Astronomy – Celestial Coordinates Right ascension (the celestial-sphere analog of terrestrial longitude) is conventionally measured in which units?
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Adegrees
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Btime-hours, minutes and seconds
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Conly hours
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Donly seconds
Answer
Correct Answer: time-hours, minutes and seconds
Explanation
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:
Recall RA’s linkage to Earth’s rotation period (sidereal day).Map 24 h to 360° to understand the conversion (1 h = 15°).Therefore, RA units are hours, minutes, seconds of time.Eliminate degree-only or partial time-unit options.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