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Archive for April, 2013

Zodiacal light

Under very good observing conditions, there are some interesting lights in the night sky.

The first is the zodiacal light, so named because it is a faint lane of light across the sky in the area of the zodiac, the ring of 12 constellations through which the Sun seems to move, — that is, if you think of the Sun circling Earth, as people imagined through all the years when the constellations were getting their names.

But not only the Sun; let me say more.

Where are the planets?

The planets, including Earth, orbit the Sun as if they were sitting on a disk that spins round with the Sun at the center. That is, they don’t orbit every which way in three dimensions, but sedately, one orbit beyond the next so that the solar system is flat as a pancake – even flatter. It’s not really two-dimensional, of course, but it’s pretty flat, and if Earth spun on an axis that was vertical with respect to the solar system, the planets would always pass directly overhead.

Because the Earth is tilted on its axis, however, the passing of the planets changes over the space of the year. Thus, if we look out into the system from Earth, on the equinox, (either March 21 or September 21, any planet crossing the sky will be visible about as far down from the zenith as the latitude of the observer.

Thus: if you are on the equator, the planets will pass right over your head; if you are at latitude 23°, they will be high in the sky, about 67° above the horizon. (90 – 23 = 67)  In Venice (Italy), or Minneapolis (Minnesota), at latitude 45°, planets will be halfway between the zenith and the horizon. From the poles, the planets will be right at the horizon and not actually visible.

At all other times of the year, there has to be a correction for the tilt of the Earth. In our northern winter, Earth’s axis northern is much closer to the disk of the solar system, and the planets are quite high in the sky. At the same time, for southerners, planets are low in the sky. In our summer, it is the opposite: planets are to be found closer to the horizon in Europe Asia, and America, but higher in the sky for Australia and for most of South America and Africa.

You can get a planisphere, or star wheel and check this for yourself. It is fairly easy to make one, and also easy to buy one. The complication of making one is that they have a slightly different construction for different latitudes.

star wheel

With or without a star wheel, we learn that the zodiac is the band of twelve constellations above our tropics, and the zodiacal light is a faint band of illumination in this area. It is believed (by almost everybody) that this dust might have been dropped by comets on their way close in to the sun; or it may be infalling dust from broken-up asteroids, or perhaps it is leftover from the beginning of the solar system. These would all be reasonable dust sources. Any such combination of sources could make a kind of pancake of dust orbiting the Sun, and that pancake could be visible from earth as a pale streak across the night sky. Wikipedia has an image by Y Beletsky, working from the Southern Astronomical Observatory in Paranal, in northern Chili.

Zodiacal light

Zodiacal light observed from the southern hemisphere, Paranal, Chili, November 19, 2009

I presume that the bright star at the top of the image is Venus. And in case you are wondering whether it is dawn, it is not; I believe it is near sunset. The zodiacal light does give a false impression of dawn — remember the zodiac is the set of constellations where the Sun travels.

And here is another image, close to sunrise, with both Venus and Jupiter caught in its glow.

Astrophoto: Zodiacal Light with Venus and Jupiter

Picture by Felipe Gallego, taken in northern Spain

But could the zodiacal light really be a dust ring around Earth instead of a dust ring around the Sun? It would look like a streak either way; it would be in the zodiac either way. If it were around the Earth, we might conclude that the Phoebe ring is visible and it has been seen: it has simply been misidentified.

This is not an easy question, and I cannot answer it, but I can offer a reason to examine it further.

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Where’s the Ring?

If there are Earth Rings, why don’t we see them?

There are many reasons why we might not see them, and basically, they fall into two categories: reasons why the rings would be inconspicuous, and claims that, actually, we do see them, we just don’t understand what we are seeing.

First of all, here are a few reasons why the rings would be inconspicuous.

  1. The rings we are talking about would have been relatively faint in the late 20th century, when our telescopes developed most rapidly, because this was generally a time of high solar activity. Even the whole 20th century had considerable activity as well as the end of the 19th. The rings we speak of must be replenished constantly, so they might have been faint at any time, but presumably they would have been faint at this time.
  2. These rings would generally be low on the horizon for observers in the northern hemisphere. From most places they would be obscured either by natural haze or by whatever city light is glowing all over the southern horizon from most places. So the rings wouldn’t be easy to see; a casual observer would not notice them. A sophisticated space probe might, but those are aimed into the deep sky, not low on the horizon.
  3. We are not looking for them. The common consensus about the Moon is that it is too cool to be volcanic. This is not a universal consensus, and my Father who was considered a reputable scientist in every other way, did not share it. But it was broad enough to govern the choice of research projects over a considerable period of time. Since the rings depend on replenishment from lunar volcanic material, they cannot exist unless the Moon is active. If they don’t exist, we need not look for them.

