Shallow Thoughts : : Apr

Akkana's Musings on Open Source Computing, Science, and Nature.

Fri, 27 Apr 2012

Venus is at its brightest -- why? And how to calculate it

Venus has been a beautiful sight in the evening sky for months, but at the end of April it's reaching a brightness peak, magnitude -4.7.

By then, if you look at it in a telescope or even good binoculars, you'll see it has waned to a crescent. That's a bit non-obvious: when the moon is a crescent, it's a lot fainter than a full moon. So why is Venus brightest in its crescent phase?

It has to do with their orbits. The moon is always about the same distance away, about 385,000 km or 239,000 miles (I've owned cars with more miles than that!), though it varies a little, from 362,600 km at perigee to 405,400 km at apogee.

When we look at the full moon, not only are we seeing the whole Earth-facing surface illuminated, but the central part of that light is reflecting straight up off the moon's surface. When we look at a crescent moon, we're seeing light that's near the moon's sunrise or sunset point -- dimmer and more spread out than the concentrated light of noon -- and in addition we're seeing less of it.

Venus, in contrast, varies its distance from us immensely. We can't see Venus when it's "full", because it's on the other side of the sun from us and lost in the sun's glow. It'll next be there a year from now, in April of 2013. But if we could see it when it's full, Venus would be a distant 1.7 AU from us. An AU is an Astronomical Unit, the average distance of the earth from the sun or about 89 million miles, so Venus when it's full is about 170 million miles away. Its disk is a tiny 9.9 arcseconds (an arcsecond is 1/3600 of a degree) -- about the size of Mars this month.

In contrast, when we look at the crescent Venus around the end of this month, although we're only seeing about 28% of its surface illuminated, and that only with glancing twilight rays, it's much closer to us -- less than half an AU, or about 45 million miles -- and its disk extends a huge 37 arcseconds, bigger than Jupiter this month.

Of course, eventually, as Venus pulls between us and the sun, its crescent gets so slim that even its huge size can't compensate. So its peak brightness happens when those two curves cross, when the disk is somewhere around 27% illuminated, as happens at the end of this month and the beginning of May.

Exactly when? Good question. The RASC Handbook says Venus' "greatest illuminated extent" is on April 30, but PyEphem and XEphem say Venus is actually brighter from May 3-8 ... and when it emerges from the sun's glare and moves into the morning sky in June, it'll be slightly brighter still, peaking at magnitude -4.8 in the first week of July.)

Tracking Venus with PyEphem

When I started my Shallow Sky column this month, I saw the notice of Venus's maximum brightness and greatest illuminated extent in the RASC Handbook. But I wanted more details -- how much did its distance and size really change, when would the brightness peak again as it emerged from the sun's glare, when would it next be "full"?

PyEphem made it easy to calculate all this. Just create an ephem.Venus() object, calculate its values for any date of interest, then print out parameters like phase, mag, earth_distance and size. In just a few minutes of programming, I had a nice table of Venus data.

import ephem

venus = ephem.Venus()

print '%10s   %6s %6s %6s %6s' % ('date', '%', 'mag', 'dist', 'size')
def print_venus(when) :
    venus.compute(when)
    fmt = '%02d-%02d-%02d   %6.2f %6.2f %6.2f %6.2f'
    trip = when.triple()
    print fmt % (trip[0], trip[1], trip[2],
                 venus.phase, venus.mag, venus.earth_distance, venus.size)

# Loop from the beginning of 2012 through the middle of 2013:
d = ephem.date('2012')
end_date = ephem.date('2013/6/1')
while d < end_date :
    print_venus(d)
    # Add a week:
    d = ephem.date(d + ephem.hour * 24)

I've found PyEphem very handy for calculations like this -- and it's great to be able to double-check listings in other publications.

Tags: , ,
[ 13:44 Apr 27, 2012    More science/astro | permalink to this entry | comments ]

Tue, 24 Apr 2012

Firefox stopped accepting remote commands

When I upgraded to Firefox 11 a month or so ago, I got a surprise: I couldn't invoke firefox from other applications any more. Clicking on a link in an app such as xchat just gave me the Firefox Profile Manager dialog, instead of opening the link in the browser I was already running.

