Momentum

In astronomy, students often ask why the Earth continues to rotate at a more or less constant rotational speed, and why it continues to point more or less at a fixed star (currently Polaris) over the course of a year's orbit around the Sun. The answer is that in the absence of a significant torque (rotation changing externally applied force), the Earth's rotation (and its angular momentum) is unchanged. This means that the Earth continues to rotate at a constant angular speed (one rotation every 23 hours and 56 minutes), and it continues to point to the (faraway) star Polaris as it orbits around the Sun. In truth, the angular momentum of the Earth is affected by a net, combined, unbalanced torque on it by the Sun and Moon. The results of that torque are 1) that the Earth's rotational speed changes very gradually over a long period of time, and 2) that the Earth's polestar changes over a long period of time.Regarding the former effect, the average Earth solar day was about 23 hours a "mere" 250 million years ago. Regarding the latter effect, the Earth changes polestars gradually over a cyclic period of 26,000 years, because its axis of rotation very gradually wobbles. This latter effect is called precession.