On which date are both the Sun’s declination and right ascension equal to zero? Choose the correct calendar date associated with δ = 0° and RA = 0 h for the Sun.

Introduction / Context:
The Sun’s equatorial coordinates provide a convenient description of seasonal changes. At the vernal equinox, the Sun crosses the celestial equator heading north and the coordinate origin is defined such that RA = 0 h at this instant.

Given Data / Assumptions:

• Equinox dates vary slightly year to year (March 20–21 and September 22–23).
• At both equinoxes, solar declination δ = 0°.

Concept / Approach:
Although δ = 0° at both equinoxes, right ascension values differ: RA = 0 h at the vernal equinox (first point of Aries), and RA ≈ 12 h at the autumnal equinox (first point of Libra). Therefore, the unique date when both δ = 0° and RA = 0 h is the March equinox.

Step-by-Step Solution:
Identify the coordinate origin: first point of Aries → RA = 0 h.Match it to the calendar event: vernal equinox, about March 21.Hence choose March 21 as the correct date.

Verification / Alternative check:
Standard almanacs list RA(☉) ≈ 0 h at the instant of the March equinox each year, with δ ≈ 0° by definition of the celestial equator crossing.

Why Other Options Are Wrong:

• September equinox has δ = 0° but RA ≈ 12 h, not 0 h.
• Solstices (June, December) have |δ| ≈ 23.5° and RA far from 0 h.
• Aphelion is unrelated to equatorial coordinates.

Common Pitfalls:
Assuming RA = 0 h at both equinoxes; only the March equinox sets the RA zero point.

Final Answer:
March 21 (around the vernal equinox)