On the December solstice, the sun is directly overhead at the Tropic of Capricorn. On the June solstice, it is bright 24 hours a day south of the Antarctic Circle (66.5 degrees north). The sun is 47 miles from the zenith at midday. On the March equinox, the sun is directly over the South Pole for half of the planet; for the other half, it's in the night sky.
The Antarctic Circle divides the continent of Antarctica into two regions: the southern summer and the northern winter. All seasons are considered part of the Antarctic environment, but temperatures vary with the location of one's distance from the ice sheet. Average annual temperature is -3 degrees Celsius (27 degrees Fahrenheit), but it can reach as high as +15 degrees C (59 degrees F) near the coast and drop as low as -180 degrees C (320 degrees F) inside the Antarctic ice cap.
Scientists have learned a lot about Earth's climate by studying data collected by instruments on board ships that have traveled to various parts of the world's oceans. But because there is no land surface to reflect light away from the planet or a atmosphere to filter out solar radiation, scientists believe that any attempt to send humans to Mars would require the use of technology that could protect them from these dangers.
The ship is probably the most common way that scientists study Earth's climate.
The sun is straight above Capricorn's Tropic of Capricorn (the 23.5 degree S parallel of latitude). On this day, the North Pole has 24 hours of darkness, while the South Pole has 24 hours of daylight. Spring officially begins in the Northern Hemisphere on March 20 or 21, when the Earth is neither inclined toward or away from the sun. In other words, the vernal equinox has arrived. The days are getting longer and the snow is beginning to melt around the pole.
As spring moves north, the days continue to get longer, but more importantly, the nights start to shorten. By July 4, the summer solstice has arrived at both the North and South Poles. It is now light all day and night at the North Pole, and dark all day and night at the South Pole.
During the summer months, no part of Antarctica experiences permanent night. At both the South Pole and the North Pole, the sun will set every night and not rise again until late January or early February. However, at the South Pole, this period of nighttime lasts for about six months, while at the North Pole it only takes three weeks before the days begin to grow longer again.
The winter season at both the South Pole and the North Pole is also known as "dark" because it is during these months that the sun is below the horizon for most of the time.
On the equinoxes, the equator is the circle where the sun is directly above at midday. The Arctic and Antarctic Circles intersect at +66.8 degrees latitude. This implies that the sun will not be visible from the Arctic Circle on December 21, when it is directly above the Tropic of Capricorn at noon. It will only be slightly below the horizon at noon.
In contrast, the Antarctic Circle is at -60.0 degrees latitude: the sun is completely hidden below the southern hemisphere at midday on January 1.
These are the only times per year when the sun is directly over the equatorial plane at midday. At other times of the year, the sun is either higher in the sky than the equator, or lower. In fact, it's always higher in the northern hemisphere during the summer months, and always lower in the south during the winter months.
The reason for this is simple geometry: if you take a vertical slice through the Earth's center, there must be a distance between the two sides of the slice. The only way to do this is with a curve: the Earth is spherical, so its surface is curved like a ball. If you stare at a map of the world and try to find the exact place on Earth's surface where the lines separating the different countries are exactly perpendicular to the ground, you'll see that they don't meet at one point but instead form a circle called the Equator.
Six months later, the South Pole is tilted at approximately 23.4 degrees away from the Sun. The Sun's vertical overhead rays move to their northernmost location, the Tropic of Cancer (23deg27' N), on this day of the winter solstice in the Southern Hemisphere. At the same time, its southernmost location, the Tropic of Capricorn (15deg18' S), is reached on the summer solstice in the Northern Hemisphere.
The angle that the Sun makes with the horizon varies throughout the day at the South Pole. It reaches a maximum of 90 degrees in mid-day sun, and a minimum of 3.5 degrees in early morning and late evening when the sky is still dark.
The distance between the Earth and the Sun is called an astronomical unit (AU). At the average distance of the Earth from the Sun (1 AU), the full Moon appears about half-full because the far side of the Moon is always facing us.
At the South Pole, the astronomical unit is a much longer number because it refers to the distance between the center of the Earth and the center of the Sun. From the surface of the Earth, the Sun looks like it is sitting directly above the center of the Sun, but actually it is about 865,000 miles (1440,000 km) away. The distance between the Earth and the Sun is called a solar radius (Rsun).
During the summer solstice on December 21st, the Antarctic Circle sees a 24-hour period where the sun is above the horizon. The cause of this phenomena is that the Earth's axis is inclined by 23.5 degrees. This means that at the South Pole, the sun is directly over the equator, which is why it can rise at its highest point and set at its lowest point during the summer months.
During the winter solstice on June 22nd, the Antarctic Circle experiences its opposite condition: night follows day, and the sun is below the horizon for most of the day.
The reason for this is the same as the previous case: the Earth's axis is tilted by 23.5 degrees from the vertical direction. However, due to the fact that the South Pole is located in Antarctica, where there is no daylight during the winter months, it stays under the shadow of the Earth for most of the year. Only in the spring and autumn do short periods of sunlight reach the South Pole.
An interesting fact about the Antarctic Circle is that it does not follow the exact same path each year. As the ice in Antarctica moves around due to wind, rain, and other factors, so too does the location of the Antarctic Circle. If you were to travel there right now, the line would be going up or down depending on the current position of the ice.