The constellation Ursa Minor includes Polaris (the North Star) as a member. An observer just north of the equator might

Question

BlackTom AI · Accepted Answer

Circumpolar stars must have declination δ greater than 90° − φ, where φ is the observer’s latitude. That condition guarantees a star never dips below the horizon as the Earth rotates.

Relevant knowledge points:
- Declination (δ) is the celestial equivalent of latitude; the observer’s latitude (φ) determines which declinations are always above the horizon (circumpolar).
- Circumpolar condition for northern hemisphere: δ > 90° − φ.
- Polaris (the North Star) has δ ≈ +89.3° and lies in Ursa Minor, but the rest of Ursa Minor has somewhat lower declinations.

Step-by-step reasoning and calculation:
1. For an observer “just north of the equator” take φ to be small (for example φ = 0.5° as a representative “just north” value).
2. Compute the circumpolar threshold: 90° − φ = 90° − 0.5° = 89.5°.
3. Compare Polaris’s declination: δPolaris ≈ +89.3°.
4. Because 89.3° < 89.5°, Polaris would not be strictly circumpolar for φ = 0.5°; it will dip below the horizon and thus rise and set. The rest of Ursa Minor has even lower declinations, so they likewise rise and set.

Why the correct answer is correct:
- Given a latitude just north of the equator the circumpolar zone is essentially nonexistent (90° − φ is very near 90°), so virtually no northern constellation is permanently above the horizon. Ursa Minor’s stars are not all north enough to remain always above the horizon, so the entire constellation will rise and set each sidereal day.

Why the other options are incorrect:
- “See Ursa Minor throughout the night, every night of the year.” That would require Ursa Minor to be entirely circumpolar (δ > 90° − φ), which it is not at near-equatorial latitudes.
- “See Ursa Minor rise and set on some nights throughout the year.” Rising and setting of stars is a daily (diurnal) phenomenon, not seasonal; this option confuses daily motion with seasonal visibility.
- “Never see Ursa Minor.” Incorrect because Ursa Minor is visible from northern latitudes—the observer would see it, and it will rise and set.

Conclusion: Therefore an observer just north of the equator would see Ursa Minor rise and set every night. This matches the provided correct answer.

The constellation Ursa Minor includes Polaris (the North Star) as a member. An observer just north of the equator might单项选择题

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