Meteor Impact

[Eternity]

What speed would a 10^9 kg heavy meteor need to have to slow down the rotation of the earth that a day lasts 25 hours? Did the day length on the earth vary drastically (hours) during its history (after Theia) due to meteor impacts (and not due to the distance to the sun)? (The day is 1 mikrosecond shorter after the earthquake in Chile)

[Chris Peterson]

What speed would a 10^9 kg heavy meteor need to have to slow down the rotation of the earth that a day lasts 25 hours?

No collision could significantly alter the rotation rate of the Earth. The energy required to do that would destroy the Earth. (This sort of stuff may have gone on in the earliest stages of the formation of the Solar System, but the Earth hasn't been melted by a collision since the Moon was created.)

Did the day length on the earth vary drastically (hours) during its history (after Theia) due to meteor impacts (and not due to the distance to the sun)?

No. The gradual increase in the length of the day (which is seen in the geological record) is attributable purely to the transfer of Earth's angular momentum to the Moon.

(The day is 1 mikrosecond shorter after the earthquake in Chile)

This is also a simple result of the conservation of angular momentum. Earthquakes shift crustal mass, which can either speed up or slow down the Earth's rotation by tiny amounts. The net effect over a long time will be zero, however.

[neufer]

What speed would a 10⁹ kg heavy meteor need to have to slow down the rotation of the earth that a day lasts 25 hours?

No collision could significantly alter the rotation rate of the Earth. The energy required to do that would destroy the Earth. (This sort of stuff may have gone on in the earliest stages of the formation of the Solar System, but the Earth hasn't been melted by a collision since the Moon was created.)

Well, it would certainly destroy most of the life on Earth at least.

Note, however, that in the ~220 million years that it will take Earth to orbit the Milky Way
tidal interactions will have moved the Moon ~8,000 km further away
and will have lengthened an Earth day to ~25 hours long.

Did the day length on the earth vary drastically (hours) during its history (after Theia) due to meteor impacts (and not due to the distance to the sun)?

No. The gradual increase in the length of the day (which is seen in the geological record) is attributable purely to the transfer of Earth's angular momentum to the Moon.

The variation of length of day versus geological time

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The Length of the Day: A Cosmological Perspective
Volume 1 PROGRESS IN PHYSICS January, 2009
Arbab I. Arbab, Department of Physics, Faculty of Science, University of Khartoum

We have found an empirical law for the variation of the length of the Earth’s day with geologic time employing Wells’s data. We attribute the lengthening of the Earth’s day to the present cosmic expansion of the Universe. The prediction of law has been found to be in agreement with the astronomical and geological data.

The length of the day is found to be 6 hours when the Earth formed.

<<Studies of growth in fossil corals have made a fascinating contribution to geochronology involving past ocean tides and the inferred relationship between the earth and the moon. The moon pulls ocean water in a tidal bulge which moves westward as the earth rotates toward the east. The tidal drag acts as a brake on the spinning earth, gradually slowing it down. Early in the eighteenth century, Edmund Halley, Astronomer Royal of England, noted that there was a discrepancy between the recorded locations of ancient eclipses of the moon and their predicted places of observation. He pointed out that the differences could be resolved by assuming a slowing down of the rate of rotation of the earth. Modern astronomers have confirmed his theory, and by precise methods have found that the earth is now slowing at the rate of 0.002 seconds per century. This seems very little, but can be appreciable over tens of millions of years.

The slowing of the earth's spin decreases the number of days in the year and causes the moon to draw away from the earth, thus conserving the [angular momentum] within the earth-moon system. The rate of recession of the moon away from the earth is now calculated at about [3.8] centimeters per year. Until recently there was no way to test these astronomical deductions, but paleontology now provides an independent test.

The discovery was made by John W. Wells at Cornell University, a leading investigator of living and fossil corals. Wells knew that the skeletons of corals (and many other kinds of invertebrates) display parallel growth rings similar to the annual growth of trees. He was able to show that annual bands of living corals are themselves made of narrow lines which closely correspond to one day's growth. With his fossil corals, he reported in 1963 that specimens of Devonian age averaged about 400 lines per year, and Carboniferous corals about 380. Subsequent investigations by intrigued paleontologists have shown that the number of daily growth increments per year in corals and molluscs has indeed been decreasing through geological time. Astronomers had already calculated that average tidal friction would allow 425 days per year in the Cambrian and 400 days per year for the Devonian. As well as providing evidence of close agreement between these two scientific approaches, John Wells' work also provides a measure of the antiquity of fossils in years, totally independent of radiometric methods.

Following up Wells' discovery, Colin T. Scrutton, of the British Museum (Natural History), found what appeared to be monthly bands in Devonian corals equivalent to the intervals between times of the full moon. He calculated 13.03 lunar months in a Devonian year of 399 days. This work has opened up a whole new field of historical research involving the earth-moon relationship.>>

(The day is 1 microsecond shorter after the earthquake in Chile)

This is also a simple result of the conservation of angular momentum. Earthquakes shift crustal mass, which can either speed up or slow down the Earth's rotation by tiny amounts. The net effect over a long time will be zero, however.

The current (historical) increase in the length of a day by 1.7 seconds per century is actually somewhat less than the 2.4 seconds per century contribution caused by tidal interactions. This is because the Earth's mantle is still experiencing a post-glacial polar rebound from the last ice age:

<<During glaciation, water is taken from the oceans, whose average position is nearer the equator, and deposited as ice over the higher latitudes closer to the poles, which is closer to the rotational axis. This causes the Moment of Inertia of the Earth-ice-water system to decrease and just like the rotating figure skater bringing her arms closer to her body, the earth should spin faster. During deglaciation, the melted ice water returns to the oceans - farther from the rotational axis - and thus causing the Earth’s spin to slow down. Also, the mantle rocks flow in a direction opposite to that of the water, but the rate is much slower. After the end of deglaciation, the dominant mass movement is from the return flow of the mantle rocks back to the glaciated areas at high latititude, making the shape of the Earth less oblate. This process would, in isolation, lead to an increase in the rotation speed of the Earth and therefore to a decrease of the length of day. Lambeck estimated that the isolated effect of post-glacial rebound on the length of the day would be a decrease of about 0.7 milliseconds per century. This process of nontidal acceleration of the rotation of the earth is corroborated by observations of the satellite LAGEOS and is generally attributed to glacial isostatic adjustment.>>