Sunday marks the longest day of the year in the Northern Hemisphere. This day, known as the solstice, signifies the beginning of astronomical summer for regions north of the equator. In contrast, the Southern Hemisphere experiences its shortest day, ushering in the start of winter.
The term “solstice” originates from Latin, combining “sol” for sun and “stitium” meaning pause or stop. During the summer solstice, the sun reaches its highest point in the sky, creating its longest arc. After this peak, the sun begins a gradual retreat, causing the days to shorten slightly each day until late December.
Throughout history, humans have celebrated solstices with festivals and monuments. Examples include Sweden’s midsummer eve celebrations and Stonehenge, built to align with the sun during the solstices.
Earth’s Orbit and Sunlight Distribution
As Earth orbits the sun, it tilts at an angle, causing uneven distribution of sunlight between the northern and southern halves for most of the year. Solstices occur when Earth’s tilt is most extreme, either toward or away from the sun. This results in significant differences in sunlight and the most unequal distribution of day and night hours.
During the Northern Hemisphere’s summer solstice, the Earth’s tilt favors the sun, creating the longest day and the shortest night. This event occurs between June 20 and 22; this year, it is on June 21. The winter solstice occurs when the upper half of the Earth leans farthest from the sun, resulting in the shortest day and longest night, falling between December 20 and 23.
The Equinox Explained
Equinoxes occur when the Earth’s tilt is neutral relative to the sun, leading to equal sunlight distribution across both hemispheres. The sun appears to rise due east and set due west. The word “equinox” derives from Latin, meaning equal night. On the equinox, day and night are almost equal in duration, with slight variations depending on location.
The Northern Hemisphere’s autumnal equinox can occur between September 21 and 24, while the vernal equinox can occur between March 19 and 21. The equinox is marked by the moment when the sun is directly overhead at the equator.
Comparing Meteorological and Astronomical Seasons
Astronomical seasons are defined by Earth’s movements around the sun, while meteorological seasons are based on weather patterns. Meteorologists divide the year into three-month seasons according to temperature cycles. This calendar sets spring to start on March 1, summer on June 1, fall on September 1, and winter on December 1.
The Associated Press Health and Science Department receives backing from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP remains solely accountable for all content.
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