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If our moon is leaving us at 1 inch per year, And itīs orbit is eliptical

Anonymous Coward
User ID: 7175
12/02/2005 10:28 PM
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If our moon is leaving us at 1 inch per year, And itīs orbit is eliptical
Wouldnīt it also be getting an inch closer per year?

This websiteīs first article contradicts another on itīs site. First it says the moonīs orbit is circular then it says it is eliptical. I wish they would pick a lie and stick to it. Like most of you guys.

That website must be maintained by debunkers.

Is the Moon moving away from the Earth? When was this discovered?

I heard in the TV that moon is moving away from the earth towards the sun. Why is that happening? And when was this exactly discovered?

The Moonīs orbit (its circular path around the Earth) is indeed getting larger, at a rate of about 3.8 centimeters per year. (The Moonīs orbit has a radius of 384,000 km.) I wouldnīt say that the Moon is getting closer to the Sun, specifically, though--it is getting farther from the Earth, so, when itīs in the part of its orbit closest to the Sun, itīs closer, but when itīs in the part of its orbit farthest from the Sun, itīs farther away.

The reason for the increase is that the Moon raises tides on the Earth. Because the side of the Earth that faces the Moon is closer, it feels a stronger pull of gravity than the center of the Earth. Similarly, the part of the Earth facing away from the Moon feels less gravity than the center of the Earth. This effect stretches the Earth a bit, making it a little bit oblong. We call the parts that stick out "tidal bulges." The actual solid body of the Earth is distorted a few centimeters, but the most noticable effect is the tides raised on the ocean.

Now, all mass exerts a gravitational force, and the tidal bulges on the Earth exert a gravitational pull on the Moon. Because the Earth rotates faster (once every 24 hours) than the Moon orbits (once every 27.3 days) the bulge tries to "speed up" the Moon, and pull it ahead in its orbit. The Moon is also pulling back on the tidal bulge of the Earth, slowing the Earthīs rotation. Tidal friction, caused by the movement of the tidal bulge around the Earth, takes energy out of the Earth and puts it into the Moonīs orbit, making the Moonīs orbit bigger (but, a bit pardoxically, the Moon actually moves slower!).

The Earthīs rotation is slowing down because of this. One hundred years from now, the day will be 2 milliseconds longer than it is now.

This same process took place billions of years ago--but the Moon was slowed down by the tides raised on it by the Earth. Thatīs why the Moon always keeps the same face pointed toward the Earth. Because the Earth is so much larger than the Moon, this process, called tidal locking, took place very quickly, in a few tens of millions of years.

Many physicists considered the effects of tides on the Earth-Moon system. However, George Howard Darwin (Charles Darwinīs son) was the first person to work out, in a mathematical way, how the Moonīs orbit would evolve due to tidal friction, in the late 19th century. He is usually credited with the invention of the modern theory of tidal evolution.

So thatīs where the idea came from, but how was it first measured? The answer is quite complicated, but Iīve tried to give the best answer I can, based on a little research into the history of the question.

There are three ways for us to actually measure the effects of tidal friction.

* Measure the change in the length of the lunar month over time.

This can be accomplished by examining the thickness of tidal deposits preserved in rocks, called tidal rhythmites, which can be billions of years old, although measurements only exist for rhythmites that are 900 million years old. As far as I can find (I am not a geologist!) these measurements have only been done since the early 90īs.

* Measure the change in the distance between the Earth and the Moon.

This is accomplished in modern times by bouncing lasers off reflectors left on the surface of the Moon by the Apollo astronauts. Less accurate measurements were obtained in the early 70īs.

* Measure the change in the rotational period of the Earth over time.

Nowadays, the rotation of the Earth is measured using the Very Long Baseline Interferometry, a technique using many radio telescopes a great distance apart. With VLBI, the positions of quasars (tiny, distant, radio-bright objects) can be measured very accuarately. Since the rotating Earth carries the antennas along, these measurements can tell us the rotation speed of the Earth very accurately.

However, the change in the Earthīs rotational period was first measured using eclipses, of all things. Astronomers who studied the timing of eclipses over many centuries found that the Moon seemed to be accelerating in its orbit, but what was actually happening was the the Earthīs rotation was slowing down. The effect was first noticed by Edmund Halley in 1695, and first measured by Richard Dunthorne in 1748--though neither one really understood what they were seeing. I think this is the earliest discovery of the effect.

[link to curious.astro.cornell.edu]

Why do the size and brightness of the full moon change?

It was explained to me that the giant full moon of this last April 16th appeared so bright and large because the moon was the closest to the Earth that it ever has been or ever will be (within a thousands of years kind of span.) How is this possible?

The Moonīs orbit around Earth isnīt a perfect circle - itīs actually fairly elliptical - about 5.5% eccentricity. This means thereīs a fairly large difference between the perigee (when the Moon is at the closest point in its orbit) and apogee (when the Moon is at its farthest). This means that the Earth-Moon distance varies by about 13,000 miles either direction of the average distance. So if the full moon occurs at or near perigee, it appears noticeably larger in the sky than if it occurs at apogee, and it also it is brighter, because the amount of light received by the Earth from the Moon depends not only upon the amount of light the Moon gives off, but also how far the Earth is from the Moon. The farther the Moon, the smaller the fraction of the Moonīs light that reaches Earth. I should add, however, that while this is a significant effect, all full moons are large and bright, so itīs difficult to tell the difference without being able to look at a perigee and apogee full moon side by side. This year, the lunar perigee occurred only hours from the full moon on April 16th. It was the closest full moon of the year, but not the closest the Moon has been to the Earth in recent times. The nearest perigee recently was in 1912. For a much more detailed explanation, check out this site - it even has a link to a perigee and apogee calculator so if you want to observe this phenomenon youīll know when to take a look!

What other factors affect the brightness of the full moon?

There are several other factors that affect the brightness of the full moon. When the Earth (and therefore the Moon) is at its perihelion, the closest point in its orbit to the Sun, the sunlight that reflects off the Moon is slightly more intense, causing the full moonīs brightness to increase by about 4%, which is imperceptible by the human eye.

The brightness of any object, including the moon, in the sky increases with its height in the sky. When an object is directly overhead, its light strikes the ground at a right angle, and the intensity of light is the same as the intensity in the beam. However, when an object is nearer to the horizon, its light strikes the ground at an angle, and the same amount of light is spread out over a larger area. Therefore, less light per unit area reaches the ground from an object near the horizon. Also, the closer the moon is to the horizon, the more atmosphere the light must travel through to reach the observer. This means that more of the moonīs light is absorbed or scattered by the atmosphere. The height of the moon in the sky results from a combination of the latitude you are observing from and the declination of the moon.

When the moon is closer to opposition, that is, the point exactly opposite the Sun (at which point there is a lunar eclipse because the Sunīs light is blocked by the Earth and does not reach the Sun), it is brighter. This is called the opposition effect. It is believed to be caused mainly by shadow hiding. The closer the moon is to opposition, the smaller the shadows cast by objects on its surface, and the brighter it appears. For more information on the opposition effect, check out this website.

Finally, atmospheric conditions have a great effect on the brightness of the full moon. The full moon on a clear night will be much brighter than if there is a lot of dust, smog or clouds.

Happy Full Moon watching!

[link to curious.astro.cornell.edu]