Watch the Sky: November has always been a good time to watch the night sky -- the air is often clear, and it's not yet as chilly as it gets as we move into winter. This year, the sky's November visual displays are expected to be particularly spectacular: We will have meteor showers/storms and perhaps Aurora Borealis displays, the latter of which are only rarely visible in our latitudes.

Meteors: On the night of November 18, our planet will pass through some of the more dense dust streams left by Comet Temple-Tuttle, and we should be able to see hundreds of "meteors" in the hours of darkness. There will be fewer, but perhaps still a good show, on the nights immediately preceding and following the 18th. The best time to watch will be right around local midnight.

Meteors, as you probably know, are bits of stuff -- usually small bits -- left behind by passing comets. Wherever a comet has been in any of its previous orbits, there are dust trails. As comets go looping around the sun, some of the cometary material boils off, and it's pushed away from the comet by the solar wind. That's what makes up the comet's tail, which is only visible as the comet approaches the sun but which really stretches out all along its orbit. We don't ever want to be right where a comet is, but we can get really neat light shows when we pass through a place where a comet was on a previous orbit. Some comets, like Temple-Tuttle, have orbits that routinely cross the orbit of the Earth. Comet orbits are fairly stable (unless they are "perturbed" by the gravity of a planet) so their dust trails tend to bunch up like coils of ribbons. It has only been in the last few years that we have actually been able to tell where those ribbons are and predict when the Earth will intersect with them. This year the earth will fly through several of the more recent and therefore more prominent ribbons left behind by Temple-Tuttle. North and Central America and Australia will get the best shows -- it will be about midnight there when earth cuts directly through some ribbons, but there should be enough meteors to go around for the whole world. 1999 was a vintage year for Leonid meteors, and 2001 will probably be better. Stick around for 2002 -- it should be best of all. (Temple-Tuttle meteors are called Leonids because they appear to be coming from the Leo constellation.)

Meteor Internet links:

Temple-Tuttle/Leonid Meteor "storm warning": http://www.spacedaily.com/news/leonid-01a.html

Leonids 2001: http://eclipse.span.ch/leonids.htm

Leonid Meteor Showers: http://www.atnf.csiro.au/asa_www/info_sheets/leonids.html

Explanation of the Temple-Tuttle/Leonid dust trails: http://www.arm.ac.uk/leonid/dustexpl.html

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Aurora Borealis: The Northern Lights (Aurora Borealis in Latin) are streaks, blobs, curtains, or waves of light of various colors that are caused by particles ejected by the Sun that get caught in the earth's magnetic field. First, people thought that the Sun emitted only visible light. Somewhere in the 19th century, we figured out that the Sun's radiation spectrum spanned the whole electromagnetic range, of which visible and invisible light is only a small part. Its only been in the past few years that scientists have proven that other things besides the massless photons (which make up the electromagnetic radiation spectrum) are also always streaming away from the sun. These other things -- atomic and sub-atomic particles that do have mass -- are what makes up the "solar wind", and they are what cause the Earth's Aurora Borealis (Northern Hemisphere) and Aurora Australis (Southern Hemisphere).

The solar wind blows at a fairly stable rate except under special circumstances. When there is a "magnetic storm" on the surface of the Sun, there is a chance for a "Solar mass ejection" that can vastly increase the speed and density of the solar wind. The mechanism is not entirely clear, but the most promising theory is that some magnetic mechanism inside the sun causes big clumps of particles to leap away from the Sun's surface. In most cases, they appear to swing back down to the surface to form those spectacular loops we've all seen pictures of. (If you really have never seen one, go to http://www.spacescience.com/headlines/y2000/ast20jan_1.htm to see NASA pix taken by the Solar and Heliospheric Observatory (SOHO) -- wait for the animation to load, watch it, and then click around on the links, including the picture, where a closer view is available.)

Sometimes, the magnetic force that spits out the clumps of particles is so great that they are ejected at a speed higher than the Sun's "escape velocity" -- they escape the Sun's gravity and go streaming out into space toward the planets. If the ejection is on the side of the Sun facing toward the Earth, it looks like a smoke ring billowing out around the Sun and coming in our direction. Because the billows are made up of particles that have mass, they travel at less than the speed of light.

The physics of all this is rather complicated and not fully understood, but the simple part is that bigger Solar Mass Ejections make bigger and brighter Auroras, and that the ejections seem to be biggest in the 12 to 18 months after peaks in the eleven year sunspot cycles. That's where we are now. For other reasons that are also only beginning to be understood, the passing particles seem to have the greatest interaction with our magnetic field and upper atmosphere around the Spring (Vernal) and Fall (Autumnal) Equinoxes -- March 21 and September 21. We're still close enough to the Fall Equinox to get maximum Auroras for our Solar Mass Ejections. There was a huge ejection on October 18 and another on October 25. The October 18 ejection produced Aurora Borealis displays as far south as the Carolinas in the US -- much further south than we are here -- and the October 25 ejection was even bigger. More ejections are expected in weeks to come, so look north at night and see if you can spot the glow.

We get about a 48-hour warning from that SOHO satellite and from another called ACE (Advanced Composition Explorer) that measures the composition and speed of the ejections. Both satellites are in synchronous "halo orbits" around the Sun that seem to park them in fixed spots between the Earth and the Sun. (For an explanation of how that works, go to http://www-spof.gsfc.nasa.gov/Education/wlagran.html.) We can get an advance warning because the ejections get to the satellites two days before they get to us (more or less, depending on the speed of the ejection), and the satellites can then send us light-speed computerized radio signals that take only a few minutes to get here. We have pictures of the ejections two days before the ejections themselves get here.

That's why we know when to look for Auroras and whether they might be visible at our latitudes. To sign up for the mailing list for free electronic notices of when the Ejections (and other space phenomena) are coming, go to: http://science.nasa.gov/news/subscribe.asp?checked=sngswtc. (Other free NASA mailing lists are on the same page.)

More Aurora Borealis/Solar phenomena links:

Seasonal Aurora peaks: http://science.nasa.gov/headlines/y2001/ast26oct_1.htm?list95764

The Aurora Page (with Links to other pages): http://www.geo.mtu.edu/weather/aurora/

Space Weather and real time info from SOHO and ACE: http://www.spaceweather.com/

NASAs ACE Mission: http://www.srl.caltech.edu/ACE/

SOHO: http://sohowww.nascom.nasa.gov/

NASA Space Science for all operating NASA missions and a lot more: http://spacescience.nasa.gov/missions/opmsns.htm#SOHO

Browse the NASAKIDS Page or sign your kids up for the free NASA KIDS Update: http://kids.msfc.nasa.gov/

Go to http://www.mmdtkw.org/Veneto2001.html for other recent articles.