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## Is the angle of the axis that the earth rotates on created by or affected by the sun?

David B.
Featured 8 months ago

Although the angle of the Earth's axis of rotation is not created by the sun, it is affected by the sun's gravity causing it to precess.

#### Explanation:

The angle of the Earth's axis of rotation is not created by the sun. It is a result of the residual angular momentum of the materials that coalesced to create the planet, and other events since that time, such as collisions, etc. The planets of the solar system have a wide range of angles and rotation directions:

The angle of the axis is affected by the sun, through a process known as precession . This precession is caused by the torque applied to the planet by the sun's gravity.

This causes the direction of the axis to move in a circle when viewed by an observer on the Earth. To the Earth-bound observer, the axis of rotation is viewed as the direction of the pole. The following picture shows how the north pole has shifted position through our view of the stars through the years (-2000 = 200 B.C., +2000 = 2000 A.D.). Right now, at 2016 A.D. the north pole points very near the star Polaris which is at the end of the handle of the Little Dipper .

## Is the earth slowly getting closer to the sun as it orbits it? If not, what keeps it from being drawn into the sun?

Oscar L.
Featured 7 months ago

Actually the planets are slowly moving away from the Sun. But the effect is very small, only about 0.01% in a billion years for Earth.

#### Explanation:

There are two main mechanisms driving the planets away from the Sun, according to http://curious.astro.cornell.edu/about-us/41-our-solar-system/the-earth/orbit/83-is-the-distance-from-the-earth-to-the-sun-changing-advanced.

First is the tidal friction effect. The Sun rotates on average about once per thirty Earth days (the Sun is not rigid and its rotation rate varies with latitude). The Earth takes about 365 days to orbit the Sun. As is better known with the Earth versus the Moon, the difference in periods of rotation and revolution means tidal friction transfers energy from the faster cycle (Sun rotating) to the slower one (Earth orbiting). So the Sun is gradually slowing its rotation and the Earth is slowly moving outward. The other planets are moving outward for the same reason. But the Sun is rather far away and its rotation is too slow to have a major impact. The source quoted above states that the tidal effect is pushing the Earth away from the Sun at only about one micrometer per year.

The second effect reported by Cornell University's site is the loss of mass the Sun experiences as hydrogen is fused to helium. The helium has less mass than the hydrogen it comes from, and the difference is the energy output from the Sun, according to Einstein's formula $E = m {c}^{2}$. The decreasing mass of the Sun is pushing the Earth away at about 1.5 cm per year, a much stronger interaction than the tidal effect. But it still amounts to only a change of 0.01% in a billion years.

With the recent verification of gravitational waves, we know that gravitational wave emission is tending to make the planets spiral inwards. But gravitational wave emission has almost no impact on the motion of the planets. Planets are moving so slowly and with such weak gravitational interactions that gravitational wave emission is ten orders of magnitude less than the direct effect of the Sun losing mass.

All told, the net result is the planets are moving away from the Sun, but only very slowly. As noted above, the dominant effect amounts to only 0.01% in a billion years for Earth.

## What is the repeating universe theory?

George C.
Featured 5 months ago

See explanation for a few thoughts...

#### Explanation:

I think the term "repeating universe theory" could have several different interpretations.

Let us look at an couple of possibilities.

Suppose the nature of the universe is such that it will stop expanding and eventually experience a "big crunch".

Suppose further that such a "big crunch" will be automatically followed by another "big bang" with the same amount of matter/energy, etc. We could call that a "repeating universe theory", but perhaps there's something more...

If such a cycle is inevitable then there are some theories that we might attach to it:

(1) The theory that the following "big bang" will of necessity be identical to that which started this cycle, and will result in exactly the same sequence of events. Such a theory is almost certainly false.

(2) The theory that there are a large but finite number of possible initial configurations for a "big bang" in such a recurring cycle, so if an infinite number of "big bang" - "big crunch" cycles occur then at least one configuration will repeat. OK so far, but does the initial configuration determine all following events? - probably not.

