Final answer:
Astronomers use 'seeing' to describe the atmospheric conditions for telescope observations, with 1 arcsecond indicating a small angle of distortion caused by the atmosphere, resulting in good quality images of the stars.
Step-by-step explanation:
When an astronomer mentions that the seeing last night was 1 arcsecond, they are referring to the quality of the Earth's atmosphere for astronomical observations. The term 'seeing' characterizes the atmospheric conditions and their effect on the clarity of images observed. An arcsecond, which is an angle measuring 1/3600th of a degree, signifies how much the atmosphere has distorted the light from celestial objects. Since there are 360 degrees in a full circle, 1 arcsecond is a very small angle indeed — so small that a US quarter appearing to have a diameter of 1 arcsecond would imply it's being viewed from about 5 kilometers (3 miles) away. A seeing of 1 arcsecond is considered quite good, as it suggests that the atmospheric conditions were stable enough to only distort the incoming starlight by a minimal amount, providing relatively sharp images of the stars.