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Joe G

Earth's rotation to the sun

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Hi all

I was at an astronomy discussion a while ago & part of the discussions was about Sun Spots & lack of them. I'm aware that when the Moon moves around the Earth , only one side is always visible to us. As the Earth moves around the sun, is it similar or can we view all areas of the Sun during that yearly rotation?

Hopefully that makes sense & thanks in advance to any input from more experience observers here

Joe

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No. The moon also rotates around it's axis, the same way other planets, including earth do.

We only see one side of the moon because we are gravitationally locked, or rather moon is to the earth. This means that moon's rotational period is synced to that of the earth and that is the reason we only see one side.

This phenomena is not "strict/firm" - moon "wobbles" a bit - sometimes it's a bit "ahead", and some times it's a bit "behind" - rotation period is not fixed due to this gravitational locking. This phenomena is called Lunar libration. It consists out of couple of factors - like relative tilt, elliptical orbit, etc ...

Here is in depth explanation of this:

https://en.wikipedia.org/wiki/Libration

Sun on the other hand rotates around it's axis, and approximate duration of this rotation is between ~24.5 and 38 days. This does not mean that Sun varies in its rotational speed, the difference is due to Sun not being solid object, so different parts of it rotate at different speeds.

Here is more on the topic:

https://en.wikipedia.org/wiki/Solar_rotation

Phenomena of sun spots is related to something completely different. It is related to solar cycle:

https://en.wikipedia.org/wiki/Solar_cycle

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Hi Vlaiv

Thanks for all that. If I'm reading your post right, I should be able to view the whole circumference of the Sun every 24.5 - 38 days, providing the days are cloudless of course

Again thanks

Joe

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Here is what wiki says on the topic:

Quote

At the equator, the solar rotation period is 24.47 days. This is called the sidereal rotation period, and should not be confused with the synodic rotation period of 26.24 days, which is the time for a fixed feature on the Sun to rotate to the same apparent position as viewed from Earth. The synodic period is longer because the Sun must rotate for a sidereal period plus an extra amount due to the orbital motion of Earth around the Sun. Note that astrophysical literature does not typically use the equatorial rotation period, but instead often uses the definition of a Carrington rotation: a synodic rotation period of 27.2753 days or a sidereal period of 25.38 days. This chosen period roughly corresponds to the prograde rotation at a latitude of 26° north or south, which is consistent with the typical latitude of sunspots and corresponding periodic solar activity. When the Sun is viewed from the "north" (above Earth's north pole), solar rotation is counterclockwise (eastward). To a person standing on the North Pole, sunspots would appear to move from left to right across the Sun's face.

So it really depends what you are observing. At higher latitudes surface rotation is slower reaching 38 days at poles. Problem is of course that we can't observe poles because of steep angle involved. But if we could we would need to "extend" this period, because earth also moves in the orbit around the sun for these 38 days, so features look like they need longer to complete full orbit when observed from earth because they are "trailing" the earth in it's orbital rotation. So if would take over 40 days for polar features.

Having said all of this, you need to be careful about saying something like viewing whole circumference of the Sun. Sun is dynamic system, it's not static like surface of the Moon. Some of the solar features are short lived and chances are you won't even see them - if they last while at opposite side of sun and dissappear before sun turns that side into view.

For example, sun spots can last only a few days - not enough to make the whole revolution.

Look at this wiki page:

https://en.wikipedia.org/wiki/Sunspot

There are couple of videos at the bottom of the page - they show really nicely the speed of sun spot dynamic in comparison to solar rotation. In one video group of spots appears and morphs in shape in about a quarter of a solar turn.

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Hi Vlaiv

Again thanks for all that. The origonal idea of the question arose as I was was walking over the hill road with the dog during a sunrise the other morning & I thought _ is this the same Sun I saw last week? Will I see a different Sun facade next week , God willing & with a clear morning? I don't do Solar gazing but just love looking up!

Thanks Joe

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