Astronomy Basics
Why Earth Has Seasons, Solstices, and Equinoxes
See how Earth's tilted axis changes daylight and why the two hemispheres have opposite seasons.
Prepared with AI assistance using the primary sources linked below. How Astroboat works
The key is Earth's tilted axis
Earth's rotation axis is tilted by about 23.4 degrees relative to the perpendicular to its orbital plane. As Earth travels around the Sun, that tilt changes how directly sunlight reaches each hemisphere and how long the Sun remains above the horizon.
When the Northern Hemisphere tilts toward the Sun, it has longer days and more direct sunlight, while the Southern Hemisphere has the opposite conditions. Seasonal heating is not mainly caused by Earth moving closer to or farther from the Sun.
Solstices and equinoxes mark the geometry
At a solstice, the Sun reaches its farthest apparent north or south position in the yearly cycle. At an equinox, it crosses the celestial equator. These are shared astronomical moments, but their local calendar dates can differ by time zone.
Day and night are not exactly twelve hours everywhere on an equinox. Atmospheric refraction, the Sun's apparent disk, and the definition of sunrise and sunset shift the observed balance. Latitude also changes the size of the seasonal variation in daylight.
Notice the change from one place
Pick a safe, fixed viewpoint and note the direction of sunset against a distant landmark over several weeks without looking directly at the Sun. Compare the length of your evening observing window in different months.
Treat seasonal sky guides as location-dependent. A winter constellation description written for Europe does not automatically describe the season in Australia. Use Astroboat's calendar for dates and a chart set to your location for the actual sky. Keep local weather seasons separate from these astronomical definitions.