news about the zodiac.

Source of zodiac glow identified

This result confirms what meteor astronomer Jenniskens of the SETI Institute in Mountain View, Calif., had long suspected. An expert on meteor showers, he had noticed that most consist of dust moving in orbits similar to those of Jupiter Family comets, but without having active dust-oozing comets associated with them.

Instead, Jenniskens discovered a dormant comet in the Quadrantid meteor shower in 2003 and has since identified a number of other such parent bodies. While most are inactive in their present orbit around the Sun, all have in common that they broke apart violently at some point in time in the past few thousand years, creating dust streams that now have migrated into Earth's path.

Nesvorny and Jenniskens, with the help of Harold Levison and William Bottke of the Southwest Research Institute, David Vokrouhlicky of the Institute of Astronomy at Charles University in Prague, and Matthieu Gounelle of the Natural History Museum in Paris, demonstrated that these comet disruptions can account for the observed thickness of the dust layer in the zodiacal cloud.

In doing so, they solved another mystery. It was long known that snow in Antarctica is laced with micro-meteorites, some 80 to 90 percent of which have a peculiar primitive composition, rare among the larger meteorites that we know originated from asteroids. Instead, Nesvorny and Jenniskens suggest that most antarctic micro-meteorites are pieces of comets. According to their calculations, cometary grains dive into Earth's atmosphere at entry speeds low enough for them to survive, reach the ground, and be picked up later by a curious micro-meteorite hunter.

source of information : http://esciencenews.com/articles/2010/04/15/source.zodiac.glow.identified

Source of Night Sky's Cosmic Zodiacal Glow Explained

The origin of a mysterious glow that stretches across the nighttime sky has been identified by scientists who examined the particles that make up the luminous dust cloud.

Called zodiacal light, the faint glow is caused by millions of tiny particles along the path followed by the sun, moon and planets across our sky, also known as the ecliptic.

The faint, whitish glow, which can be seen best in the night sky just after sunset and before sunrise in the spring and autumn, was first correctly identified by Joshua Childrey in 1661 as sunlight that is scattered in our direction by dust particles in the solar system.

Yet, the source of the thick cloud of dust has been a topic of debate.  In a new study, David Nesvorny and Peter Jenniskens found that more than 85 percent of the zodiacal dust originated from Jupiter family comets (so-called because their orbits are modified by their close passage to the gas giant Jupiter), rather than asteroids, as was previously thought.

"This is the first fully dynamical model of the zodiacal cloud," said Nesvorny, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "We find that the dust of asteroids is not stirred up enough over its lifetime to make the zodiacal dust cloud as thick as observed. Only the dust of short-period comets is scattered enough by Jupiter to do so."

The researchers identified the dust coming from Jupiter family comets after examining the shape of the zodiacal cloud, said Nesvorny.

"With other comets, like the Halley-type comets, they have large orbital inclinations," Nesvorny told SPACE.com. "They are coming into the inner solar system from all directions, so if these comets were producing the zodiacal cloud, it would be almost a ball, and not a disk. Telescopes, like Spitzer, all show that the zodiacal cloud is a disk. This points best to Jupiter family comets, which have more moderate inclinations."

These results confirm what Jenniskens, an astronomer with the SETI Institute in Mountain View, Calif., had long suspected. As an expert on meteor showers, Jenniskens had noticed that most consist of dust moving in orbits similar to those of the Jupiter family comets, but without having dust-shedding comets associated with them.

Instead, Jenniskens discovered a dormant comet in the Quadrantid meteor shower in 2003 and has since identified a number of other such parent bodies.

While most are inactive in their present orbit around the sun, all broke apart violently at some point in time in the past few thousand years, creating debris in the form of dust streams that have now migrated into Earth's path.

Nesvorny and Jenniskens, with the help of Harold Levison and William Bottke of the Southwest Research Institute, David Vokrouhlicky of the Institute of Astronomy at Charles University in Prague, and MatthieuGounelle of the Natural History Museum in Paris, demonstrated that these comet disruptions can account for the observed thickness of the dust layer in the zodiacal cloud.

In doing so, they also solved another mystery.

It was long-known that snow in Antarctica is laced with micrometeorites, some 80 to 90 percent of which have a peculiar primitive composition, rare among the larger meteorites that originated from asteroids.

