of its revolution, just as missiles placed in slings are kept from falling by being whirled around in a circle.

923D continues: “For each thing is governed by its natural motion unless it be diverted by something else.”

This bears on the programme in three ways.

• It answers the question of the missing sling. The arrow-and-sling note observes that the ancient debate on motion is almost entirely rectilinear while Newton defines centripetal force with a stone in a sling at Definition V. Plutarch is the exception, and he applies the sling to exactly the case Newton applies it to, the retention of a heavenly body in its orbit.
• The inference runs the other way from Newton’s. For Plutarch the whirling prevents falling, so speed supplies a sustaining effect; for Newton the sling’s cord pulls the stone inward, and the natural motion it diverts is the straight line. The same apparatus supports opposite accounts, and what separates them is which motion is taken to need no cause. The second clause, that each thing is governed by its natural motion unless diverted, is as close to an inertial principle as the passage comes, with “natural motion” still doing the work that inertia later does.
• Newton read it. The Classical Scholia to Book III cite Plutarch among the ancients said to have known the doctrine of gravitation, and Gregory’s 1702 preface, the printed witness this collection holds, belongs to the same project. So the sling is not merely available in antiquity; it is available in a text Newton was mining for ancient authority while writing the very propositions the sling models.

Passage anchors

Coverage and limits

Reading level passage for 923C–D in the Cherniss and Helmbold translation only. The Greek is not consulted, and the Stephanus letters are read off the marginal markers in the LacusCurtius text rather than from a critical edition; both are verification targets. The claim that the Classical Scholia cite this work is recorded at metadata level from the scholarship listed in H02 seeds, chiefly Casini 1984 and McGuire and Rattansi 1966, none of which has been read here. Whether the scholia cite this passage rather than the work in general is open, and it is the question a historian would ask first.