I couldn't find anything written about it, so I've been putting up with it, copying each link then switching to the desktop where Firefox is running and middleclick-pasting it into the browser. But this morning, I did a new round of searching, and finally found the answer, in bug 716110. and its duplicate, 716361.

Quoting from bug 716110::

[The developers] changed the -no-remote flag's behavior in a
surprising, backward incompatible way. Before, it just meant "start a
new instance." Now, it also means "don't listen for remote commands."
Apparently the change went in for Firefox 9, because of bug 650078.

Indeed, that was the problem. I have multiple Firefox profiles, so I use -no-remote -P profilename when I start Firefox, so each profile doesn't conflict with one that might already be running.

But with Firefox 9 or later, you can't do that. Instead, run your first, primary profile without -no-remote; then if you start up other profiles later, run them with -no-remote so they don't conflict with the first one. That works okay for my typical usage, fortunately: I have a main Firefox window I run all day, and only start up other profiles for short periods.

But since not everyone uses this model, fortunately, some upcoming Firefox version will fix the problem by adding a new runtime flag, -new-instance, to do what -no-remote used to do: start up a window for a new profile, rather than talking to the running Firefox. Here's the new --help text:
-no-remote Do not accept or send remote commands; implies -new-instance.\n
-new-instance Open new instance, not a new window in running instance.\n
The web Command Line Options page doesn't seem to have been updated yet, but perhaps it will when the Firefox with the fix is released.

Of course, it would have been much simpler if Firefox just honored the -P flag and used whatever profile it was given, as suggested by a commenter in bug 650078. But bsmedberg replies that the complexity of the code makes that difficult.

The new arguments look more sensible than the old -no-remote, though it's frustrating that it was so hard to find information about changes like this. All three bugs are filled with comments from people who, like me, lost a lot of time trying to figure out what broke and how to launch URLs remotely after the change. Thanks to Ryan for clarifying the issue and filing the bug to fix the problem, and to Jed, who added the new flag with his first Mozilla patch. Hooray for open source!

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[ 10:26 Apr 24, 2012    More tech/web | permalink to this entry | comments ]

Sat, 21 Apr 2012

Android WebView can't goBack from a page with iframes

I've been fighting a bug in Android's WebView class for ages: on some pages, clicking FeedViewer's back arrow (which calls WebView::goBack()) doesn't go back to the previous page. Instead, it jumps to some random position in the current page. If you repeat it, eventually, after five or so tries (depending on the page), eventually goBack() will finally work and you'll be back at the previous page.

It was especially frustrating in that it didn't happen everywhere -- only in pages from certain sites. I saw it all the time in pages from the Los Angeles Times and from Make Magazine, but only rarely on other sites.

But I finally tracked it down: it's because those pages include the HTML <iframe> tag. Apparently, if a WebView is on a page (at least if it's a local page) that contains N iframes, the first N calls to goBack will jump somewhere in the document -- probably the location of the associated iframe, though I haven't verified that -- and only on the N+1st call will the WebView actually go back to the previous page.

The oddest thing is, this doesn't seem to be reported anywhere. Android's bug tracker finds nothing for webview iframe goback, and web searching hasn't found a hint of it, even though I see this in Android versions from 1.6 through 2.3.5. How is it possible that other people haven't noticed this? I wonder if it only happens on local file:// URLs, and not many people use those.

In any case, I'm very happy to have found the answer at last. It was easy enough to modify FeedMe to omit iframes (and who wants iframes in simplified HTML anyway?), and it's great to have a Back button that works on any page.

Tags: ,
[ 19:56 Apr 21, 2012    More programming | permalink to this entry | comments ]

Wed, 18 Apr 2012

Mounting a Samsung Galaxy Player on Linux

My new toy: a Samsung Galaxy Player 5.0!

