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Archive for November, 2009

Texhash

In LaTex, after you have downloaded the necessary files (styles, …), you have to make these visible to TeX. This is accomplished by the following two steps.

Create a private tex hierarchy.
Under your home directory, create a subdirectory “texmf”, underneath that directory create another one called “tex”. and under this one create a “latex” directory. Thus, you should have a tree of the form

$HOME/texmf/tex/latex/

Place document class files (extension .cls), packages and any other custom style files for use by LaTeX (extensions .sty or .tex) into this directory.

If custom bibtex style files (extension .bst) have been provided, place these into a similar tree of the form

$HOME/texmf/bibtex/bst/

for *.bib files, create  a directory

$HOME/texmf/bibtex/bib/

and put .bib files there.

all *.fd files into the directory $TEXMF/tex/latex/

Run the “texhash” program. Simply type “texhash” at the prompt. This will create a database of files inside your texmf directory. The database file is called, appropriately enough, “ls-R”, and is located in the top level texmf directory, i.e., the file is $HOME/texmf/ls-R . The ls-R file is an ordinary text file and can be inspected with an editor or a pager like more or less. If the texhash run was successful, this file should contain a listing of all files under your private texmf directory.

Whenever you add new files to your texmf tree, be sure to run texhash. For efficiency reasons, TeX does not search for files, but only consults the ls-R database; if a (non-standard) file is not listed in this database, TeX will not find it.

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The Perseids is one of the best meteor showers, associated with the comet Swift-Tuttle. The Perseids are so-called because the point they appear to come from, called the radiant, lies in the constellation Perseus.  The stream of debris is called the Perseid cloud and stretches along the orbit of the comet Swift-Tuttle. The cloud consists of particles ejected by the comet as it travels on its 133-year orbit. Most of the dust in the cloud today is around a thousand years old. However, there is also a relatively young filament of dust in the stream that was pulled off the comet in 1862. The rate of meteors originating from this filament is much higher than for the older part of the stream.

The Perseid meteor shower has been observed for about 2000 years, with the earliest information on this meteor shower coming from the Far East.

Double meteors from Perseids 2007, passing through the summer triangle.

The shower is visible from mid-July each year, with the peak in activity being between August 9 and 14, depending on the particular location of the stream. During the peak, the rate of meteors reaches 60 or more per hour. They can be seen all across the sky, but because of the path of Swift-Tuttle’s orbit, Perseids are primarily visible in the northern hemisphere. As with all meteor showers, the rate is greatest in the pre-dawn hours, since the side of the Earth nearest to turning into the sun scoops up more meteors as the Earth moves through space.

The shower is visible from mid-July each year, with the peak in activity being between August 9 and 14, depending on the particular location of the stream. During the peak, the rate of meteors reaches 60 or more per hour. They can be seen all across the sky, but because of the path of Swift-Tuttle’s orbit, Perseids are primarily visible in the northern hemisphere. As with all meteor showers, the rate is greatest in the pre-dawn hours, since the side of the Earth nearest to turning into the sun scoops up more meteors as the Earth moves through space.

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Install something in python

If you have a package with a python installer, this is usually how you should proceed:

$ python setup.py build
$ python setup install

In some case, you have to switch to root  before doing that.

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from a desperate scientist

“Mea culpa, mea culpa, mea maxima culpa, I’m an idiot….  Here is the reality. I used  a = .15 and b = .05, and the result is attached. Now I’m going
to get the rope that is in the main entrance hall, and hang myselfup. You will have to include me as co-author, but with this “+” sign that you can do in tex, which means DEAD. ”

It is real !

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Python: things to install 1

You have to install Python and a couple of necessary libraries on your computer. If you use Linux, most of them are accessible thought your package manager. There are three packages which are especially required for the astronomical data analysis: 1) Pyfits, provided by STSCT/NASA, to read/write the FITS file, 2) NumPy and SciPy for working with numeric arrays and mathematical operation, and 3) Matplotlib, a collection of MatLab routines to plot a variety of data. Some extra packages, relevant for astronomical data reduction, will be explained below.

PyFITS

PyFITS provides an interface to FITS formatted files under the Python scripting language. It is useful both for interactive data analysis and for writing analysis scripts in Python using FITS files as either input or output. PyFITS is a development project of the Science Software Branch at the Space Telescope Science Institute. The manual provides a tutorial on how to use PyFITS with FITS images and FITS tables, along with an extensive description of all the methods (currently) available for working with FITS files.

I will show examples of Pyfits in a later post.

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Leonids 2009

The Leonids 2009 meteor shower did not show any sort of storm or outburst. It was rather a normal meteor shower witha  peak activity about 100 (comparable to Perseids). Well, we might have to wait quite a bit, say 20 years to the next expected outburst. The following plots are from IMO webpage, showing the zenith hourly rate.

The graph below shows the ZHR (Zenithal Hourly Rate), which is the number of meteors an observer would see under a very dark sky with the radiant of the shower in zenith.


ZHRmax = 88 based on 894 Leonids reported in 341 intervals, assuming population index r = 2.3

Peak of the Leonids activity, 2009

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A meteor is the visible streak of light that occurs when a meteoroid enters the Earth’s atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km. Millions of meteors occur in the Earth’s atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between about 40 and 75 miles (65 and 120 kilometers) above the earth. They disintegrate at altitudes of 50 to 95 km. 

For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres) the visibility is due to the air friction that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second. A relatively small percentage of meteoroids hit the Earth’s atmosphere and then pass out again: these are termed Earth-grazing fireballs.

Meteors may occur in showers, which arise when the Earth passes through a trail of debris left by a comet, or as “random” or “sporadic” meteors, not associated with a specific single cause. In an active meteor shower like Perseids, almost one meteor can be observed per minute during the peak time.

 

APOD: 2008 January 3 - Geminids in 2007. It seems that all of the meteors are coming from a single point, the radiants.

The radiant or apparent radiant of a meteor shower is the point in the sky, from which (to a planetary observer) meteors appear to originate. The Perseids, for example, are meteors which appear to come from a point within the constellation of Perseus. An observer might see such a meteor anywhere in the sky but the direction of motion, when traced back, will point to the radiant. A meteor that does not point back to the known radiant for a given shower is known as a sporadic and is not considered part of that shower.



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