What is a constellation as astronomers define it today? What does it mean when an astronomer says, "I saw a comet in Capricorn last night?"

A constellation, in astronomy, is a conventional grouping of stars, whose position in the night sky is apparently invariable. The peoples, generally of ancient civilizations, decided to link them through imaginary strokes, thus creating virtual silhouettes on the celestial sphere. From 1928, the International Astronomical Union (UAI) decided to officially regroup the celestial sphere into 88 constellations with precise limits, such that every point in the sky would be within the limits of a figure. When an astronomer says he saw a comet in Capricorn last night, it means that he saw a comet in the direction of the constellation of Capricorn.

carla slowed down in the beginning of her trip, stopped, and then sped up

because if the tool that you are using is not balanced it there would be false mass.