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All planets spin on tilted axes. Earth's 23.5° tilt relative to its orbital plane is the cause of our seasons. Compared to Uranus’s nearly 90° tilt or Jupiter’s modest 3°, Earth's inclination creates a pronounced seasonal cycle: each pole enjoys half the year in summer warmth and the other half in winter chill. The resulting seasonal progressions in the Northern and Southern Hemispheres mirror one another, converging on the equinoxes.
The vernal and autumnal equinoxes, when day and night are almost equal, occur each year within a roughly three‑day window, though the exact calendar date shifts. For example, the September equinox fell on September 23 in 2011 and on September 22 in 2012. The solstices exhibit a similar pattern, marking the points of greatest axial tilt relative to the Sun.
While the term "equinox" denotes a date, the underlying event—the Sun crossing the celestial equator—takes place at a single instant. Almanacs record this moment in Greenwich Mean Time (GMT) or Coordinated Universal Time (UTC). Observers around the globe must translate that universal timestamp into their local time zone to witness the event, which can occur during daylight for some regions and at night for others.
In theory, day and night should be of equal length on the equinox, but the equator never experiences perfect symmetry, and higher latitudes often find their equal lengths on days adjacent to the equinox. Two atmospheric effects cause this discrepancy. First, refraction bends sunlight, allowing the Sun to be visible before it rises and after it sets. Second, the Sun’s disk has angular width; sunrise begins when its leading edge touches the horizon and sunset ends when its trailing edge disappears. Together, these factors extend apparent daylight by more than six minutes.
