Sun’s path: Understanding Solstices & Equinoxes
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Equinox and Solstice
1. Basic concepts
Earth’s tilt and revolution
• Earth’s axis is tilted at 23.5 degrees relative to its orbital plane (the ecliptic).
• This tilt revolution around the Sun causes seasons.
2. Equinox
• From Latin aequus (equal) nox (night).
• Occurs twice a year when the Sun is directly over the Equator.
• Day and night are nearly equal across the globe.
Dates:
• Spring (Vernal) Equinox: Rs March 21 (Northern Hemisphere).
• Autumnal (Fall) Equinox: Rs September 22-23 (Northern Hemisphere).
Features:
• Sun rises exactly in the east and sets exactly in the west.
• Length of day ≈ length of night (12 hrs).
• Marks change of seasons:
Vernal equinox → Spring begins in Northern Hemisphere, Autumn in Southern.
Autumn equinox → Autumn begins in Northern Hemisphere, Spring in Southern.
3. Solstice
• From Latin sol (sun) stitium (stopped).
• Occurs when the Sun is farthest north or south of the Equator.
• Marks the longest and shortest days.
Dates:
• Summer Solstice: Rs June 21 (NH has longest day, SH has shortest).
• Winter Solstice: Rs December 21-22 (NH has shortest day, SH has longest).
Features:
• Sun is overhead at Tropic of Cancer (June solstice) or Tropic of Capricorn (December solstice).
• Extreme difference in day and night length.
4. Today’s context – Autumn Equinox (Sept 22, 2025)
• On this day, the Sun is directly overhead at the Equator.
• Marks the beginning of Autumn in the Northern Hemisphere (India included).
• Southern Hemisphere enters Spring.
• Day and night are roughly equal everywhere.
• After today:
Northern Hemisphere → days grow shorter, nights longer (winter approaching).
Southern Hemisphere → days grow longer (summer approaching).
5. Geographical & astronomical significance
• Equinoxes and solstices divide the solar year into four parts.
• Used in agriculture (sowing/harvesting seasons).
• Basis for many calendars and festivals worldwide.
• Equinox is important for celestial navigation.
6. Cultural & historical significance
• Many civilisations tracked equinoxes and solstices to frame calendars.
• Stonehenge (UK) and Maya temples (Chichen Itza, Mexico) align with these solar events.
• In India, harvest festivals and rituals often align with seasonal changes.
7. UPSC relevance
• Frequently asked under geography (world & Indian physical geography).
• Linked to: Latitude & seasons, tropics and Equator significance, time and calendar systems
• Possible framing in Prelims:
“On equinox, the Sun is directly overhead at…?” (Answer: Equator).
“Longest day in Northern Hemisphere occurs on…?” (Answer: 21 June).
Mains: Seasonality, agriculture, cultural geography, environment.
Summary for UPSC:
• Equinox → Equal day & night, Sun overhead at Equator (March & Sept).
• Solstice → Longest/shortest day, Sun overhead at Tropics (June & Dec).
• Today (Sept 22, 2025) → Autumn Equinox → marks start of Autumn in NH, Spring in SH.
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