Zero Shadow Day - Outcome of Geophysical cycles, basis of Biological rhythms

A Zero Shadow Day (ZSD) is an astronomical phenomenon where the Sun is directly overhead (at its zenith), causing vertical objects to cast no shadow at local noon. This event is entirely driven by the geophysical cycle of Earth's revolution around the Sun and its axial tilt, occurring strictly within tropical latitudes.

Key Geophysical Drivers

• Earth’s Axial Tilt (23.5 degrees): The primary cause is the (23.5 degrees) tilt of Earth's rotation axis relative to the plane of its revolution around the Sun.
• Apparent Movement of the Sun (Uttarayan & Dakshinayan): As the Earth orbits the Sun, the Sun appears to move north and south across the sky between 23.5 degrees N (Tropic of Cancer) and 23.5 degrees S (Tropic of Capricorn) over a year.
• Declination Matching Latitude: A zero shadow day occurs when the declination of the Sun matches the latitude of a particular location

Frequency and Timing

• Occurrence: The phenomenon happens twice a year for any location within the tropics (between \(\pm 23.5^{\circ }\) latitude).
• Timing: One event occurs during the northward movement of the sun (Uttarayan, March–June) and the other during the southward movement (Dakshinayan, July–September).
• Duration: The event is extremely brief, lasting only a few minutes or seconds, and occurs exactly at local noon.
• Location Constraints: Outside the tropics (>23.5 latitude), the Sun is never directly overhead, so zero shadow days do not occur.

Ref: https://astron-soc.in/outreach/activities/zero-shadow-day/