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FAQ - Earth

1. How fast is the Earth's rotation at Tucson, AZ?

Tucson’s latitude is about 32.1 degrees. So, while the speed at the equator is about 1038 miles/hour at the equator, it is about 879 miles/hour at the latitude of Tucson.

2. For how long have we known that Earth’s axis and rotation change with each major earthquake? Do other geological activities, like volcanoes, affect tilt and rotation?

While Newton predicted the wobble of the Earth (because it is not a perfect sphere) more than 300 years ago, the wobble was not actually measured until 1891 by Seth Chandler. There isnt a reliable source that discusses the effects due to earthquakes. Also, be aware that the numbers quoted in newspapers are calculations, not actual measurements. One could assume that only the largest volcanic eruptions in Earth's history would cause polar changes. It is important to remember that this is actually a motion of the crust of the Earth relative to the spin axis of the Earth, not a real shift in the Earth’s axis.

3. Does Earth’s orbit speed up and slow down when its closer/farther from the Sun, like the Moon’s orbit?

Yes! Earth orbits the Sun at an average distance of 149.6 million kilometers every 365.2564 days. The orbital speed of the Earth averages 29.78 km/s (107,200 km/hr), which is fast enough to move its own diameter length (about 12,600 km) in seven minutes (and the distance to the Moon (384,000 km) in four hours-- taken, in part from Wikipedia). On January 3, 2011, when the Earth was at its closest to the Sun (perihelion, 147.1 million kilometers), it was traveling at 30.29 km/s. On July 4, 2011, when the Earth was farthest from the Sun (aphelion, 152.1 million kilometers) it was traveling at 30.29 km/s.

To further the discussion on a slightly more advanced level-- recall the motion of the Moon. Because of its elliptical orbit, it is moving faster when closest and slowest when farthest, but its rotation rate is constant. Therefore, we see more than half of the Moon (the rotation falls behind when the Moon is closest, but ahead when farthest). Similarly, when the Earth is closest to the Sun, the rotation of the Earth falls behind and it takes a little longer for the Sun to go from noon to noon (the day is slightly longer than 24 hours). The opposite is true when the Earth is farthest away. The maximum difference is about 7.7 seconds, but because it adds up from one day to the next, the noon as determined by the Sun or a sundial will be off by as much as 7.7 minutes. There is an additional factor that complicates this-- the tilt of the orbit of the Earth.

Images of the Sun taken at the same time (8:30 am) and location over a one year period

An analemma, similar to the one in front of the Flandreau Science Center in Tucson, AZ

4. When Earth’s rotation alters, say due to earthquakes, does it impact the Moon’s speed?

The speed of the Moon around the Earth is dependent only on its distance and the total mass of the Earth-Moon system (Kepler and Newton). So, changing the rotation of the Earth by a microsecond or so (the Japan earthquake shortened the day by 1.8 microseconds) will not affect the motion of the Moon. From the numbers for the previous two great earthquakes: Chile, 1.2 microseconds and Sumatra, 6.8 microseconds(!), all three shortened the day (redistribution of mass within the Earth). Note that an earthquake can change the rotation rate of the Earth or move the crust of the Earth relative to the spinning Earth (move the pole on the surface of the Earth), but it cannot change the direction that the axis of the Earth points. That takes an external force, like the gravity of the Moon or Sun.

5. Does time of day (23 hr 56 min) ever fluctuate +/- minutes based on Earth’s activity?

Major earthquakes have altered the length of the day by only microseconds. However, over time, there is a bigger effect—tides. The Earth rotates in one day and the Moon orbits in 27.3 days. Think of the Earth spinning with the Moon raising tides. What do you get? Friction. What does this friction do? It slows down the Earth. However, angular momentum needs to be conserved (think of a spinning skater bringing in her/his arms, she/he spins faster). Therefore, the angular momentum of the slowing Earth is transferred to the Moon—the Moon moves away from the Earth. Is this measurable? Yes! The astronauts left mirrors on the Moon for scientists to bounce laser light. They have measured that the Moon is moving away from us at a rate of 3.8 centimeters a year. This is consistent with the measurement that the Earth is slowing down by about 2.3 milliseconds per century. This may not seem like much, but scientists have looked at tidal deposits from 620 million years ago and find that the day was about 22 hours long and the Moon revolved around the Earth in 6% less time (it was closer) than it does now.

6. Are there other things that help cause tides other than the Moon?

Tides are raised on all objects that are orbiting each other. So on Earth tides are raised by both the Moon and the Sun. When these two sets of tides occur at the same time (new moon and full moon) they add together and we call it a spring tide. When the two sets of tides are 90 degrees apart on Earth (first and third quarter moon) they slightly cancel each other and we call it a neap tide.

7. What are the differences in interior composition between Earth and Mars? How are they different? How are they similar?

Earth and Mars have fairly similar compositions, with roughly equal amounts of rock and iron in their interiors. Even taking compression into account, Earth’s density is still somewhat larger than Mars’, indicating either somewhat more iron or fewer less-dense materials (water?) than Mars. Mars’ surface is largely covered with basalt, a volcanic rock covering much of Earth’s surface as well. Earth has large amounts of granite in its crust, which Mars appears to lack. Carbonate (as in limestone) is common on Earth, and has been recently discovered on Mars as well, though how much of it below ground is not known. This implies the existence of past water activity on Mars. Mars presently lacks oceans, but appears to have a lot of water buried below the surface. Again, how much is not well known. 

8. Once the Earth formed, how did the various and vast variety of life forms begin?

This question is in the realm of biology and can be answered best by biologists!

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