On March 21 each year, all towns in the southern hemisphere experience?

Analyze the question: March 21 is the March (vernal) equinox. The question asks what happens at all places in the southern hemisphere on that date.

Explain the concept: An equinox happens twice a year, around March 21 and September 23. On these days the sun's vertical rays fall directly on the Equator. The Earth's axis is tilted, but neither pole leans toward or away from the sun. As a result, the circle of illumination passes through both poles and cuts every line of latitude into two equal halves.

Step by step:

1. On March 21, the overhead sun is at 0° latitude (the Equator).
2. The boundary between day and night (the circle of illumination) runs from pole to pole.
3. Every place on Earth therefore spends half of its 24-hour rotation in sunlight and half in darkness.
4. Day and night each last about 12 hours everywhere. The word "equinox" means "equal night".

Checking the options:

• A. Complete daylight for 14 hours: Wrong. Day length is about 12 hours, not 14.
• B. Partial darkness: Wrong. This is not a feature of the equinox.
• C. Equal length of day and night: Correct. This is the defining feature of an equinox, and it applies to both hemispheres.
• D. Longer period of twilight: Wrong. Twilight is longest near the poles and at the solstices, not at the equinox in all southern towns.

Answer: C. Equal length of day and night.

Tip: Remember "Equinox = Equal". On March 21 and September 23, the sun is overhead at the Equator, and day and night are both about 12 hours everywhere. For the solstices (June 21 and December 22), day and night are unequal, so don't mix them up.