Solar Eclipse Total Skyscape (APOD 20 Sep 2008)

[apodman]

http://antwrp.gsfc.nasa.gov/apod/ap080920.html

Before someone complains that the sun is slightly off center and that the picture is monochrome, let me be the first to say that this is a great picture. Catchy at first glance, the depth of content is incredible once you look at it closely. Astronomy? It's a total solar eclipse, so (duh) yeah. Of the Day? A few weeks old, recent enough for me. Picture? Let me repeat that this is a great picture, the kind of eye candy this APODer is looking for. If someone doesn't list this one in the Favorite APOD thread, I might.

I've tried to get good looks at the Beehive cluster all my life, but it's hard to pick out in even slightly light-polluted skies. And here's a sharp picture of it in the daytime. Better yet in the large view at full size.

[orin stepanek]

As Yogi said; "deja vu all over again."
http://apod.nasa.gov/apod/ap080808.html

http://asterisk.apod.com/vie ... hp?t=14388

I think this picture is neat also. 8) I like the fact that you can see stars behind the sun that normally we have to wait until the Earth's orbit make them visible.
Orin

[jimsaruff]

Would someone please tell me a few things about the sun?

I assume it rotates, but how fast and does its 'north' pole move or wander about as ours does? If so, how far, how quickly and why?

Also, how did the earth develop a spin inclined (declined?) from the sun's and why has it kept it?

Thanks for your time.

[Case]

I assume [the sun] rotates, but how fast

From the Sun wiki:
"As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles. This behaviour is known as differential rotation. The period of this actual rotation is approximately 25 days at the equator and 35 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days."

and does its 'north' pole move or wander about as ours does? If so, how far, how quickly and why?

The axis of rotation is inclined to 7.25° to the ecliptic. I doubt that there are ways to gather information on how that was several thousands of years (or more) ago, as solid surface geology does not apply to the Sun.

Also, how did the earth develop a spin inclined (declined?) from the sun's and why has it kept it?

From the Obliquity wiki:
"The Earth's axial tilt varies between 22.1° and 24.5°, with a 41,000-year period, and at present, the tilt is decreasing. In addition to this steady decrease, there are also much smaller short term (18.6 years) variations, known as nutation."
Because the magnetic poles are so close to the poles of the rotation axis, one would assume a causal relation, although I do not know the details.
It may have have been a totally different angle extremely long ago. I don't think it is valid to say the Earth "has kept it".

[apodman]

Okay, I see I've simultaneously duplicated some of Case's material. But I'll post it all anyway.

Solar Rotation

I assume it rotates, but how fast and does its 'north' pole move or wander about as ours does? If so, how far, how quickly and why?

http://solarscience.msfc.nasa.gov/sunturn.shtml

The Sun rotates on its axis once in about 27 days. This rotation was first detected by observing the motion of sunspots as shown in the animation to the left. The Sun's rotation axis is tilted by about 7.25 degrees from the axis of the Earth's orbit so we see more of the Sun's north pole in September of each year and more of its south pole in March.

Since the Sun is a ball of gas, it does not have to rotate rigidly like the solid planets and moons do. In fact, the Sun's equatorial regions rotate faster (taking only about 24 days) than the polar regions (which rotate once in more than 30 days). The source of this "differential rotation" is an area of current research in solar astronomy.

http://www.enchantedlearning.com/subjec ... tion.shtml

The Sun rotates around an axis which is roughly perpendicular to the plane of the ecliptic; the Sun's rotational axis is tilted by 7.25° from perpendicular to the ecliptic. It rotates in the counterclockwise direction (when viewed from the north), the same direction that the planets rotate (and orbit around the Sun).

The Sun's rotation period varies with latitude on the Sun since it is made of gas. Equatorial regions rotate faster than polar regions. The equatorial regions (latitude = 0 degrees) rotate in about 25.6 days. The regions at 60 degrees latitude rotate in about 30.9 days. Polar regions rotate in about 36 days.

I would expect some miniscule precession of the sun's axis (small because of its relatively large mass compared to its gravitationally bound orbiting objects), but I can't find a reference to it. I did the easy searches and will leave that one to you. Don't be afraid of google.

Earth's Axis

Also, how did the earth develop a spin inclined (declined?) from the sun's and why has it kept it?

http://en.wikipedia.org/wiki/Axial_tilt#Values

The Earth's axial tilt varies between 22.1° and 24.5° (but see below), with a 41,000-year period, and at present, the tilt is decreasing. In addition to this steady decrease, there are also much smaller short term (18.6 years) variations, known as nutation.

For the complete history, again don't be afraid of google.

[Tatiana]

Great job, both of you, on answering the questions!

I'll expand a little bit more on the last one. The earth's spin axis precesses over time just like a spinning top does. Only instead of the pull downward of gravity on the spinning top, the forces that cause this motion of the Earth are tidal forces of the sun and moon. This is called "nutation", which is Latin for "nodding". The earth's spin axis draws little circles over a time period that's measured in only thousands of years. It's kind of cool, really, because usually stuff like this takes millions of years to happen, but you can see the effects of this one in historical time.

So even though right now there's a relatively bright star near the celestial pole, which is Polaris, of course, that wasn't true until recently, historically speaking, and won't be true in a few thousand years either. We're sort of lucky in living during a time where there's a pole star.

Another funny thing about nutation is it causes the phenomenon called "the precession of the equinoxes", meaning that the point on the sky that corresponds to the moment each spring that the days and nights are exactly the same length (the equinoxes) moves in only a few thousand years as well. This wouldn't be so weird except that our coordinate system in the sky, the right ascension and declination that corresponds to latitude and longitude on earth, has as its zero point this same point on the sky that corresponds to the spring equinox. So our map of the sky is constantly moving. Star catalogs from my youth are already out of date, and we have to redo them (putting out a new 'era') every few years.

So funny that we picked a coordinate system that's constantly moving. =)

[apodman]

Star catalogs from my youth are already out of date, and we have to redo them (putting out a new 'era') every few years.

So funny that we picked a coordinate system that's constantly moving.

To expand a little further:

When "they" laid out the coordinates 2000-or-so years ago, the Vernal Equinox (the point where the "rising" ecliptic meets the celestial equator) was at the cusp (border) of Aries and Pisces. The Equinox precesses backwards through each of the 12 constellations of the ecliptic (the Zodiac) in about 2150 years. So far, it has worked its way almost all the way through Pisces - hence the imminent dawning of the Age of Aquarius (astronomically we are still in the age of Pisces).

I think the current "era" is "Epoch 2000" and the previous one was "Epoch 1950" as far as the major star charts go. With advancements in computing 1950-2000 and what might happen before 2050, I'm thinking we'll switch to a real-time system - except of course for printed copies - before "Epoch 2050" hits. There are always unpredictable changes in ephemeris time to deal with either way due to fluctuations in the Earth's rotation.