What are the vernal and the autumnal equinoxes? What are the summer and winter solstices? How are these four points related to the ecliptic and the celestial equator?

The vernal equinox and autumnal equinox are two points in the year where the tilt of Earth's axis is neither pointing towards nor away from the Sun. During these times, both the Northern and Southern Hemispheres are receiving the Sun's rays equally. This means that day and night are approximately the same length everywhere on Earth. The vernal equinox occurs around March 20th and the autumnal equinox occurs around September 22nd. The summer solstice and winter solstice, on the other hand, occur when the Sun is at its highest or lowest point in the sky at noon. This occurs when the tilt of Earth's axis is most inclined towards or away from the Sun. The summer solstice is the longest day of the year and occurs around June 21st, whereas the winter solstice is the shortest day of the year and occurs around December 21st.

The ecliptic is the path the Sun appears to trace across the sky throughout the year. The celestial equator is the projection of Earth's equator onto the sky. The points where the ecliptic and the celestial equator intersect are the equinoxes. On these days, the Sun appears to move directly from east to west. At the solstices, the Sun appears to stand still before reversing direction. The ecliptic's maximum deviation from the celestial equator occurs at the solstices.

Revisit the definitions and relationship among vernal equinox, autumnal equinox, summer solstice, winter solstice, the ecliptic, and the celestial equator. Make sure you understand how they are linked to each other.