(a) at what times of year are the periods of daylight and darkness the same everywhere on the earth? What are these times called? (b) Where is the noon sun directly overhead at these times?
The word, "equinox," means equal nights. The vernal equinox is around March 21, and the autumnal equinox is around September 21.
The noon sun is directly over the Equator at the equinoxes.
(a) The times of year when the periods of daylight and darkness are approximately the same everywhere on Earth are called the equinoxes. There are two equinoxes in a year: the March equinox, which occurs around March 20th or 21st, and the September equinox, which occurs around September 22nd or 23rd. During these equinoxes, the length of day and night is nearly equal across the entire planet, with roughly 12 hours of daylight and 12 hours of darkness.
To understand why the periods of daylight and darkness are the same during the equinoxes, it's helpful to consider the tilt of Earth's axis. Earth's axis is tilted by about 23.5 degrees relative to its orbit around the Sun. This tilt creates the changing seasons and the difference in daylight hours throughout the year. However, during the equinoxes, the tilt of Earth's axis is not oriented towards or away from the Sun, resulting in equal day and night lengths.
(b) During the equinoxes, the noon sun is directly overhead at the Earth's equator. The equator is an imaginary line that divides the Earth into the Northern and Southern Hemispheres. Since the equinoxes mark the time when the Sun is crossing the equator, the noon sun is at its highest point directly above the equator during these times.
It's important to note that the position of the noon sun varies throughout the year. During the equinoxes, it is directly overhead at the equator. As the seasons change, the noon sun gradually shifts its position between the Tropic of Cancer and the Tropic of Capricorn, which are located at about 23.5 degrees north and south of the equator respectively. This shifting of the noon sun's position is responsible for the differences in daylight hours at different latitudes throughout the year.