Celestial coordinates — definition of right ascension (R.A.) Right ascension of a celestial body is its equatorial angular distance measured in which sense from which reference point?
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AWestward from the first point of Libra
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BEastward from the first point of Aries (vernal equinox)
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CWestward from the first point of Aries
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DEastward from the first point of Libra
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EEastward from the autumnal equinox in Virgo
Answer
Correct Answer: Eastward from the first point of Aries (vernal equinox)
Explanation
Introduction / Context:Right ascension (R.A.) and declination (Dec.) form the standard equatorial coordinate system used in astronomy and astro-surveying, analogous to longitude and latitude on Earth.
Given Data / Assumptions:
- Equatorial coordinate system referenced to the celestial equator.
- Zero reference defined by the vernal equinox (first point of Aries).
Concept / Approach:
R.A. is measured along the celestial equator eastward from the vernal equinox to the hour circle passing through the body. Declination is the angular distance north or south of the celestial equator. This convention mirrors terrestrial longitude and latitude but on the celestial sphere.
Step-by-Step Solution:
Identify zero point → first point of Aries (vernal equinox).Define direction → measured eastward along celestial equator.Hence R.A. = eastward arc from Aries to the object's hour circle.Verification / Alternative check:
Almanacs and star catalogs universally adopt this definition of R.A., typically expressed in hours, minutes, and seconds of time (0–24 h).
Why Other Options Are Wrong:
- Options stating “westward” or referencing Libra are inconsistent with the standard definition.
Common Pitfalls:
- Confusing R.A. (along the equator) with ecliptic longitude (along the ecliptic).
Final Answer:
Eastward from the first point of Aries (vernal equinox).