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What could we learn from the rings?

There is now a weather forecast based on a simulation of the relationship between the Earth, the Sun, and the hypothesized Earth rings. The simulation is complex and hard to watch; that’s why I saved it until I could lay out some background. Today I would like to explain what it might contribute to a weather forecast, but let me begin by reiterating that this is a hypothesis, not a prophecy. There are many influences on weather; this is a hypothesis that Earth rings could be one of them, and such rings would be more important during years of quiet sun; in fact, the better I understand them, the more important they seem so: disclaimer finished.

Of course, the rings won’t change the mountains near you; they won’t stop “lake effect” snow; they won’t make the Earth revolve backwards around the Sun or shift the equator or stop the jet stream (though they might guide the Hadley cells…) But after all the things we can’t change have been taken into account, there’s still a lot of weather that still takes us completely by surprise and seems to have no cause at all. Some of it fits into the ring model. It’s worth looking to see how much.

Here are a few things to help you watch the simulation.

Where am I?

First of all, notice the position of our northern hemisphere. The simulation begins in January, and you are lobserving Earth and its rings from the Sun. Jan 1 is very close to the winter solstice, and the South Pole is in full view; the North Pole is in back.

Second, notice that there are two sets of rings, two discs, one equatorial, and the other tilted slightly across the equator. The tilted set slips from side to side each month. That’s because it’s the Phoebe ring, and its center is actually the gravitational center of the Earth-Moon system, rather than the center of the Earth. As the Moon goes around the Earth each month, the uptilt of the disc follows it and so does the center of rotation. Watch how the Phoebe rings slip from left to right and back again because this will help you distinguish them from the equatorial rings.

Third, because the Earth is tilted 23° with respect to the sun, and because your point of view in this video is as if you stood on the Sun, the tilt of the equatorial ring system also shifts, making the rings very confusing sometimes. Majestic, but confusing!

Key: the equatorial disc of rings shifts its angle but not its center; it tilts, but it does not slip from side to side.

The equatorial ring:

  1. In January, you are looking at the rings from the south, so they cast their shadows north.
  2. In spring, you have one day (March 20) of looking at those rings edge-on. This is always a possible storm time, but noticing the heaviness of the Phoebe ring suggested that it would also be cold, and that this cold would persist for a while. See below. Notice also that the North Pole is coming into view.
  3. In summer, the equatorial ring shadow is cast south as you look at the disk from the north. An equatorial ring system would make the winters colder but would not affect the summers. Here is the North Pole in full view.
  4. On September 22, there is another day of edge-on for the equatorial rings, and then they begin again to cast their shadows north again; if the equatorial rings are heavy, they will make the winter colder, but they will never affect the summer.
  5. Because both sets of rings are sometimes tilted, their motions can be very confusing. Note that the equatorial rings are the ones that do not slide from left to right.
  6. As winter progresses, the equatorial disk gradually spreads out across the background of the Earth, until it is fully opened around Dec 22; then it begins to fold back again.

The Phoebe ring

  1. As the simulation begins, the Phoebe ring is casting its shadow north along with the equatorial ring.
  2. As you approach May 7, the Phoebe ring is folding up. Its increasingly heavy and localized shadow might be expected to cause storms, which would eventually move southwards with its maximum shadow. This is the reason for the forecast of a rough April, which is certainly being fulfilled in South Dakota. (The ring forecast was made January 1.)
  3. May 7, the Phoebe ring is edge-on to the Sun, and everything north of the tropics is out of its shadow.
  4. After that, its shadow falls south, which mostly affects the southern hemisphere but also the equator, at least for a while. In any case, it leaves the North Temperate zone clear. All summer, you see the Phoebe ring sliding from right to left, from left to right, but always with its shadow southward. That’s why the ring forecast is for a relatively mild and warm summer. For us. For the US, and also for Europe and the northern hemisphere generally. At the same time, it could mean a colder winter for the southern hemisphere, which faces both rings from May to September.
  5. On October 30-31, the Phoebe disc of rings is edge-on. Again, this deep shadow could cause storms, and at the same time, the equatorial rings have been spreading chill northwards all month. From then on, the northern hemisphere has both sets of shadows, with the Phoebe rings swinging majestically from side to side, but continuing to supply a north-falling shadow. It is hard to say exactly how the two discs will interact, especially since we don’t know their detailed structure – we don’t know which rings are heaviest and how far out.