So far I love it. It's everything my old Archos 5 wanted to be when it grew up, except the Archos never grew up. It's the same size, a little lighter weight, reliable hardware (no random reboots), great battery life, fast GPS, modern Android 2.3, and the camera is even not too bad (though it certainly wouldn't tempt me away from my Canon).

For the record, Dave got a Galaxy Player 4.0, and it's very nifty too, and choosing between them was a tough choice -- the 4-inch is light and so easy to hold, and it uses replaceable batteries, while the 5-inch's screen is better for reading and maps.

USB-storage devices don't register

I love the Galaxy ... but there's one thing that bugs me about it. When I plug it in to Linux, dmesg reports two new storage devices, one for main storage and one for the SD card. Just like most Android devices, so far.

The difference is that these Samsung devices aren't fully there. They don't show up in /proc/partitions or in /dev/disk/by-uuid, dmesg doesn't show sizes for them, and, most important, they can't be mounted by UUID from an fstab entry, like

UUID=62B0-C667   /droidsd    vfat   user,noauto,exec,fmask=111,shortname=lower 0 0
That meant I couldn't mount it as myself -- I had to become root, figure out where it happened to show up this time (/dev/sdf or wherever, depending on what else might be plugged in), mount it, then do all my file transfers as root.

I found that if I mounted it explicitly using the device pathname -- mount /dev/sdf /mnt -- then subsequently the device shows up normally, even after I unmount it. So I could check dmesg to find the device name, mount it as root, unmount as root, then mount it again as myself using my fstab entry. What a pain!

A kernel expert I asked thought it looked like the Samsung is pretending to be a removable device, but only "plugging in" when the system actually tries to access it. Annoying. So how do you get Linux to "access" it?

Udev: still an exercise in frustration

The obvious solution is a udev rule. Some scrutiny of /lib/udev/rules.d/60-persistent-storage.rules found some rules that did this intriguing thing: IMPORT{program}="ata_id --export $tempnode".

Naturally, this mysterious ata_id is undocumented. It's hidden in /lib/udev/ata_id, and I found this tiny ata_id man page online since there's none available in Ubuntu. Running ata_id /dev/sdf seemed to do what I needed: it made the device show up in /proc/partitions and /dev/disk/by-uuid, and after that, I could mount it without being root.

I created a file named /etc/udev/rules.d/59-galaxy.rules, with the rule:

KERNEL=="sd[b-g]", SUBSYSTEMS=="usb", ATTRS{idVendor}=="04e8", SYMLINK+="galaxy-%k-%n", IMPORT{program}="ata_id --export $tempnode"

When I tested it with udevadm test /block/sdf, not only did the rule trigger correctly, but it actually ran ata_id and the device became visible -- even though udevadm test states clearly no programs will be run. How do I love udev -- let me count the ways.

But a reboot revealed that udev was not actually running the rule when I actually plugged the Galaxy in -- the devices did not become visible. Did I mention how much I love udev?

Much simpler: a shell alias

But one thing I'd noticed in all this: side by side with /dev/disk/by-uuid is a directory called /dev/disk/by-id. And the Samsung devices did show up there, with names like usb-Android_UMS_Composite_c197022a2b41c1ae-0:0.

Faced with the prospect of spending the rest of the day trying random udev rules, rebooting each time since that's the only way to test udev, I decided to cheat and find another way. I made a shell alias:

alias galaxy='sudo sh -c "for d in /dev/disk/by-id/usb-Android*; do /lib/udev/ata_id --export \$d; done"'

Typing galaxy will now cause the Samsung to register both devices; and from then on I can mount and unmount them without needing root, using my normal fstab entries.

Update: This works for the Nook's main storage, too -- just add x/dev/disk/by-id/usb-B_N_Ebook_Disk* to the list -- but it doesn't work reliably for the Nook's SD card. The SD card does show up in /dev/disk/by-id along with main storage; but running ata_id on it doesn't make its UUID appear. I may just change my fstab entry to refer to the /dev/disk/by-id device directly.

