Earth’s unique characteristics

• Liquid water at the surface

  • Earth has lakes, rivers, and oceans of water. Unfortunately, it also has tsunamis and hurricanes. The oceans cover 70% of Earth's surface.

• Plentiful amounts of oxygen in the air

  • No other planet's atmosphere contains more oxygen than Earth's, which has a 21% oxygen content.

• Plate tectonics

  • Huge moving rock plates make up the crust of the Earth; when these plates collide, earthquakes take place, and new mountains develop. The seafloor spreads as new crust forms at the midocean ridges, far below the water.

• Active volcanoes

  • Huge volcanic landforms like the Hawaiian Islands are created by hot, molten rock that rises from below the earth's surface. Every day, a volcano erupts somewhere on the planet.

• Life, intelligent or otherwise

  • You be the judge of intelligence, but the Earth is abundant with life, ranging from single-celled amoebas, bacteria, and viruses to flowers, trees, fish, birds, insects, and mammals.

Northern Lights

• Auroras emerge when electron fluxes from the magnetosphere of the Earth fall on the atmosphere below, causing oxygen and other atoms to shine.

• Glow: The simplest aurora. It looks like thin clouds are reflecting moonlight or city lights.

• Arc: Shaped like a rainbow but with no sunlight to make one. A steady or pulsating green arc is the most common type of arc, but sometimes dim red arcs appear.

• Curtain: Also called drapery. This spectacular aurora resembles a billowing curtain at a theater, but nature is the star of the show.

• Rays: One or more long, thin bright lines in the sky, appearing like faint beams from the heavens.

• Corona: High overhead, a crown in the sky, with rays emanating in every direction.

Earth’s Regions

• Lithosphere: Our planet's rocky terrain.

• Hydrosphere: The water in the oceans, lakes, and elsewhere on Earth.

• Cryosphere: The frozen regions — Antarctica and Greenland’s ice caps.

• Atmosphere: The air from ground level up to hundreds of miles.

• Biosphere: All living things on Earth.

• Magnetosphere: Plays a vital role in protecting Earth from many of the dangerous emanations from the Sun.

  • Earth’s Radiation Belt: Regions where electrically charged particles — bounce back and forth above Earth, trapped in its magnetic field.

  • Geomagnetic Field: Moving streams of molten iron in the outer core generate a magnetic field that reaches out through the whole planet and far into space.

  • Cosmic Rays: High-speed, high-energy particles that come from explosions on the Sun and from distant points in space.

Geomagnetic Field

• Makes the needle of a compass point toward north (or south).

• Provides an invisible guidance system for homing pigeons, some migratory birds, turtles, salmon, various ants and bees (among other bugs), and even some ocean-dwelling bacteria.

• Forms the magnetosphere far above Earth.

• Shields Earth from incoming electrically charged particles from space, such as the solar wind and many cosmic rays.

Earth’s Time, Seasons, and Age

• The rotation of Earth was the original basis of our system of measuring time, and we now know that the orbital motion of Earth and the tilt of its axis produce the seasons.

• Mean Solar Time: The length of the day, 24 hours, is the average time it takes for the Sun to rise and set and rise again.

  • A year consists of approximately 365 days, the time that it takes Earth to make one complete orbit around the Sun.

• Sidereal Days: Earth turns once in 23 hours, 56 minutes, and 4 seconds concerning the stars.

• Sidereal Clocks: Measures sidereal time by registering 24 sidereal hours during an interval of 23 hours, 56 minutes, and 4 seconds of mean solar time.

• Universal Time (UT): Also known as Greenwich Mean Time (GMT); Common standard time, and simply the standard time at Greenwich, England.

• Coordinated Universal Time (UTC): The official international standard.

• Axis: The line through the North and South poles; it isn’t perpendicular to the plane of Earth’s orbit around the Sun.

  • It is tilted by 23 ½° from the perpendicular to the orbital plane.

• Polaris: The north star; the star Alpha Ursae Minoris, located in the Little Dipper asterism of the Little Bear constellation, Ursa Minor.

• Vernal equinox: On the first day of spring, the Sun crosses from “below” (south) the equator to “above” (north).

• Summer solstice: The Sun reaches the farthest point north on the ecliptic.

• Autumnal equinox: The Sun crosses the equator going back down south, and fall begins.

• Winter solstice: The Sun gets as far south as possible on the ecliptic.

• Radioactive Isotopes: It turns into another isotope of the same element, or into a different element, at a rate determined by the half-life of the radioactive substance. If the half-life is 1 million years.

• Parent Atoms: The original radioactive isotope atoms.

• Radioactive Dating: An approach to date minerals and rocks that makes use of radioactive isotopes.

• Volcanism: The eruption of molten rock from within Earth, including the formation of new volcanoes.

• Meteorites yield radioactive dates as old as 4.6 billion years. These are considered debris from asteroids, and asteroids are thought to be debris from the very early solar system, when the planets first formed.