But it looks cold. And if you watch the simulation at the youTube site, there is a longer and more detailed forecast underneath. It’s from Lucy Hancock, the architect of this whole

There is also a forecast for next March being harsh like this April, but I don’t have the simulation for that. That’s when the Phoebe ring-shadow falls north again, not on the same date as this year, but earlier.

All of this is based on the work of Lucy Hancock, who has her own blog on earthrings.

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You may remember that in 2008 and 2009, there was a lot of talk about the quiet sun. There were days and days without sunspots. People who knew a little history remembered that there was a “Little ice age” in Europe centered on the Maunder Minimum, the 70 years when there were no sunspots at all. Well, maybe with our better instruments they would have seen some sunspots, but basically, there were no visible spots for at least 70 years, from 1645-1715.

Because it was so cold at that time, people wonder whether sunspots make for warm weather; and conversely, whether a lack of sunspots could make for cold weather. Perhaps I should call it cold climate because the effect is not instant – we don’t get hot weather the day of a sunspot, and it was actually cold not just for 70 years, but for about 500 years, until the mid-19th century.

So take a look at the sunspot history of the 19th and 20th century. Faithfully, every eleven years, there are lots of sunspots, and then fewer in the years between. A sunspot maximum, a sunspot minimum, regular as a clock.

Sunspot cycles from 1750 to 2000. Notice how regular they are. Notice the low spot in the mid-19th century; notice the high spot in the late 1950’s.

Now, 2008, or maybe mid-2007, was supposed to be the minimum for the current sunspot cycle, so it was okay to be short of sunspots those years. But this minimum was a little too minimal for comfort. There were 200 days in a row with no sunspots. And 2008 stretched into 2009 with the spots still very sparse. Now we are at 2013, which should be the solar maximum, and we have some sunspots every day, but not very many and not very bright.

Here is the sunspot cycle through 2012.

Some people are hoping that the real max will come later; maybe the little bump we see  halfway through 2011 was a fake max or the first part of a twin max.

Maybe.

Maybe not.

The red line that marks the prediction is looking more hopeful than plausible. This doesn’t look like much of a max, and the signs that we should already be seeing for the onset of the next solar cycle are seriously subdued.

While some people are still talking about global warming, others are saying that, yes, maybe there was some warming in the late 20th century, which was also a time of nice sun cycles you’ll notice, but there hasn’t been a speck of warming for 15 years, and if history is any indication, we could be in Big Trouble, not with warm, but with cold.

So are we still talking about Earth rings?

We are still talking about Earth Rings.

Two things can make dust fall out of the ring. One is that there is some natural decay in any orbit. Eventually, gravity is going to win all the way and the dust is going to fall. Even the Moon itself is subject to this: it will come home; don’t worry, not soon.

But another influence on space dust would be interaction with the solar wind. The solar wind is the stream of particles that flows out of solar storms. The particles in the solar wind are small, not even atoms, but they are fast and charged, and if there is a ring, they are bound to disturb its particles, and that will thin it out, blast by blast. An unstable orbit is more likely to dump its contents. The YORP effect.

With a quiet sun, however, rings could be expected to grow heavy and dense.

That is the reason for certain parts of the ring forecast. It is based on the position of the rings, which is a very simple calculation based on physics, not on observation: if there is a disc of rings, we can say exactly where it must be.

Second, the forecast is based on the hypothesis that the rings are thickening. If there are rings, they would be expected to thicken during solar quietude. This means more shading and a colder effect on the Earth.

The ring forecast calls for a warm, still summer (likely to involve re-initiation of global warmism), then the onset of autumn as usual, maybe a bit on the harsh side.

