EARTH STATS Statistics | Atmosphere | Seasons EARTH STATISTICS Mass (kg) . . . . . . . 5.97e24 Mass (Earth = 1) . . . . . 1.000 Equatorial radius (km) . . . . 6,378 Equatorial radius (Earth = 1) . . 1.000 Mean Density (gm/cm^3) . . . . 5.52 Rotational period (hours) . . . 23.9345 Tilt of axis (degrees) . . . . 23.45 ATMOSPHERE Nitrogen . . . . . . . 78% Oxygen . . . . . . . . 21% Carbon Dioxide . . . . . . 0.03% Water Vapor . . . . . . . up to 4.0% The Earth's hydrosphere, or water layer, is what makes our planet unique in the solar system. Approximately three quarters of Earth's surface is covered by liquid water. Some of this water evaporates and condenses around dust, salt, or pollen grains that are blown into the atmosphere. These condensation nuclei are the beginnings of our clouds. Cloud are classified by their color, shape, and altitude. Most clouds consist of condensed water vapor - in other words, liquid water. Clouds that form very high in the sky, however, tend to form from ice crystals. These clouds are cirrus clouds. They are very thin, wispy clouds. Clouds play an important role for our terrestrial readers. They supply rain and snow and help keep in the infrared heat that is radiating from earth. Carbon dioxide also helps trap the heat and keep it in the atmosphere. This is called the greenhouse effect. Clouds and carbon dioxide in our atmosphere help moderate the daily temperature fluctuations on Earth. Daily temperature changes are at an extreme in deserts. The difference between the hot days and cool nights in these areas is because there are very few clouds and little water vapor in the air to keep in the heat. Usually you can see a sharp line of clouds in satellite images. This line is called a front, and is a boundary between different types of air (for example: hot/dry meeting cool/moist). The passing of a front usually means a change in weather. SEASONS Since the orbit of the Earth around the sun is almost circular, the distance from the sun is nearly constant and is NOT a factor in causing seasons on Earth. Seasons on earth result because Earth is tilted on an axis. Since it is tilted, one pole will point toward the sun as it revolves in its orbit. When the North Pole is tilted toward the sun it is summer in the northern hemisphere. At this time the North Pole receives 24 hours of daylight. The South Pole is in darkness for 24 hours. The sun's direct rays are shining at 23.5 degrees latitude, the Tropic of Cancer. On the first day of summer the sun is directly overhead here. The first day of summer for the northern hemisphere is called the summer solstice. It occurs June 21 or 22. This has the longest number of daylight hours in the North. Summer officially lasts until the first day of autumn. The first day of autumn for the northern hemisphere is called the autumnal equinox. It occurs September 22 or 23. At this time the sun is directly overhead at the equator, and day and night are equal length. Autumn officially ends in December. The first day of winter for the northern hemisphere is called the winter solstice. It occurs December 21 or 22. This day has the shortest number of daylight hours in the North. The sun at this time is shining overhead at 23.5 degrees south latitude, at the Tropic of Capricorn. Winter ends in March. The first day of spring for the northern hemisphere is called the vernal equinox. Because of the tilt of Earth and its revolution around the sun, the sun's direct rays are once again at the equator and day and night are equal length. This occurs March 20 or 21.