Tags: , ,
[ 12:43 Apr 18, 2012    More linux/kernel | permalink to this entry | comments ]

Mon, 09 Apr 2012

Quick Guide to Android ADB

I've been fiddling with several new Android devices, which means I have to teach myself how to use adb all over again.

adb is the Android Debug Bridge, and it's great for debugging. It lets you type commands on your desktop keyboard rather than tapping them into the soft keyboard in Android's terminal emulator, it gives you a fast way to install apps, and most important, it lets you get Java stack traces from crashing programs.

Alas, the documentation is incomplete and sometimes just plain wrong. Since I don't need adb very often, I always forget how to use it between sessions, and it takes some googling to figure out the tricks. Here are the commands I've found most helpful.

Start the server

First you have to start the adb, and that must be done as root. But adb isn't a system program and probably lives in some path like /home/username/path/to/android-sdk-linux_x86/tools. Even if you put it in your own path, it may not be in root's. You can probably run it with the explicit path:

$ sudo /path/to/android-sdk-linux_x86/tools/sudo adb start-server
or you can add it to root's path:
# export PATH=$PATH:/path/to/android/android-sdk-linux_x86/tools
# adb start-server

If you're also running eclipse, that probably won't work the first time, because eclipse may also have started an adb server (that gets in the way when you try to run adb manually). if you don't see "* daemon started successfully *", try killing the server and restarting it:

# adb kill-server
# adb start-server
* daemon not running. starting it now on port 5037 *
* daemon started successfully *

Keep trying until you see that "* daemon started successfully *" message.

Connecting

$ adb usb

Occasionally, this will give "error: closed". Don't panic -- sometimes this actually means "I noticed something connected on USB and automatically connected to it, so no need to connect again." It's mysterious, and no one seems to have an explanation for what's really happening. Anyway, try running some adb commands and you may find you're actually connected.

Shell and install

The most fun is running an interactive shell on your Android device.

$ adb shell
It's just busybox, not a full shell, so you don't have nice things like tab completion. But it's still awfully useful.

You can also install apps. On some devices (like the Nook, where I haven't found a way to allow install from non-market sources), it's the only way to install an apk file.

$ adb install /path/to/appname.apk

If the app is already installed, you'll get an error. Theoretically you can also do adb uninstall first, but when I tried that it just printed "Failure". But you can use -r for "reinstall":

$ adb install -r /path/to/appname.apk

There is no mention of either uninstall or -r in the online adb documentation, though adb -h mentions it.

Update: To uninstall, you need the full name of the package. To get the names of installed packages (another undocumented command), do this: adb shell pm list packages

Debug crashes with logcat

Finally, for debugging crashes, you can start up a logcat and see system messages, including stack traces from crashing apps:

$ adb logcat

Logcat is great for fixing reproducible crashes. Sadly, it's not so good for random/sporadic crashes that happen during the course of using the device.

You're supposed to be able to do adb logcat -s AppName if you're only interested in debugging messages from one app, but that doesn't work for me -- I get no output even when the app runs and crashes.

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[ 10:32 Apr 09, 2012    More tech | permalink to this entry | comments ]

Tue, 03 Apr 2012

Displaying equations on the web

How do you show equations on a web page? Every now and then, I write an article that involves math, and I wrestle with that problem.

The obvious (but wrong) approach: MathML

It was nearly fifteen years ago that MathML was recommended as a standard for embedding equations inside an HTML page. I remember being excited about it back then. There were a few problems -- like the availability of fonts including symbols for integrals, summations and so forth -- but they seemed minor. That was 1998.

Now, in 2012, I found myself wanting to write an article involving an integral, so I looked into the state of MathML. I found that even now, all these years later, it wasn't widely supported.

In Firefox I could show some simple equations, like x = 0 x x and x = b ± b 2 4 a c 2 a

But when I tried them in Chromium, I learned that webkit-based browsers don't support MathML. At all. The exception is Safari: apparently Apple has added some MathML support into their browser but hasn't contributed that code back to webkit (yet?)