Tomorrow, we will look at a simulation to explain and extend the ring forecast.

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No, I didn’t spell it wrong, and it’s not slang.

The YORP effect is named after four scientists who worked out the way that photons can cause small space objects to spin. Now, small can mean a lot of things. If we are talking about asteroids, we are talking about objects of a few miles or a few tens of miles in diameter. Since many asteroids are clumps of rocks, rather than a single rock, a spin could overbalance the light hold of gravity and cause them to break up. In fact, there are many double asteroids, and the YORP effect seems to be the reason.

By the way, the four scientists are Yarkovsky, O’Keefe, Radzievskii, and Paddack. I knew you wanted to know.

And just for the record, there is one asteroid, 1862 Apollo, which gains about 4 minutes a year in its rotations. Whopping spin! If the earth slowed like that, we’d squeeze another month into the calendar every 10,000 years. If it sped up like that, we’d lose a month and be standing still by now.

But YORP isn’t just a matter of speeding up or breaking up asteroids. We have been talking about dust in Earth rings. Individual grains of dust can also have a spin. The spin comes about because the photons hit the dust grains and push them around. Furthermore, they heat the grains unevenly because they are grains, not spheres, and then the grains give off the heat again, with the same kind of thrust we have on a rocket when it’s burned gases go out the back. It’s a very small thrust, but remember that grains of dust in space are also small, and are not floating in the sort of atmosphere that could quickly damp a tiny thrust. They’re out there in a vacuum, and whatever motion begins goes on and on. If photons hit them repeatedly – when photons hit them, all day every day, they can begin to spin and then spin up a little more and a little more. Then when they bump each other, as they are bound to do, they are going to get all ground up.

Thus, the YORP effect makes the ring material get dustier over time. We’ll come back to the dust.

Phoebe ring

I have been trying to find out the reason for the name Phoebe for the second set of rings, the ones whose orbit is tilted towards the Moon. Wiki informs us that Saturn has a moon named Phoebe, and just inside the orbit of Phoebe is a ring that is clearly composed of dust from Phoebe. It is clear for several reasons, but the simplest is that Phoebe orbits Saturn in the opposite direction of most of its other moons, and the dust in this nearby ring orbits in the same unusual direction.

So the hypothesized Moon-oriented ring is called the Phoebe ring.

And I figured out how to embed the YouTube I linked before.

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The Phoebe Ring

So we have asked whether the Earth has a ring made of dust, and we suggested the Moon might supply the dust. But then… if the dust comes from the Moon, why does it form an equatorial ring around the Earth? Why isn’t it in a ring tilted towards the Moon?

Because that’s the natural way for rings to arrange themselves. No matter where the stuff comes from, if it gets into an orbit around the Earth, it will eventually get into an equatorial orbit. James Clerk Maxwell had this in his equations. The bits of dust (or rocks, whatever comes) will bump into each other as long as their orbits are irregular; once they get into an equatorial orbit, they will settle quietly and stop jostling each other so much. There will be, as we will see, still some jostling for various reasons.

So, the grains of dust will “tend” to fall into equatorial Earth orbit. But they will indeed start out in an orbit oriented towards the Moon, even an Earth-Moon orbit, circling the gravitational center of the Earth-Moon system. Step by step, the dust will fall into an Earth orbit oriented towards the Moon, and then into an equatorial Earth orbit. So: a handful of volcanic dust sweeps up into the non-atmosphere of the Moon and starts to orbit the Moon. Pretty quickly, the Earth begins to rob the Moon dust, pulling everything towards and then into Earth orbit, an orbit tilted towards the Moon, an orbit crossing the Earth’s equator but not centered on it.

Gradually, grain by grain, the dust settles into an equatorial orbit.

How long does that take?

I don’t know. A while. But we hypothesize that if there are Earth rings made of Moon dust, (nothing else could reasonably supply enough dust,) there are probably two sets of rings. One is an equatorial disc; the other disc is oriented to the Moon and for now, we are calling it the Phoebe Ring. We can draw the rings in different colors to make things clear, but in fact, both discs contain the same dust, younger, more recently arriving dust in the Phoebe ring, older dust in the equatorial ring.

