Earth–Sun Geometry – Why day length varies through the year The changing duration of daylight across seasons on Earth is primarily due to which astronomical factors?

Introduction / Context:
Day length (photoperiod) varies by season and latitude. Understanding which motions of Earth control this variation is fundamental to climatology, phenology, and the design of solar energy systems.

Given Data / Assumptions:

• Earth spins once per day (rotation) and orbits the Sun once per year (revolution).
• Earth’s rotational axis is tilted ~23.5° relative to the orbital plane (the ecliptic).
• We consider seasonal day-length differences, not just the alternation of day and night.

Concept / Approach:
Rotation creates the basic cycle of day and night but by itself would not produce seasonal changes in day length. The variation arises because the tilted axis points toward or away from the Sun at different points in Earth’s revolution. This changes the Sun’s declination, shifting the circle of illumination and the height of the midday Sun, lengthening days in one hemisphere while shortening them in the other.

Step-by-Step Solution:
Recognize that “variation” (longer/shorter days) demands a mechanism beyond simple rotation.Combine axial tilt with annual revolution to change solar declination through the year.Select the option explicitly naming revolution plus axial tilt (~23.5°).

Verification / Alternative check:
At solstices, one hemisphere is maximally tilted toward the Sun (long days) while the other is tilted away (short days). At equinoxes, day and night are nearly equal worldwide, consistent with the tilt + revolution explanation.

Why Other Options Are Wrong:

• Rotation alone: produces day/night but not seasonal variation in length.
• Precession: changes the orientation of the axis slowly over millennia; not the seasonal driver.
• Zero tilt circular orbit: would yield minimal seasonal variation and nearly constant day length at a given latitude.

Common Pitfalls:
Attributing seasonal day-length changes to Earth–Sun distance; eccentricity plays a minor role compared to axial tilt.

Final Answer:
Earth’s revolution around the Sun together with its axial tilt of about 23.5°