On the equinox, the sun is above the horizon for 12 hours, thus the length of daylight and nighttime are equal. This means every location on Earth has 12 hours of daylight and 12 hours of nighttime. If the equinox follows winter, it is called the vernal equinox and is the first day of spring. Spring is the season during the year between winter and summer.
In the Northern Hemisphere, vernal equinox occurs on March 20 or 21. In the Southern Hemisphere, vernal equinox occurs on September 22 or 23.
Seasons are opposite for Earth’s two hemispheres. If it is spring
in the Northern Hemisphere is would be autumn in the Southern Hemisphere and vise versa.
The date for the vernal equinox is different for different years. The equinox would fall on exactly the same day each year if Earth completed its orbit in exactly 365 days. But it actually takes about 365.25 days. The extra quarter of a day means that the equinoxes occur 6 hours later each year, thus the date the equinox falls on changes from year to year.
Each day following spring equinox the number of hours of daylight increases a few minutes. Finally the first day of summer arrives and it is the day with greatest amount of daylight during the year. This day is called the summer solstice.
Big Book of Science Experiments
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