Phoebe Ring

Now things get interesting, because the Phoebe ring does not follow our seasons. It continues to track the Moon until its dust either falls into the equatorial orbit or falls to Earth altogether. This is going to be very messy. The new ring-shadow is on a journey, which may cause it to fall north in winter or summer, which may bring it edge-on at any time of the year, or rather it will become edge-on at various times as the years go by. It may be more or less coordinated with the equatorial ring or completely at odds.

How well do you know the Moon calendar?

The Phoebe Ring will be cyclic because the motions of the Moon are cyclic, but it won’t be tracking our seasons, which is really the only cycle most of us notice. (Do you actually know, right now, the present phase of the Moon?) If the Phoebe Ring exists, it will, when thick, cause storms at times of the year that can only seem completely random.

Think about the date of Easter and how it floats. It’s the one part of our cultural life that is still based on the Moon. The calculation is:

  1. it’s the first Sunday
  2. after the first full Moon
  3. after the vernal equinox.

The vernal (spring) equinox is March 21 or 22. Any of the next 28 days can be the first full Moon after that. And the next Sunday could be 6 days later. This means that the date of Easter could be any one of 34 days, and that, in turn, should help you recognize that the interaction of the solar year and a Moon ring can vary quite a bit.

The tides are another reminder that the Moon affects the earth, and their phases are enormously complex. They are caused by the gravity of the Moon itself, not by rings, but because of them, we do pay attention to the tides and to the phases of the Moon; it is a cycle for which we have charts and guides.

It is not an easy cycle.

Here’s a simulation to get used to the idea of a ring… and its shadow…  and maybe peek at two rings. youtube=http://www.youtube.com/watch?v=YwO93KeCrmI

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What if Earth had an equatorial ring”

The first set of rings we need to visualize simply orbits the equator of the earth. It’s not just one ring, however; it’s a whole raft of them, arranged concentrically.

To get an idea of what I am saying, look at the Saturn ring images. Here is especially good one from Astronomy Picture of the Day,  (a site that will send you daily spectacular images if you sign up.) This particular image was taken last December by our Cassini spacecraft, from the underside of Saturn’s rings, the dark side, away from the Sun. The dark band in the image is the thickest band of ring material – a bright ring as seen from Earth. Note how that dark band is casting a shadow on the lower hemisphere of Saturn.

Now let’s think about such a ring around Earth.

Earth orbits the sun, from west to east around the Sun. If its axis were perpendicular to the plane of its orbit, the Sun would simply cast the ring shadow on the equator, spring, summer, fall, and winter.

hypothetical rings

It would have weather consequences, but they would hardly be noticed because they would be the same year round and every year.

In fact, however, the axis of the Earth’s rotation is actually tipped 23° and this means that the Sun would fall differently on the rings at different seasons.

  • Near the time of spring and fall equinoxes, the rings would be oriented edge-on to the Sun, so that their shadow would not be spread out at all; it would be a thin line across the globe at the equator. A narrow and heavy ring shadow might depress the natural height of the constant equatorial ring of clouds at the equinoxes, and this could have a weather effect itself.

earth rings

  • In the summer (our summer) the sun would shine atop the rings, casting a shadow on Australia and the southern hemisphere while we have no shadow at all. This would make Australia’s winter cold, which winters are anyway, but if the ring is thicker, the winter would be even colder than usual. Our own summer would not be affected. It would just be warm.
  • In winter, (our winter) the ring would be now illumined from below and Australia’s summer would be unaffectedly warm while our winter would be colder than usual especially if the rings were very heavy with dust. So far, this does not seem very significant. The rings make winter colder. Who would notice, since winters are always cold and we can’t see the rings anyway?
  • Between the pencil-thin and the all-spread-out-in-one-hemisphere orientations, there are all the variants. This is interesting. Think of the time shortly after the pencil-thin shadow of the equinox. Now there is a wider shadow, gradually spreading north in the winter, gradually spreading south in the summer. Even if it’s so diaphanous we can’t see it, it will still have a definite edge, depending on how thick the ring is, and any shadow’s edge is going to be windy. The air in the shadow is cooled, the cool air sinks and slips in below the warm air nearby. That makes wind. Big winds make storms. Since it is all coordinated with the seasons anyway, it is not particularly noticeable, but it is real.

This is the simple part of the ring hypothesis. It’s worth taking a few minutes to get it straight, because the next part is very complicated.

 

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