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Earth Earth is the only planet we know that has life on it. Earth is a surface planet. We are stuck inside a huge gravity well that can kill (by a fall); Earth has the greatest gravity of any surface planet. Greatest density of any planet -- 5.5. Density can be thought of as the amount of weight of an object in a certain volume. For example, a lead ball has a high density, but a chunk of balsa wood has a low density. Mercury's (5.427 density) could have been the most dense planet if it were not for Earth's higher internal pressures (due to its greater size) compressing the material inside. Earth may be the roundest planet in the solar system. Oblateness measures the amount flattening of a globe due to its rotation pulling material closer to the equator. Earth has a oblateness of 0.00335. Earth is the only planet known to have an ocean on its surface. Titan may also have one, but we don't know for sure. Somewhere in the interior of most planets and large moons, there is a liquid region deep within the astronomical body. With some worlds we may not need to look far under the surface to find liquid water such as on Enceladus of Saturn or Europa of Jupiter. Enceladus may have even had water volcanos. Europa of Jupiter, may also have an ocean of water underneath its crust. Earth is the only surface planet to have nearly equal tides from two different sources. The tides from the Moon are slightly more than twice the force of the tides from the Sun. Earth is the only known world with plate tectonics. Plates are large land masses that move around and form volcanos and earthquakes at their boundaries. A meteorite bounced off the atmosphere on August 10, 1972. At magnitude -15, 80 m in diameter, it was detected at a height of 76 km, reaching a low altitude of 58 km and becoming undetectable at just over 100 km over Alberta being visible for 1 minute 41 seconds. If the meteorite was just 50 to 100 km nearer, it would have caused considerable damage. The greatest 'cometary' shower (with no associated meteor shower) was on October 9, 1933 when the recent passage of Comet P/Giacobiov-Zinner produced a high count of 350 meteors/minute. The greatest meteor shower is the Leonids. Normally it is a regular shower, but sometimes every 33 years it is something more. On November 17, 1966 some observers in the central USA saw about 5000 meteors in twenty minutes. |