Time scales in astronomy – identify the single incorrect statement Which one of the following statements about apparent solar time, mean solar time, and sidereal time is incorrect?

Introduction / Context:
Astronomical time scales are defined by meridian transits of reference bodies or directions. Apparent solar time uses the true Sun, mean solar time uses a fictitious mean Sun moving uniformly, and sidereal time uses the vernal equinox (first point of Aries).

Given Data / Assumptions:

• Transit means crossing of the local meridian.
• “Upper transit” is the culmination above the horizon; “lower transit” is the opposite crossing.
• Standard surveying/astronomy convention uses upper transits to define these time scales.

Concept / Approach:
By convention, apparent solar noon occurs at the upper meridian transit of the true Sun. Mean solar time is keyed to the upper transit of the mean Sun to smooth out seasonal variations. Local sidereal time equals the hour angle of the first point of Aries and is measured from its upper meridian transit.

Step-by-Step Solution:
Check (a): apparent solar time from upper transit → correct.Check (b): mean solar time from upper transit of mean Sun → correct.Check (c): sidereal time from upper transit of Aries → correct.Check (d): claims lower transit defines sidereal time → incorrect → select as answer.

Verification / Alternative check:
Local sidereal time at any moment is the hour angle of Aries; when Aries is on the upper meridian, LST = 0 h (mod 24 h), confirming the reference.

Why Other Options Are Wrong:

• (a), (b), and (c) follow standard definitions.
• (e) is broadly true; each scale references a meridian transit (upper), so it is not the incorrect one.

Common Pitfalls:
Confusing upper with lower transit or mixing apparent and mean Sun concepts.

Final Answer:
Sidereal time is measured from the lower transit of the first point of Aries