What angular unit represents the apparent size of a US dime held on the Moon when viewed from Earth?
Answer
One arcsecond
To illustrate the extreme precision required in positional measurements for astrometry, a direct comparison is made using angular units. A whole circle is divided into 360 degrees, a degree into 60 arcminutes, and an arcminute into 60 arcseconds. The text specifies that if one were to hold a US dime on the surface of the Moon, its apparent angular size as perceived from Earth would equate to approximately one arcsecond. Detecting Earth-mass planets often requires measurements down to microarcseconds, which are significantly smaller units than this terrestrial reference.
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