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Solstices: The Science Behind the Longest and Shortest Days of the Year

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21-Jun-2024 | Akai Negi

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Daily Motion of the Earth

Ever wonder why we have day, night, and changing seasons? It's all due to Earth's dance moves: rotation (spinning on its axis) gives us 24-hour days, and revolution (orbiting the Sun) takes a year. The Earth's 66½° axis tilt creates our seasons, and because it's spherical, only one hemisphere is lit at a time. These movements set the rhythm of life on Earth!

Day and night happen because the Earth rotates. The side facing the Sun experiences day, while the side facing away experiences night. This boundary, called the circle of illumination, doesn't align with Earth's axis. One full rotation takes 24 hours, giving us an Earth day.

If one side of the Earth always faced the Sun, it would be unbearably hot, while the other side would be freezing. Fortunately, the Earth revolves around the Sun, taking 365¼ days to complete an elliptical orbit. This revolution, combined with the Earth's constant axial tilt, causes seasonal changes. We simplify the year to 365 days, ignoring the extra six hours. This tilt and orbit create our four seasons: spring, summer, autumn, and winter.

Solstice

On Earth, solstices are defined by solar declination—the latitude where the sun is directly overhead at noon. These events happen twice annually when the sun's declination aligns with the Tropic of Cancer in the northern hemisphere and the Tropic of Capricorn in the southern hemisphere. During the June solstice (between June 20 and June 22), the solar declination is about 23.5°N (Tropic of Cancer). During the December solstice (between December 20 and December 23), the solar declination is about 23.5°S (Tropic of Capricorn).

A solstice is an event where a planet’s poles are tilted most extremely toward or away from the star it orbits. Derived from the Latin words "sol" (sun) and "sistere" (to stand still), solstices are astronomical events that mark the two points in the year when the sun appears to stand still in its northward or southward movement before reversing direction. These events are responsible for the longest and shortest days of the year and play a crucial role in the changing seasons.

Solstices are sometimes referred to as the “summer solstice” and the “winter solstice,” though these terms vary between the Northern and Southern Hemispheres. The summer solstice marks the longest day of the year, with the most sunlight hours, while the winter solstice marks the shortest day, with the fewest daylight hours.

The June solstice marks the onset of summer in the Northern Hemisphere and winter in the Southern Hemisphere. Conversely, the December solstice signals the arrival of winter in the Northern Hemisphere and summer in the Southern Hemisphere.

Summer Solstice

During the Summer Solstice on June 21st, unique astronomical alignments create distinct seasonal changes:

  • The Northern Hemisphere tilts towards the Sun.
  • The Sun's rays directly align with the Tropic of Cancer, increasing heat in these regions.
  • Areas near the poles receive less heat due to the slanting angle of the Sun's rays.
  • The North Pole experiences continuous daylight for about six months beyond the Arctic Circle.
  • This period marks summer north of the equator, with the longest day and shortest night of the year.
  • Conversely, the Southern Hemisphere experiences winter, with longer nights and shorter days.

Winter Solstice

On December 22nd, a special alignment occurs known as the Winter Solstice:

  • The Tropic of Capricorn (23½° S) experiences direct sunlight.
  • The South Pole leans towards the Sun, causing the Sun's rays to fall vertically at the Tropic of Capricorn.
  • A significant portion of the Southern Hemisphere is illuminated, leading to summer with longer days and shorter nights.
  • Conversely, the Northern Hemisphere experiences winter, with shorter days and longer nights.

Equinox

On March 21st and September 23rd, the equinoxes mark instances when the Earth's axis is neither tilted towards nor away from the Sun, positioning the equator to receive direct sunlight. This astronomical alignment results in nearly equal durations of day and night globally, a phenomenon aptly named from the Latin "aequus" (equal) and "nox" (night).

During these times, the Sun is directly overhead at noon at the equator, spreading its rays evenly across the Earth's surface. This celestial event signifies autumn in the Northern Hemisphere and spring in the Southern Hemisphere on September 23rd, with the opposite occurring on March 21st. The equinoxes demonstrate the precise interplay of the Earth's axial tilt and orbital path around the Sun, influencing seasonal changes and the balance of daylight hours worldwide.

Solstices and Equinoxes in 2024

  • Spring equinox 2024 occurred on 20 March at 03:07 UTC
  • Summer solstice 2024 occurred on 20 June at 20:51
  • Autumn equinox 2024 to occur on 22 September at 12.44
  • Winter solstice 2024 to occur on 21 December at 09:21

Extraterrestrial Solstices

All planets in our solar system have solstices, influenced by their axial tilt, orbital eccentricity, and distance from the sun.

  • Venus, with a minimal axial tilt of just 3°, experiences little seasonal variation, and its solstices are about three months apart.
  • Mars has an axial tilt similar to Earth’s (24°) but a more elliptical orbit. This greater orbital eccentricity and axial tilt result in significant seasonal variations, with its solstices occurring approximately 11 months apart.

Cultural Significance of Solstices

Throughout history, solstices have held profound cultural significance. Ancient civilizations across the globe built monuments and devised calendars to mark these celestial events.

  • Stonehenge: One of the most famous prehistoric monuments aligned with the solstices is Stonehenge in England. This ancient stone circle is believed to have been used for ceremonial purposes, aligning with the sunrise on the Summer Solstice and the sunset on the Winter Solstice.
  • Machu Picchu: The Incan citadel in Peru is another marvel of ancient engineering, with structures that align with the solstices. The Intihuatana stone, for example, casts no shadow at noon on the day of the Winter Solstice, marking the event with precision.
  • Cultural Festivals: Many cultures celebrate the solstices with festivals and rituals. In Scandinavia, Midsummer festivals coincide with the Summer Solstice, featuring bonfires, dancing, and feasting. In contrast, the Winter Solstice is marked by celebrations such as Yule, a festival with origins in ancient Germanic traditions that has influenced modern Christmas customs.

Modern Observations and Scientific Research

In contemporary times, the solstices are still celebrated and studied extensively. Modern technology allows for precise observations and a deeper understanding of these phenomena.

  • Astronomical Observations: Scientists use advanced instruments to monitor the position of the sun and measure the exact moment of solstices. These observations help refine our understanding of Earth's orbital mechanics and contribute to more accurate calendar systems.
  • Climate Research: Understanding the solstices also aids in climate research. By studying historical climate patterns in relation to solstices, scientists can better predict future climatic changes and develop strategies for mitigating their impacts.
  • Space Missions: Space missions to other planets consider solstices when planning landings and operations. For instance, knowing the solstice dates on Mars helps mission planners understand seasonal changes and plan activities accordingly.

Conclusion

Solstices are a testament to the intricate interplay between Earth's axial tilt, orbit, and the sun's position. These astronomical events are not merely markers of the longest and shortest days but are fundamental to the rhythms of our natural world. From influencing climate and weather patterns to shaping ecosystems and inspiring cultural traditions, solstices hold a profound significance. As we continue to explore and understand our universe, the solstices remain a reminder of our planet's dynamic relationship with the sun, a relationship that governs the passage of time and the cycles of life on Earth.

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