APOD: Apollo 14 Heads for Home (2020 Feb 01)

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APOD: Apollo 14 Heads for Home (2020 Feb 01)

Post by APOD Robot » Sat Feb 01, 2020 5:05 am

Image Apollo 14 Heads for Home

Explanation: When leaving lunar orbit in February 1971, the crew of Apollo 14 watched this Earthrise from their command module Kittyhawk. With Earth's sunlit crescent just peaking over the lunar horizon, the cratered terrain in the foreground is along the lunar farside. Of course, while orbiting the Moon, the crew could watch Earth rise and set, but the Earth hung stationary in the sky over Fra Mauro Base, their landing site on the lunar surface. Rock samples brought back by the Apollo 14 mission included a 20 pound rock nicknamed Big Bertha, later determined to contain a likely fragment of a meteorite from planet Earth.

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sillyworm 2

Re: APOD: Apollo 14 Heads for Home (2020 Feb 01)

Post by sillyworm 2 » Sat Feb 01, 2020 1:52 pm

In the article for the meteorite link they mention that at one time the Moon was 3 times closer to Earth.How did they come to this conclusion and about what time frame do they figure that was? Could the oceans have been present at that time?

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Re: APOD: Apollo 14 Heads for Home (2020 Feb 01)

Post by Chris Peterson » Sat Feb 01, 2020 2:55 pm

sillyworm 2 wrote:
Sat Feb 01, 2020 1:52 pm
In the article for the meteorite link they mention that at one time the Moon was 3 times closer to Earth.How did they come to this conclusion and about what time frame do they figure that was? Could the oceans have been present at that time?
There is geological evidence of the distance between the Earth and Moon going back at least 2.5 billion years. That evidence comes from the effects of tides in shifting sediments, now captured in sandstone. (Diurnal patterns are similarly captured, allowing us to directly measure the changing length of the day, i.e. the slowing of Earth's rotation.) At that time the Moon was about 300,000 km away, compared with about 400,000 today. The increasing distance from Earth is well understood in terms of basic physics. Going back earlier, however, gets into the realm of knowing how the Moon was created. Most people believe it was the product of an impact, and if so, models tell us that the initial debris (4.5 billion years ago) was very close to the Earth- just 20,000 to 30,000 km. If so, basic physics allows for calculating the orbital dynamics after that... until the theory meets the reality of the fossil record after the first two billion years.
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Re: APOD: Apollo 14 Heads for Home (2020 Feb 01)

Post by neufer » Sat Feb 01, 2020 3:32 pm

Chris Peterson wrote:
Sat Feb 01, 2020 2:55 pm
sillyworm 2 wrote:
Sat Feb 01, 2020 1:52 pm

In the article for the meteorite link they mention that at one time the Moon was 3 times closer to Earth. How did they come to this conclusion and about what time frame do they figure that was? Could the oceans have been present at that time?
There is geological evidence of the distance between the Earth and Moon going back at least 2.5 billion years. That evidence comes from the effects of tides in shifting sediments, now captured in sandstone. (Diurnal patterns are similarly captured, allowing us to directly measure the changing length of the day, i.e. the slowing of Earth's rotation.) At that time the Moon was about 300,000 km away, compared with about 400,000 today. The increasing distance from Earth is well understood in terms of basic physics. Going back earlier, however, gets into the realm of knowing how the Moon was created. Most people believe it was the product of an impact, and if so, models tell us that the initial debris (4.5 billion years ago) was very close to the Earth- just 20,000 to 30,000 km. If so, basic physics allows for calculating the orbital dynamics after that... until the theory meets the reality of the fossil record after the first two billion years.
Currently, the Earth's rotation contributes only ~10% of the total angular momentum of the Earth/Moon system.

It is probably assumed (heuristically?) that the Moon initially orbited so close that the Earth's rapid rotation (i.e., ~once every 7 hours) contributed ~50% of the total angular momentum of the Earth/Moon system. That would have placed the Moon about 3 times closer to Earth (with a corresponding reduced angular momentum of 3-1/2).
https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Tidal_evolution wrote:
<<The mass of the Moon (about 1:81 of Earth's) is sufficiently large, and it is sufficiently close, to raise tides in the matter of Earth. In particular, the water of the oceans bulges out towards and away from the Moon. The average tidal bulge is synchronized with the Moon's orbit, and Earth rotates under this tidal bulge in just over a day. However, Earth's rotation drags the position of the tidal bulge ahead of the position directly under the Moon. As a consequence, there exists a substantial amount of mass in the bulge that is offset from the line through the centers of Earth and the Moon. Because of this offset there exists a torque between Earth and the Moon. Since the bulge nearer the moon pulls more strongly on it than the bulge further away, this torque boosts the Moon in its orbit and slows the rotation of Earth. As a result of this process, the mean solar day, which is nominally 86,400 seconds long, is actually getting longer when measured in SI seconds with stable atomic clocks.

In addition to the effect of the ocean tides, there is also a tidal acceleration due to flexing of Earth's crust, but this accounts [currently] for only about 4% of the total effect when expressed in terms of heat dissipation. This mechanism has been working for 4.5 billion years. There is geological and paleontological evidence that Earth rotated faster and that the Moon was closer to Earth in the remote past. Tidal rhythmites are alternating layers of sand and silt laid down offshore from estuaries having great tidal flows. Daily, monthly and seasonal cycles can be found in the deposits. This geological record is consistent with these conditions 620 million years ago: the day was 21.9±0.4 hours, and there were 13.1±0.1 synodic months/year and 400±7 solar days/year. The average recession rate of the Moon between then and now has been 2.17±0.31 cm/year, which is about half the present rate. The present high rate may be due to near resonance between natural ocean frequencies and tidal frequencies.>>
https://en.wikipedia.org/wiki/Earth%27s_rotation#Tidal_interactions wrote:
<<Over millions of years, Earth's rotation has been slowed significantly by tidal acceleration through gravitational interactions with the Moon. Thus angular momentum is slowly transferred to the Moon at a rate proportional to r−6, where r is the orbital radius of the Moon. This process has gradually increased the length of the day to its current value.

This gradual rotational deceleration is empirically documented by estimates of day lengths obtained from observations of tidal rhythmites and stromatolites; a compilation of these measurements found that the length of the day has increased steadily from about 21 hours at 600 Myr ago to the current 24-hour value. By counting the microscopic lamina that form at higher tides, tidal frequencies (and thus day lengths) can be estimated, much like counting tree rings, though these estimates can be increasingly unreliable at older ages.

The current rate of tidal deceleration is anomalously high, implying Earth's rotational velocity must have decreased more slowly in the past. Empirical data tentatively shows a sharp increase in rotational deceleration about 600 Myr ago. Some models suggest that Earth maintained a constant day length of 21 hours throughout much of the Precambrian. This day length corresponds to the semidiurnal resonant period of the thermally-driven atmospheric tide; at this day length, the decelerative lunar torque could have been canceled by an accelerative torque from the atmospheric tide, resulting in no net torque and a constant rotational period. This stabilizing effect could have been broken by a sudden change in global temperature. Recent computational simulations support this hypothesis and suggest the Marinoan or Sturtian glaciations broke this stable configuration about 600 Myr ago; the simulated results agree quite closely with existing paleorotational data.>>
Last edited by neufer on Sat Feb 01, 2020 4:27 pm, edited 1 time in total.
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Re: APOD: Apollo 14 Heads for Home (2020 Feb 01)

Post by orin stepanek » Sat Feb 01, 2020 4:14 pm

Awesome view of a crescent Earth! 8-) :b:
Orin

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