Besides that, MathML is ridiculously hard to use. Here's the code for that little integral:

<math xmlns="http://www.w3.org/1998/Math/MathML">
<mrow>
<semantics>
  <mrow>
    <msubsup>
      <mo>&int;</mo>
      <mn>x = 0</mn>
      <mi>&#x221E;</mi>
    </msubsup>
    <mfrac>
      <mrow>
        <mo>&dd;</mo>
        <mi>x</mi>
      </mrow>
      <mi>x</mi>
    </mfrac>
  </mrow>
</semantics>
</mrow>
</math>

Ugh! You can't even specify infinity without using an HTML numeric entity. And the code for the quadratic equation is even worse (use View Source if you want to see it).

Good ol' tables

Several years ago, I wrote about the Twelve Days of Christmas and how to calculate the total number of gifts represented in the song.

I needed summations, and I was rather proud of working out a way to use HTML tables to display all the sums and line up everything correctly. It wasn't exactly publication-quality graphics, but it was readable.

More recently, I worked out a way to do exponentials that way, and found a hint about how to do integrals:

now
P (t)  dt
P0 =————
1 + t
0

Looks a little better than the tiny MathML version. But the code isn't any easier to read:

<table border="0" cellpadding="0" cellspacing="0">
<tr><td><td align="center"><small><i>now</i></small></td><td></td><td></td></tr>
<tr>
 <td>
 <td rowspan="3" valign="middle"><font size="6" style="font-size:3em" class="bigsym">&#8747;</font>
 <td align="center"><i>P</i>&nbsp;(<i>t</i>)</td>

 <td rowspan="3" valign="middle">&nbsp;<i>dt</i></td></tr>
<tr><td>P<sub>0</sub> =<td align="center">&mdash;&mdash;&mdash;&mdash;</td></tr>
<tr><td><td align="center">1 + <i>t</i></td></tr>
<tr><td><td valign="top"><small><i>0</i></small></td><td></td><td></td></tr>
</table>

The solution: MathJax

And then I discovered MathJAX. It was added recently to the Udacity forums, and I think it's also what MITx is using for their courses.

MathJax is fantastic. It's an open-source library that lets you specify equations in readable ways -- you can use MathML, but you can also use LaTEX or even ASCII math like `x = (-b +- sqrt(b^2-4ac))/(2a) .`

It uses Javascript: you put your equations in the text of the page with delimiters like $$ around them (you can control the delimiters), then run a function that scans the page content and replaces any equations it sees with pretty graphics. (Viewers using NoScript or similar extensions will need to allow mathjax.org to see the equations, unless you make a local copy of the mathjax.org libraries, which you probably should anyway if you're using a lot of equations.)

For displaying those graphics, MathJax might use MathML, HTML and CSS, or whatever, depending on the user's browser ... but you don't have to worry about that. (Alas, even in Firefox, MathML rendering isn't up to par so MathJax doesn't use it by default, though you can specify it as an option if you know your equations render well.)

Here's that integral again, using LaTeX format: $$ P_0 =\int_0^\infty \frac {P(t) dt}{1 + t} $$ and $$ x = {-b \pm \sqrt{b^2-4ac} \over 2a} $$

It's beautiful! And although I don't know LaTex at all -- been wanting an excuse to learn it -- I put together that integral with five minutes of web searching. (The quadratic code came from a MathJax demo page.) Here's what the code looks like:

$$ P_0 =\int_0^\infty \frac {P(t) dt}{1 + t} $$

$$ x = {-b \pm \sqrt{b^2-4ac} \over 2a} $$

MathJax is even smart enough to notice the code there is in a <pre> tag, so I didn't have to find a way to escape it.

I'm sold! The MathJax team has really put together a nice package, and I think we'll be seeing it on a lot more websites. If you want to try it, start here: Getting Started with MathJAX.

Tags: , , , ,
[ 15:45 Apr 03, 2012    More science | permalink to this entry | comments ]

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