Ecliptic–equator crossing — motion of the Sun at the First Point of Aries (vernal equinox) At the First Point of Aries (the vernal equinox), in which sense does the Sun move relative to the celestial equator?

Introduction / Context:
Understanding the Sun’s apparent annual motion is fundamental in astronomy and geodetic astronomy. The ecliptic intersects the celestial equator at two equinox points. At the vernal equinox, also called the First Point of Aries, the Sun crosses the celestial equator heading into the northern celestial hemisphere. This event sets the zero of right ascension and is tied to seasonal changes on Earth.

Given Data / Assumptions:

• The First Point of Aries denotes the Sun’s position at the March (vernal) equinox.
• Declination describes angular distance north/south of the celestial equator.
• Right ascension increases eastward from this point.

Concept / Approach:
At the vernal equinox the Sun’s declination changes sign from negative (south) to positive (north) as it crosses the celestial equator. Therefore, the Sun moves from south of the equator to north of the equator. This moment defines RA = 0 h for the Sun and marks the start of astronomical spring in the northern hemisphere.

Step-by-Step Solution:

Verification / Alternative check:
Seasonal declination tables show the Sun at δ = 0° on about March 20–21 while increasing to +23.4° by June solstice, confirming the northward crossing.

Why Other Options Are Wrong:

• “Northward” (a) is directionally true but incomplete; (c) correctly states the crossing from south to north.
• “Southward” or “from north to south” describe the autumnal equinox, not the vernal equinox.
• “None of these” is incorrect because (c) is correct.

Common Pitfalls:
Confusing vernal with autumnal equinox; mixing northern/southern hemisphere seasonal naming; ignoring that RA origin is tied to this crossing.

Final Answer:
From south of the equator to north of the equator