These motions explain such phenomena as the day, the year, seasons, phases of the moon, eclipses, and tides.
Gravity influences the motions of celestial objects. The force of gravity between two objects in the universe depends on their masses and the distance between them.
The orbit of each planet is an ellipse with the Sun located at one of the foci.
Earth is orbited by one moon and many artificial satellites.
Earth's coordinate system of latitude and longitude, with the equator and prime meridian as reference lines, is based upon Earth's rotation and our observation of the Sun and stars.
Earth rotates on an imaginary axis at a rate of 15 degrees per hour. To people on Earth, this turning of the planet makes it seem as though the Sun, the moon, and the stars are moving around Earth once a day. Rotation provides a basis for our system of local time; meridians of longitude are the basis for time zones.
The Foucault pendulum and the Coriolis effect provide evidence of Earth's rotation.
Earth revolves around the Sun with its rotational axis tilted at 23.5 degrees to a line perpendicular to the plane of its orbit, with the North Pole aligned with Polaris.
During Earth's one-year period of revolution, the tilt of its axis results in changes in the angle of incidence of the Sun's rays at a given latitude; these changes cause variation in the heating of the surface. This produces seasonal variation in weather.
Seasonal changes in the apparent positions of constellations provide evidence of Earth's revolution.
The Sun's apparent path through the sky varies with latitude and season.
Approximately 70 percent of Earth's surface is covered by a relatively thin layer of water, which responds to the gravitational attraction of the moon and the Sun with a daily cycle of high and low tides.
cosmic background radiation
a red-shift (the Doppler effect) in the light from very distant galaxies.
The stars differ from each other in size, temperature, and age.
Our Sun is a medium-sized star within a spiral galaxy of stars known as the Milky Way. Our galaxy contains billions of stars, and the universe contains billions of such galaxies.
The characteristics of the planets of the solar system are affected by each planet's location in relationship to the Sun.
The terrestrial planets are small, rocky, and dense. The Jovian planets are large, gaseous, and of low density.
Impact events have been correlated with mass extinction and global climatic change.
Impact craters can be identified in Earth's crust.
Earth's early atmosphere formed as a result of the outgassing of water vapor, carbon dioxide, nitrogen, and lesser amounts of other gases from its interior.
Earth's oceans formed as a result of precipitation over millions of years. The presence of an early ocean is indicated by sedimentary rocks of marine origin, dating back about four billion years.
Water is returned from the atmosphere to Earth's surface by precipitation. Water returns to the atmosphere by evaporation or transpiration from plants. A portion of the precipitation becomes runoff over the land or infiltrates into the ground to become stored in the soil or groundwater below the water table. Soil capillarity influences these processes.
The amount of precipitation that seeps into the ground or runs off is influenced by climate, slope of the land, soil, rock type, vegetation, land use, and degree of saturation.
Porosity, permeability, and water retention affect runoff and infiltration.
The evolution of life caused dramatic changes in the composition of Earth's atmosphere. Free oxygen did not form in the atmosphere until oxygen-producing organisms evolved.
Fossil evidence indicates that a wide variety of life-forms has existed in the past and that most of these forms have become extinct.
Human existence has been very brief compared to the expanse of geologic time.
The characteristics of rocks indicate the processes by which they formed and the environments in which these processes took place.
Fossils preserved in rocks provide information about past environmental conditions.
Geologists have divided Earth history into time units based upon the fossil record.
Age relationships among bodies of rocks can be determined using principles of original horizontality, superposition, inclusions, cross-cutting relationships, contact metamorphism, and unconformities. The presence of volcanic ash layers, index fossils, and meteoritic debris can provide additional information.
The regular rate of nuclear decay (half-life time period) of radioactive isotopes allows geologists to determine the absolute age of materials found in some rocks.
These plate boundaries are the sites of most earthquakes, volcanoes, and young mountain ranges.
Compared to continental crust, ocean crust is thinner and denser. New ocean crust continues to form at mid-ocean ridges.
Earthquakes and volcanoes present geologic hazards to humans. Loss of property, personal injury, and loss of life can be reduced by effective emergency preparedness.