Transit (culmination) of a star in the northern hemisphere In practical field astronomy for surveying, a star is said to 'transit' (culminate) when which of the following conditions is met for an observer in the northern hemisphere? Clarify the condition in terms of altitude, azimuth, and meridian crossing.

Introduction / Context:
In astronomical observations used by surveyors, the term transit (or culmination) describes the instant a celestial object crosses the observer's local meridian. Recognizing this instant is essential for determining azimuths, time, and latitude with high precision.

Given Data / Assumptions:

• Northern hemisphere observer.
• Transit refers to crossing of the local meridian plane.
• Most ordinary (non-circumpolar) stars reach their highest altitude when due south.

Concept / Approach:
Transit occurs when the star crosses the meridian (a north–south great circle through the zenith). For stars that transit to the south for a northern observer, this is the moment of upper culmination, giving the maximum altitude and an azimuth of about 180° (due south). Circumpolar stars also have a lower culmination in the north, but the basic definition of transit as meridian crossing remains.

Step-by-Step Solution:
Define transit → crossing the meridian.For a northern observer, most stars cross the meridian due south → azimuth ≈ 180°.At this instant the altitude is maximized (upper culmination).Hence all listed statements describe the same moment.

Verification / Alternative check:
Star charts and almanacs mark meridian passage; measured altitudes around transit show a maximum (upper culmination) consistent with due-south direction for non-circumpolar stars.

Why Other Options Are Wrong:
Each of (a), (b), and (c) is correct for the typical northern-hemisphere case, so the inclusive choice is required.

Common Pitfalls:
For circumpolar stars, a lower culmination occurs north of the zenith; however, the definition of transit still means meridian crossing, and upper culmination remains the highest altitude event.

Final Answer:
All the above