An astronomical phenomenon that occurs twice a year when the sun appears directly overhead to viewers at the Tropics of Cancer and Capricorn. Between the South Pole and the Equator, half of the Earth is covered. The Sun never sets at these locations, so this is called a "zero-hour" sun.
During the summer months, when it is warm outside, people will usually want their houses to be as cool as possible. So they'll need to make sure that the windows are closed and the air conditioner is turned on. But since the sun is always above the horizon, there's no need to worry about it causing a problem with sunset or sunrise.
At the end of the summer, when it is cold outside, people will again want their houses to be as warm as possible. So they'll need to make sure that the windows are open and the air conditioner is off.
So in conclusion, there's no need to worry about the sun causing problems for plants or animals at either end of the temperature scale.
The Year's Second Solstice The Sun shines exactly overhead the Tropic of Cancer (latitude 23deg 30' North) in the Northern Hemisphere on the June Solstice, and directly overhead the Tropic of Capricorn (latitude 23deg 30' South) in the Southern Hemisphere on the December Solstice. These are the only times each year when the Sun is directly over the geographical poles.
During the rest of the year, the Sun is at some distance below the horizon in both the Northern and Southern Hemispheres. It reaches its lowest point in relation to the Earth's surface on the March equinoxes (around 20 degrees south in the Northern Hemisphere and 20 degrees north in the Southern Hemisphere).
The word "solstice" comes from the Latin words for "sun" and "stick", because during a solstice day the rays of the rising sun are perpendicular to the earth's surface. At other times of the year, the angle is less than 90 degrees.
In addition to the two equinoxes, there are three more occasions when the Sun is directly over the poles: the Winter and Summer Solstices and the Spring Equinox. The Winter Solstice occurs on or around December 22; the Summer Solstice on or around June 21; the Spring Equinox on or around March 20th.
You may have noticed two unusual lines of latitude on a map of the world: the Tropic of Cancer in the northern hemisphere at +23.5 degrees latitude and the Tropic of Capricorn in the southern hemisphere at -23.5 degrees latitude. These are the latitudes where the sun shines directly above once a year at noon. Every region on Earth has a pair of these tropics, one for each hemisphere.
The fact that the sun appears to be located over a constant position on the horizon during the day everywhere on Earth shows us that all we have to do to see it from east to west is go around the planet about every 24 hours. The sun is always overhead at some time during the day at these locations, since it never sets anywhere on Earth!
These are important facts to know about the earth and its location with respect to the sun, but what does it mean in practice for humans living here on Earth? Well, it means that wherever you are on Earth, no matter how far north or south, east or west, you will experience sunlight during the day and darkness during the night.
In fact, if you were able to travel to either of these special places on Earth (or even closer to home!), you would find that the seasons are quite different from those back home. It's not just that it's summer there and winter here, although that is true enough.