A shadow is a dark shape created when an opaque object blocks light rays. For objects on Earth, the sun is the primary light source, and its apparent position in the sky dictates the length and orientation of the resulting shadow. Shadow length variability results from two astronomical cycles: the Earth’s daily rotation and its yearly orbit around the sun. These cycles cause the sun’s angle above the horizon to constantly change, determining how long or short a shadow appears.
The Role of Solar Elevation
The fundamental principle governing shadow length is solar elevation, the angle of the sun above the horizon. This angle determines how directly the sun’s rays strike an object and the ground. The relationship is inversely proportional: as the angle increases, the shadow length decreases.
When the sun is low on the horizon, light rays hit an object at a shallow, oblique angle, causing the shadow to stretch out significantly. Conversely, when the sun is higher overhead, the rays strike the object and the ground more directly. This results in a shorter shadow because the object blocks a smaller horizontal area of light. This geometric relationship is measurable using trigonometry.
Shadow Length by Time of Day
The daily rotation of the Earth provides a consistent cycle of shadow lengthening and shortening. Shadows reach their longest length immediately after sunrise and just before sunset. At these times, the sun is at its lowest possible elevation, often near zero degrees above the horizon, causing light to strike at a shallow angle.
The shortest shadow of the day occurs at “true solar noon.” Solar noon is the instant the sun reaches its maximum altitude, or highest point, in the sky. At this point, the sun’s rays are hitting the earth most directly, minimizing the length of the shadow. This time rarely aligns perfectly with 12:00 PM due to factors like time zone boundaries and the “equation of time.”
Shadow Length by Time of Year
While the daily cycle dictates the shortest and longest shadows within a 24-hour period, the annual cycle determines the overall shortest and longest shadows of the entire year. This seasonal variation is caused by the Earth’s fixed axial tilt of approximately 23.5 degrees as it orbits the sun. This tilt changes the maximum solar elevation the sun can achieve at solar noon throughout the year.
The longest noon shadow of the year occurs on the Winter Solstice, which falls around December 21st in the Northern Hemisphere. On this day, the sun’s path across the sky is at its lowest annual altitude, meaning even at solar noon, the sun is lower than on any other day. Conversely, the shortest noon shadow of the year is cast on the Summer Solstice, around June 21st in the Northern Hemisphere. This is when the sun reaches its maximum annual altitude at solar noon, resulting in the most direct overhead angle and the year’s minimum shadow length.