Final answer:
Parallax is an apparent shift in an object's position when viewed from different angles, well-known in astronomy for measuring the positions of stars. It is caused by Earth's movement around the Sun and is defined as half the angle of the apparent shift measured against distant stars. The Greeks were unable to detect stellar parallax due to the large distances and smaller angles involved.
Step-by-step explanation:
The phenomenon known as parallax is an apparent shift in the position of an object when viewed from different angles or positions, and it is particularly relevant in the field of astronomy when observing celestial objects. The Greeks tried to measure stellar parallax to no avail, which is the change in the apparent direction of a nearby star when Earth is on opposite sides of its orbit around the Sun. This method of measurement relies on Earth's revolution, creating a situation where a nearby star's position appears to shift against the background of more distant stars. Astronomers define this parallax as one-half of the angle of observed shift and often measure it in arcseconds, using the baseline of 1 Astronomical Unit (AU), the average distance from the Earth to the Sun.
An example of parallax can also be observed by holding up a finger and viewing it first with one eye and then with the other, noticing that the finger appears to move. This effect decreases as the object is moved further away, illustrating how parallax allows your brain to determine the distance of objects closer to you, a concept that fails for objects far beyond a few tens of meters.