Astronomy – Celestial Coordinates Right ascension (the celestial-sphere analog of terrestrial longitude) is conventionally measured in which units?

Introduction / Context:
Celestial coordinates locate objects on the sky. Declination is analogous to latitude and is expressed in degrees. Right ascension (RA), analogous to longitude, uses a time-based unit system that reflects Earth’s rotation and the apparent daily motion of the sky.

Given Data / Assumptions:

• We need the conventional units for right ascension.
• RA measures angular distance eastward along the celestial equator from the vernal equinox.
• Astronomers relate angular sky distances to time because the sky appears to rotate 360° in ~24 hours.

Concept / Approach:
Right ascension is measured in hours, minutes, and seconds of time, where 24 h corresponds to 360°, hence 1 h = 15°, 1 min = 15 arcminutes, 1 s = 15 arcseconds. This convention simplifies telescope tracking and timing observations.

Step-by-Step Solution:

Verification / Alternative check:
Star catalogs (e.g., RA 05h 35m 17s) and planetarium software display RA in time units, confirming the standard practice.

Why Other Options Are Wrong:

• Degrees: Used for declination, not for RA by convention (though convertible).
• Only hours / only seconds: Incomplete specification; full RA uses hours, minutes, seconds.

Common Pitfalls:
Assuming both celestial coordinates use degrees. While possible to convert, the astronomical standard for RA is time-based units.

Final Answer:
time-hours, minutes and seconds