Final answer:
The parallax equation D = 1/p uses units of parsecs (pc) to measure the distance to stars, where p is the parallax in arcseconds.
Step-by-step explanation:
In the parallax equation D = 1/p, the units used to measure the distance D can be expressed in parsecs (pc), where the parallax p is given in arcseconds. A parsec is a unit of stellar distance equal to approximately 3.26 light-years or 3.1 × 10¹³ kilometers. This equation is fundamental in the field of astronomy for measuring the distances to stars based on the observed parallax. It's important to note that in a different context within physics, particularly optics, the symbol 'D' can also represent the power of a lens, which is measured in diopters, a unit denoted as (D), where 1 D equals the reciprocal of the focal length in meters (1/m).