"These micrometeorites are small meteorites that are about 100 microns in size," Nesvorny explained. "These micrometeorites are collected in the Antarctic ice, and it's been puzzling as to why they have a different composition than the large meteorites that are collected elsewhere."

Nesvorny and Jenniskens suggest that most Antarctic micrometeorites are actually fragmentations of comets, which explains the different composition from other meteorites that come from the asteroid belt. According to their calculations, cometary grains dive into Earth's atmosphere at entry speeds low enough for them to survive and reach the ground.

The study will be detailed in the April 20 issue of The Astrophysical Journal.

source of information :http://www.space.com/8251-source-night-sky-cosmic-zodiacal-glow-explained.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+spaceheadlines+%28SPACE.com+Headline+Feed%29

Have the Zodiac and star

 signs changed?

Newspaper reports of an astronomer's view that star signs cover the wrong dates have caused a hubbub. So are Pisceans really Aquarians?

Astronomy instructor Parke Kunkle has generated a wealth of online and watercooler discussion in the US and elsewhere.

He pointed out in a Minnesota newspaper that a wobble in the Earth's rotation meant that the stars were in a different position - as viewed from Earth - than they were 2,000 years ago when the Zodiac was drawn up.

Since then the story has gone viral, with many - including those with star sign tattoos - expressing concern about the possibility that they might have spent years checking the wrong horoscope. But should they be looking at the predictions for a different sign?

Astrologers say this shift in where the stars appear to be has no bearing on the "tropical zodiac" system. This is what is typically used for horoscopes in the West, and is based on segments of the sky calculated using the equinoxes on Earth.

For astronomers, astrology is mere pseudo-science, and there are plenty of people who only read their horoscope with their tongue firmly in their cheek. But even if you regard astrology as bunk, Kunkle's pronouncement still reveals an interesting astronomical phenomenon.

The stars appear to move over the years because of a 26,000-year long process called "precession". This is because of the wobble in the rotation of the Earth.

"As a consequence of this, over the last two thousand years the dates at which the Sun appears to move in front of each background constellation of stars has altered by a few days," says Dr Marek Kukula, public astronomer at the Royal Observatory, Greenwich.

"Astronomers therefore have to include the effects of precession to ensure that long-term predictions of celestial motions are accurate. The gravity of the Sun and Moon work together on the Earth's tilted axis, causing it to gradually shift.

"Every 26,000 years it slews slowly round in a circle, much like a spinning top wobbling around on a table. This additional motion is almost imperceptible on everyday timescales but over centuries it adds up."

So the stars don't move, but they appear to move over the course of centuries.

It is this process that led Kunkle to calculate that the Zodiac should have new dates.

But this is an annoying red herring for astrologers.They point out that this precession only affects "sidereal" astrology, a system commonly used in India and by other people with an interest in "Vedic" astrology. In this system signs do shift.

"People might be Aquarian under the tropical system but a Capricorn under the Vedic system," says astrologer Russell Grant.

But in the "tropical zodiac", the system used by Western astrologers, there are fixed sectors of the sky through which the movement of the sun, moon and planets is tracked to create the basis for horoscopes.

"The tropical zodiac is fixed and immovable. It is static - whatever happens to the Earth's axis."

These sectors of the sky use the equinoxes as reference points, not the constellations upon which the star signs are based.

"I'm delighted that I'm looking at the exact same sector of the sky that my Babylonian equivalent looked at," notes astrologer Jonathan Cainer.

The suggestion of a 13th sign - "Ophiuchus" - is also dismissed by astrologers. They point out that the Babylonians consciously chose 12 signs.

In other systems there have also been signs for "Arachne" and "Cetus the Whale", notes Grant.

Both the effect of precession on the zodiac and the presence of a 13th sign are stories that crop up in the press every few years, Grant explains.

And the coverage is irksome to astrologers, says Cainer.

"I'm annoyed because it treads on several raw nerves at once."

The publicising of the effect - and its use to dismiss astrology - represents the "incredible bigotry some members of the scientific community display towards astrology", says Cainer.

But for all those people that believe in astrology or set some kind of lesser emotional store on the signs of the zodiac, they can rest easy. The Tauruses are still Taurus and the Arieses are still Aries.

source of information : http://www.bbc.co.uk/news/magazine-12207811