Tarqeq’s 76-hour day

How Cassini measured the slowest spin of any irregular moon it observed.

Published 2026-10-23

On 15–16 January 2014, Cassini watched Tarqeq for 1.5 days.^[1]

Light curves

A light curve is a graph of an object’s brightness over time.^[7] As an elongated moon spins, its brightness rises and falls, revealing its rotation.

The result

Tarqeq turned out to rotate once every 76.13 ± 0.01 hours, with a roughly ellipsoidal shape — the slowest-rotating irregular moon measured by Cassini.^[1]

A 1:5 coincidence?

That period is very close to a 1:5 resonance with Titan’s orbital period, so gravity might lock Tarqeq in resonance.^[1]

Numbered citations link to the sources list on the Tarqeq overview page.