Earth is not flat, but it's not perfectly round either


Earth has never been perfectly round. The planet bulges around the equator by an extra 0.3 percent as a result of the fact that it rotates about its axis. Earth's diameter from North to South Pole is 12,714 kilometers (7,900 miles), while through the equator it is 12,756 kilometers (7,926 miles). The difference — 42.78 kilometers (26.58 miles) — is about 1/300th the diameter of Earth. This variation is too tiny to be seen in pictures of Earth from space, so the planet appears round to the human eye. Recent research from NASA's Jet Propulsion Laboratory suggests that melting glaciers are causing Earth's waistline to spread.

The days are getting longer


The length of Earth's day is increasing. When Earth was formed 4.6 billion years ago, its day would have been roughly six hours long. By 620 million years ago, this had increased to 21.9 hours. Today, the average day is 24 hours long, but is increasing by about 1.7 milliseconds every century. The reason? The moon is slowing down Earth's rotation through the tides that it helps create. Earth's spin causes the position of its tidal ocean bulges to be pulled slightly ahead of the moon-Earth axis, which creates a twisting force that slows down Earth's rotation. As a result, our day is getting longer — but not long enough to make a difference to your busy schedule.

There weren't always several continents


Earth's continents have had an on-again, off-again relationship that has lasted for millions of years. Some 800 million years ago the great tectonic plates that Earth's land masses ride upon came together, assembling the continents into a large supercontinent called Rodinia; what is now North America lay at the center of it. Rodinia eventually broke apart into many pieces that re-collided 250-500 million years ago, creating the Appalachian Mountains in North America and the Ural Mountains in Russia and Kazakhstan.

About 250 million years ago, the continents came together once again to form another supercontinent called Pangaea, surrounded by a single, worldwide ocean. Fifty million years later, Pangaea began to break apart. It split into two large land masses — Gondwanaland and Laurasia — that ultimately fragmented into the continents we know today.

Earth's icy times


About 600-800 million years ago, Earth underwent several extreme climate changes known as ice ages. The climate became so cold that some scientists believe Earth nearly or completely froze several times; this is known as the "snowball Earth" theory. There may have been four such periods of alternate freezing and thawing, triggered by reductions in greenhouse gases such as methane and carbon dioxide, during which Earth would have been covered by glacial ice from pole to pole. Because most of the sun's energy would have been reflected back into space by ice, the planet's average temperature would have been about -50 degrees Celsius (-74 degrees Fahrenheit), with the equator akin to Antarctica today. If snowball Earth did exist — a point that is hotly contested — luckily we weren't around to feel the chill, as only microscopic and simple organisms existed then.

The driest place on Earth


Ironically, the driest place in the world — the Atacama Desert in northern Chile — is next to the biggest body of water — the Pacific Ocean. Average annual rainfall in Arica, Chile, is just 0.8 millimeters (0.03 inches). It is believed that Atacama's Calama city saw no rain for 400 years until a sudden storm fell in 1972. Unlike most deserts, the Atacama is relatively cold and, in its most arid parts, does not even host cyanobacteria — green photosynthetic microorganisms that live in rocks or under stones. NASA astrobiologists travel to the Atacama to look for microorganisms that live in such an extreme environment, hoping to learn how life might exist on other planets.

Earth's gravity isn't uniform


If Earth were a perfect sphere, its gravitational field would be the same everywhere. But in reality, the planet's surface is bumpy, and water flow, ice drift and the movement of the tectonic plates beneath Earth's crust all change the pull of gravity. These variations are known as gravity anomalies. A mountain range such as the Himalayas causes a positive gravity anomaly — gravity is stronger there than it would be on a featureless perfectly smooth planet. Conversely, the presence of ocean trenches, or dips in the land caused by glaciers millennia ago, leads to negative gravity anomalies. NASA's GRACE (Gravity Recovery and Climate Experiment) mission, orbiting above us, is mapping Earth's gravitational field in unprecedented detail.

In the past, sea levels were very different


The most recent advance of ice on planet Earth began about 70,000 years ago, ended 11,500 years ago and reached its farthest extent 18,000 years ago. During this time, glaciers and sheets of ice carved out the basins of the Great Lakes and blocked rivers, diverting the courses of the Mississippi and other rivers in the U.S. So much water was trapped as ice that sea levels dropped by as much as 120 meters (390 feet), exposing parts of what is now the ocean floor. Earth's sea level has also been up to 70 meters (230 feet) higher in the past. During the last interglacial period, the sea was actually 5 to 7 meters (16 to 23 feet) higher than it is today.

Our sun has a voracious appetite


All stars, like our sun, age and eventually die. As the sun exhausts its supply of hydrogen, it will collapse under gravity, ultimately ballooning into a red giant that is 100 times bigger and 2,000 times more luminous, vaporizing Earth in the process. But don't worry; it won't happen for about five billion years.

One option is to leave the planet before this happens, but that would require as yet unimagined technology and a habitable destination. The other possibility is that, over the next few billion years, a passing star could disrupt Earth's orbit and kick it away from the sun. Scientists have suggested the odds of this are one in 100,000 — better than winning the lottery. Unfortunately, left without a sun our descendants would probably end up freezing to death.

The moon is not Earth's only companion


There are two other bodies orbiting near Earth that are sometimes referred to as moons, though they are not strictly worthy of the title. Discovered in 1986, 3753 Cruithne is an asteroid that actually orbits the sun. Since it takes the same amount of time to orbit the sun as Earth, it looks as if Cruithne is following our planet. Its orbit, when seen from the perspective of Earth, appears bean-shaped. Asteroid 2002 AA29 also orbits the sun once a year, following a more bizarre horseshoe-shaped path that brings it close to Earth (within about 5.9 million kilometers or 3.7 million miles) every 95 years. Because of its proximity to us, scientists have suggested collecting samples from AA29 and bringing them back to Earth.

The calm before the storm


It's not just an old wives' tale: under the right conditions, the calm before the storm really does exist. As a storm draws in warm, moist air — its fuel — from the surrounding atmosphere, it leaves a low-pressure area behind. Air is carried up into the storm cloud, and some of it is forced upwards by powerful drafts. These updrafts remove the hot air and push it out over the sides of the highest storm clouds, which can be up to 16 kilometers (10 miles) high. As the air then descends, it becomes warmer and drier and therefore more stable. It blankets the region below and stabilizes the air contained within, causing people within that region to notice a calm before the storm.