One Earth day is about 24 hours, or 86, seconds, long. Over the course of a year, its length varies by about one millisecond, or 1, microseconds, due to seasonal variations in the planet's mass distribution such as the seasonal shift of the jet stream. The initial data suggests Friday's earthquake moved Japan's main island about 8 feet, according to Kenneth Hudnut of the U. Geological Survey. The Earth's figure axis is not the same as its north-south axis in space, which it spins around once every day at a speed of about 1, mph 1, kph.
The figure axis is the axis around which the Earth's mass is balanced and the north-south axis by about 33 feet 10 meters. This isn't the first time a massive earthquake has changed the length of Earth's day. Major temblors have shortened day length in the past. Gross said that while we can measure the effects of the atmosphere and ocean on Earth's rotation, the effects of earthquakes, at least up until now, have been too small to measure.
The computed change in the length of day caused by earthquakes is much smaller than the accuracy with which scientists can currently measure changes in the length of the day.
However, since the position of the figure axis can be measured to an accuracy of about 5 centimeters 2 inches , the estimated centimeter shift in the figure axis from the Japan quake may actually be large enough to observe if scientists can adequately remove the larger effects of the atmosphere and ocean from the Earth rotation measurements.
He and other scientists will be investigating this as more data become available. Gross said the changes in Earth's rotation and figure axis caused by earthquakes should not have any impacts on our daily lives. There's a problem with your browser or settings. Follow this link to skip to the main content. After the solstice, it begins moving north again.
Since the Northern Hemisphere is tilted away from the Sun in December, it receives less sunlight during the course of a day.
At the solstice, the North Pole's tilt away from the Sun is greatest, so this event marks the shortest day of the year north of the equator. This effect is greatest in locations that are farther away from the equator.
In tropical areas, the shortest day is just a little shorter than 12 hours; in the temperate zone, it is significantly shorter; and places within the Arctic Circle experience polar night , when the Sun does not rise at all. Conversely, the day of the December solstice is the longest day of the year in the Southern Hemisphere.
Here, too, the effect is greater the farther a location is away from the equator. Sun times at the South Pole in December. During the course of a year, the subsolar point—the spot on the Earth's surface directly beneath the Sun—slowly moves along a north-south axis. Having reached its northernmost point at the June solstice , it starts moving southward until it crosses the equator on the day of the September equinox.
At the December solstice, which marks the southernmost point of its journey, it stops again to start its journey back toward the north. In the months leading up to the December solstice, the position of sunrise and sunset creeps southward.
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