(3) Like (2), but since the length of a "big bang" - "big crunch" cycle is finite, then if spacetime is quantised, there are a finite number of possible states through which the universe goes during a cycle. So there is a ridiculously huge but finite number of possible cycles. So in an infinite number of cycles, at least one possible sequence will repeat.

## How many AU is the nearest Galaxy from the milkyway?

George C.
Featured 5 months ago

It depends what you mean by nearest galaxy, but the conversion factor between light years and AU is about $63000$

#### Explanation:

Let's work out a conversion factor between light years and AU first.

The Sun is roughly $93000000$ miles from the Earth and light travels at about $186000$ miles per second.

So $1$ AU is about $500$ light seconds.

There are $86400$ seconds in a day and about $365.25$ days per year, hence:

$86400 \cdot 365.25 = 31557600 \text{ }$ seconds per year.

Dividing this by $500$ we arrive at:

$\frac{31557600}{500} = 63115.2 \approx 63000 \text{ }$ AU per light year.

$\textcolor{w h i t e}{}$
How far away is the nearest galaxy from the Milky Way?

The very nearest galaxy would be one of the small satellite galaxies.

There are about $50$ galaxies within about $1.4$ million light years of the Milky Way. The closest are about $1000$ light years from the edge, which by our reckoning would be about $63000000 = 6.3 \times {10}^{7}$ AU from the edge.

Perhaps of more interest is the distance to the Andromeda galaxy, our largest neighbour in the Local Group. This is about $2.5$ million light years away, which would make it about $160$ billion AU away.

## If the earth was reduced to the size of an atom how big would the galaxy be?

Parzival S.
Featured 5 months ago

$x = \frac{\left(1 \times {10}^{- 10}\right) \left(1 \times {10}^{21}\right)}{12742000} = \frac{1 \times {10}^{11}}{12742000} \cong 7848 m \approx 8 k m$

#### Explanation:

We have a ratio question - given the size of the Earth and reducing it to the size of an atom, how big will the galaxy be? That looks like:

$\frac{\text{Earth"/"atom"="galaxy}}{x}$

So first off we can solve for x:

#x=("atom"xx"galaxy")/"Earth"#

And now we need to make a decision - the size of an atom depends on which atom we're talking about. They typically run between $0.1 \to 0.5$ nanometres. I'm going to assume we're using a smaller atom (it'll make the math easier).

And now let's get some numbers:

$\text{Earth} = 12 , 742 k m = 12 , 742 , 000 m$

$\text{Atom} = 1 \times {10}^{-} 10 m$

$\text{Galaxy} = 1 , 000 , 000 , 000 , 000 , 000 , 000 , 000 = 1 \times {10}^{21} m$

And now the math:

$x = \frac{\left(1 \times {10}^{- 10}\right) \left(1 \times {10}^{21}\right)}{12742000} = \frac{1 \times {10}^{11}}{12742000} \cong 7848 m \approx 8 k m$

## What started the Big Bang? What was there before it? What is some evidence for it? Why do some people not agree?

Ricardo A.
Featured 5 months ago

#### Explanation:

We have, however many hypotheses that could explain it, such as string theory or m-theory, the problem we have right now is that all the predictions these ideas make about the universe are impossible to test with our current technology. You see, all scientific models make predictions and we can prove or disprove them by going out and making observations that confirm or deny that model.

• What was there before it?

Same as above, we have models that could explain it, but the observations are beyond our tech so far.

• What is some evidence for it?

In the 1920s Edwin Hubble (the telescope was named after him) discovered that all galaxies are flying away from us, and the further away a galaxy is from us the faster it is going away from us. This proved the Universe was expanding. Meaning it must have been smaller early on. This begat the Big Bang Theory.
There are many cosmological observations that corroborate this, and one of the best, which was predicted by the theory itself, is the Cosmic Microwave Background.

• Why do some people not agree?

Overall the school system has done a poor job of communicating the science to the people. The media also doesn't help as it is sensationalist and misrepresents the issues most of the time, leading to people distrusting science. Religion also plays a part in it as science disproves some of the stuff written in their holy books, so they (true to their doctrines) truest blind faith over knowledge.

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