What phase will Mercury have when it is at its closest to the Earth? Note that Mercury's orbit is closer to the Sun than the orbit of Earth is.

Question

BlackTom AI · Accepted Answer

When Mercury is at inferior conjunction (between Earth and the Sun), the Sun illuminates the side facing away from Earth and we see the dark, unilluminated hemisphere — this is the new phase.

1) Relevant knowledge points and concepts
- Inner planets (Mercury and Venus) orbit closer to the Sun than Earth and therefore exhibit phases like the Moon depending on Sun–planet–Earth geometry.
- Phases depend on the fraction of the planet’s sunlit hemisphere visible to an observer on Earth.
- Key configurations: inferior conjunction (planet between Earth and Sun), greatest elongation (maximum apparent separation from Sun), and superior conjunction (planet on far side of Sun).

2) Step-by-step reasoning
- Step 1: Closest approach of Mercury to Earth occurs near inferior conjunction because Mercury’s orbit brings it between Earth and the Sun.
- Step 2: At inferior conjunction the line Earth–Mercury–Sun has Mercury in the middle (roughly), so the Sun illuminates Mercury’s hemisphere facing away from Earth.
- Step 3: The observer on Earth sees the hemisphere facing Earth largely unlit, so the visible disk is essentially dark — the new phase.

3) Intermediate geometry steps (conceptual "calculation")
- Angle at Mercury between directions to Sun and to Earth ≈ 0° at exact inferior conjunction → illuminated fraction visible to Earth ≈ 0%.
- As elongation increases from 0° to larger angles, the visible illuminated fraction increases from new through crescent to gibbous and so on.

4) Why "new" is correct
- Being between Earth and Sun yields minimal visible illumination, matching the definition of the new phase.

5) Why other options are incorrect
- "none; Mercury does not show phases": false — Galileo observed Mercury’s phases, proving heliocentrism.
- "full": full requires Mercury opposite the Sun (superior conjunction), i.e., on the far side — not the closest approach.
- "crescent": a crescent appears when some sunlit portion is visible (near but not exactly at inferior conjunction); the closest approach aligns with inferior conjunction → new.

Conclusion: The correct phase at closest approach is new, which matches the provided answer.

What phase will Mercury have when it is at its closest to the Earth? Note that Mercury's orbit is closer to the Sun than the orbit of Earth is.单项选择题

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