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Difficulty: MediumThe Earth as a Planet and Earth Movements

During Earth's annual revolution around the Sun, the subsolar point (the position where the Sun is directly overhead at noon) continuously changes its latitude between the Tropics. Starting from the March equinox when the Sun is directly overhead at the Equator moving northward, arrange the following solar positions in chronological sequence throughout the year.

  1. 1Sun directly overhead at the Equator moving northward
  2. 2Sun directly overhead at the Tropic of Cancer (23.5∘N23.5^\circ\text{N})
  3. 3Sun directly overhead at the Equator moving southward
  4. 4Sun directly overhead at the Tropic of Capricorn (23.5∘S23.5^\circ\text{S})

Answer

The correct chronological sequence starting from the March equinox is: Sun directly overhead at the Equator moving northward ��� Sun directly overhead at the Tropic of Cancer → Sun directly overhead at the Equator moving southward → Sun directly overhead at the Tropic of Capricorn.
Because Earth's axis is inclined at 23.5∘23.5^\circ to its orbital plane, the overhead position of the noon Sun shifts continuously throughout the year. Starting at the March Equinox (around March 21), the overhead Sun is at the Equator moving northward. It reaches its northernmost boundary at the Tropic of Cancer (23.5∘N23.5^\circ\text{N}) during the June Solstice (around June 21). It then returns southward, crossing the Equator at the September Equinox (around September 23), and finally reaches its southernmost boundary at the Tropic of Capricorn (23.5∘S23.5^\circ\text{S}) during the December Solstice (around December 22).

Step-by-Step Solution

1
Identify the initial position of the overhead Sun at the start of the specified cycle.
The cycle begins at the March Equinox (approx. March 21), when the Sun is overhead at the Equator (0∘0^\circ) heading into the Northern Hemisphere.
The question prompt specifies starting at the March equinox with the subsolar point moving northward.
2
Determine the northernmost limit of the overhead Sun.
Three months later (approx. June 21), the Sun reaches its maximum northern latitude at the Tropic of Cancer (23.5∘N23.5^\circ\text{N}).
Earth's axial tilt of 23.5∘23.5^\circ defines the maximum latitude where the Sun can be directly overhead at noon.
3
Trace the overhead Sun's retreat back across the Equator.
Three months after the June solstice (approx. September 23), the Sun crosses the Equator again, moving southward.
This event is the Autumnal Equinox.
4
Identify the southernmost limit of the overhead Sun.
Three months after the September equinox (approx. December 22), the Sun reaches its maximum southern latitude at the Tropic of Capricorn (23.5∘S23.5^\circ\text{S}).
This event marks the December (Winter) Solstice before the Sun begins moving back north toward the Equator.

Key Concept

Seasonal migration of the subsolar point due to Earth's axial tilt and revolution
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Topics

The Earth as a Planet and Earth Movements

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