Final answer:
The first coordinate of the celestial coordinate system is called A. Right ascension (RA)
Step-by-step explanation:
Right ascension (RA) is the first coordinate of the celestial coordinate system. It is similar to longitude on Earth, but instead of using Greenwich as the starting point, the vernal equinox is used. The vernal equinox is the point where the ecliptic (the Sun's path) crosses the celestial equator. RA can be expressed in either degrees or time. Since the celestial sphere appears to turn around Earth once a day, the 360° of RA is equivalent to 24 hours, with each 15° of arc equaling 1 hour of time.
For example, the bright star Capella has approximate celestial coordinates of RA 5h = 75° and declination +50°. This means that Capella is located at an RA of 5 hours and a declination of +50°. Therefore the correct answer is A. Right ascension (RA)