The solstices and equinoxes are key points in Earth’s orbit around the Sun, and they have significant meaning when observed on the celestial sphere. Here’s how they relate:
- Equinoxes:
- Spring (Vernal) Equinox: This occurs around March 21st, when the Sun crosses the celestial equator from south to north. This marks the beginning of spring in the Northern Hemisphere and fall in the Southern Hemisphere. On this day, day and night are approximately equal in length everywhere on Earth.
- Autumnal Equinox: Occurs around September 23rd, when the Sun crosses the celestial equator from north to south, marking the start of fall in the Northern Hemisphere and spring in the Southern Hemisphere. Again, day and night are nearly equal in length.
- At both equinoxes, the Sun appears directly above the celestial equator. The celestial equator divides the sky into the northern and southern hemispheres.
- These equinoxes also mark points where the ecliptic (the Sun’s apparent path through the sky) intersects the celestial equator.
- Solstices:
- Summer Solstice: Occurs around June 21st, when the Sun reaches its highest point in the sky relative to the celestial equator. This marks the beginning of summer in the Northern Hemisphere and winter in the Southern Hemisphere. It’s the longest day of the year in the Northern Hemisphere.
- Winter Solstice: Happens around December 21st, when the Sun reaches its lowest point relative to the celestial equator, marking the start of winter in the Northern Hemisphere and summer in the Southern Hemisphere. This is the shortest day of the year in the Northern Hemisphere.
- At the solstices, the Sun is at its maximum declination, either +23.5° (for the Summer Solstice) or -23.5° (for the Winter Solstice), relative to the celestial equator.
- The Sun’s position during these times is the furthest from the celestial equator, and it marks the points where the ecliptic is farthest north and south of the celestial equator.