The vernal equinox occurs on March 20-22 and marks one of two times in the year in which the sun's rays hit Earth at 90° on the equator. The other equinox is called the autumnal equinox and takes place six months later on September 20-22.
Earth spins on an axis that it tiled at a slight angle (23.5° to be exact). Because of this tilt, different parts of the Earth receive different concentrations of sunlight at different times. During equinoxes, the entire planet receives 12 hours of daylight and 12 hours of night as the sun's rays hit Earth at 90° - however, that's not true all year.
During solstices, the sun's rays hit Earth at 90° on the tropics (23.5°N or 23.5°S). At this time, the poles either receive full sun all day or receive no sun all day, depending on the time of year.
- When the year begins, the Earth has just exited the winter solstice (Dec. 21) and is moving toward the vernal equinox. At this time, the sun's rays are hitting Earth at 90° at the Tropic of Capricorn at 23.5°S. The Antarctic circle will receive full sun all day while the Arctic circle receives no sun.
- As Earth moves toward the vernal equinox, the days grow longer in the southern hemisphere are shorter in the northern hemisphere. On March 21, the entire planet gets equal amounts of sunlight and night.
- Then, Earth starts moving toward the summer solstice. Days begin to grow longer in the northern hemisphere and shorter in the southern hemisphere. On June 21, the sun's rays hit Earth at 90° at the Tropic of Cancer at 23.5°N. At this time, the Arctic circle receives full sun and the Antarctic circle receives no sun.
- Finally, Earth moves out of the June solstice toward the autumnal equinox on September 21. At this time, the entire planet again receives equal amounts of sunlight and night.
Check out the attached video for more information on this topic and to get a visual idea of what is happening throughout the year.