Understanding Solstices and Equinoxes

The longest day of the year in the Northern Hemisphere occurs on Sunday, marking the solstice and the start of astronomical summer. This contrasts with the Southern Hemisphere, where Sunday is the shortest day of the year, signaling the start of winter.

The term ‘solstice’ originates from the Latin words ‘sol,’ meaning sun, and ‘stitium,’ which means pause or stop. The summer solstice represents the time when the sun reaches its highest point in the sky, creating its longest arc. After this, the sun begins to retreat, causing days to gradually shorten until late December.

Throughout history, solstices have been celebrated with festivals and monuments. For example, Sweden hosts midsummer eve celebrations, and Stonehenge was constructed to align with the sun’s paths at the solstices.

Understanding Earth’s Orbit

As Earth orbits the sun, its tilted angle causes sunlight and warmth to distribute unevenly between the northern and southern halves of the planet most of the year. Solstices occur when Earth is tilted most strongly towards or away from the sun, creating significant differences in sunlight and resulting in unequal day and night lengths.

During the Northern Hemisphere’s summer solstice, the Earth tilts towards the sun, producing the longest day and shortest night of the year. This event takes place between June 20 and 22, with June 21 being the date this year. Conversely, during the winter solstice in the Northern Hemisphere, Earth tilts furthest from the sun, leading to the shortest day and longest night. This occurs between December 20 and 23.

What is the Equinox?

An equinox occurs when Earth’s tilt is neither towards nor away from the sun, resulting in equal sunlight for both hemispheres. The sun rises due east and sets due west. The term ‘equinox’ is derived from Latin, meaning equal night, as day and night are nearly the same length, though slight variations may exist depending on geographical location.

The Northern Hemisphere experiences the fall, or autumnal, equinox between September 21 and 24, while the spring, or vernal, equinox occurs between March 19 and 21. The exact moment of an equinox is when the sun is positioned directly overhead at the Equator.

Differences Between Meteorological and Astronomical Seasons

Meteorological and astronomical seasons offer different frameworks for dividing the year. Astronomical seasons rely on Earth’s movement around the sun. Meteorological seasons are defined based on weather patterns. Meteorologists partition the year into three-month seasons aligned with temperature cycles, with spring starting on March 1, summer on June 1, fall on September 1, and winter on December 1.

The Associated Press Health and Science Department has received support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. AP is solely responsible for its content.