Axial parallelism
Axial parallelism is the characteristic of an orbiting body in which the direction of the axis remains parallel to itself throughout its orbit.
Earth's axial parallelism
The Earth's orbit, with its axis tilted at 23.5 degrees, exhibits approximate axial parallelism, maintaining its direction towards Polaris (the "North Star") year-round. This is one of the primary reasons for the Earth's seasons, as illustrated by the diagram to the right.[1][2][3][4]
Minor variation in the direction of the axis, known as axial precession, takes place over the course of 26,000 years.
Explanation
Early gyroscopes were used to demonstrate the principle.[5] Prior to the invention of the gyroscope, it had been explained by scientists in various ways. Early modern astronomer David Gregory, a contemporary of Isaac Newton, wrote:
To explain the Motion of the Celestial Bodies about their proper Axes, given in Position, and the Revolutions of them… If a Body be said to be moved about a given Axe, being in other respects not moved, that Axe is suppos'd to be unmov'd, and every point out of it to describe a Circle, to whose Plane the Axis is perpendicular. And for that reason, if a Body be carried along a line, and at the fame time be revolved about a given Axe; the Axe, in all the time of the Body's motion, will continue parallel to it self. Nor is any thing else required to preserve this Parallelism, than that no other Motion besides these two be impressed upon the Body; for if there be no other third Motion in it, its Axe will continue always parallel to the Right-line, to which it was once parallel.[6]
This gyroscopic effect is described in modern times as "gyroscopic stiffness" or "rigidity in space". The Newtonian mechanical explanation is known as the conservation of angular momentum.[7]
References
- ^ Petersen, J.; Sack, D.; Gabler, R.E. (2014). Fundamentals of Physical Geography. Cengage Learning. p. 18. ISBN 978-1-285-96971-8. Retrieved 2022-12-02.
- ^ The Popular Educator. John Cassell. 1856. p. 89. Retrieved 2022-12-02.
- ^ Oliver, J.E. (2008). Encyclopedia of World Climatology. Encyclopedia of World Climatology. Springer Netherlands. p. 651. ISBN 978-1-4020-3264-6. Retrieved 2022-12-02.
- ^ Rohli, R.V.; Vega, A.J. (2011). Climatology. Jones & Bartlett Learning, LLC. p. 30. ISBN 978-1-4496-5591-4. Retrieved 2022-12-02.
- ^ The Encyclopaedia Britannica: A Dictionary of Arts, Sciences and General Literature. R.S. Peale. 1890. p. 351. Retrieved 2022-12-02.
Under the title of precession instruments, various pieces of apparatus, involving the gyroscope principle, have been in use for a number of years for illustrating the precession of the equinoxes, and the parallelism of the earth's axis as it revolves round the sun.
- ^ Gregory, D. (1715). The Elements of Astronomy, Physical and Geometrical. By David Gregory ... Done Into English, with Additions and Corrections. To which is Annex'd, Dr. Halley's Synopsis of the Astronomy of Comets . J. Nicholson ... sold. p. 59. Retrieved 2022-12-02.
- ^ Giordano, N. (2012). College Physics: Reasoning and Relationships. Cengage Learning. p. 299. ISBN 978-1-285-22534-0. Retrieved 2022-12-02.