Parallax
2 of the 6 encyclopedias on this shelf carry an entry for Parallax. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
a difference in the apparent direction of a heavenly body, due to different positions of the viewpoint. Persons standing at opposite points on the earth's surface will see the same star in slightly different directions. In astronomy parallax is measured by the angle included between lines drawn from the object to the observer, and from the object to the center of the earth or other point taken as a standard center. Evidently a heavenly body immediately above the observer has no parallax. The farther away a body, the less its parallax. The parallax of the moon is 57'. That is, to the lunar observer, the earth would seem to have a diameter of 114'. If star observations be made at intervals of six months when the viewpoints are at opposite diameters of the earth's annual orbit, the parallax becomes appreciable, even for the most distant star. By the aid of trigonometry this apparent shifting of direction is made use of by astronomers in calculating the distance of heavenly bodies.
Collier's New Encyclopedia (1921)
the difference of direction of a body as seen from two different points. It is generally applied to the direction of the heavenly bodies as seen from the earth's center and from some point of its surface. The parallax is greater the nearer the body and the greater the distance between the points in a direction at right angles to that of the body. The term is also applied to the difference in direction of a body seen from different points of the earth's orbit, the longest diameter of which is insufficient in case of some fixed stars.
What Has Changed in the Last Century
The geometry the two entries describe is exactly the geometry used today, and Aiton's figure of 57' for the moon's parallax still stands. What has changed is scale and precision. When these were written only a few hundred stellar parallaxes had been measured at all, and Aiton's confidence that the six-month baseline makes the shift "appreciable, even for the most distant star" was too optimistic — Collier's 1921 wording, that the orbit's diameter is "insufficient in case of some fixed stars," was nearer the mark. The Hipparcos satellite (1989–93) fixed about 118,000 parallaxes, and the Gaia mission, launched in 2013, has produced them for roughly 1.5 billion stars at microarcsecond precision; meanwhile the astronomical unit, once derived from painstaking solar-parallax work, was simply defined as an exact number of metres by the IAU in 2012, with planetary distances now got from radar and spacecraft ranging.
Written for this edition, 2026.