17 Ocak 2011 Pazartesi

Axial Precession

In astronomy, axial precession is a gravity-induced, slow and continuous change in the orientation of an astronomical body's rotational axis. In particular, it refers to the gradual shift in the orientation of Earth's axis of rotation, which, like a wobbling top, traces out a cone in a cycle of approximately 26,000 years (called a Great or Platonic Year in astrology). The term "precession" typically refers only to this largest secular motion; other changes in the alignment of Earth's axis — nutation and polar motion — are much smaller in magnitude.

Earth's precession was historically called precession of the equinoxes because the equinoxes moved westward along the ecliptic relative to the fixed stars, opposite to the motion of the Sun along the ecliptic. This term is still used in non-technical discussions, that is, when detailed mathematics are absent. Historically, Hipparchus is credited with discovering precession of the equinoxes. The exact dates of his life are not known, but astronomical observations attributed to him by Ptolemy date from 147 BC to 127 BC.

With improvements in the ability to calculate the gravitational force between planets during the first half of the 19th century, it was recognized that the ecliptic itself moved slightly, which was named planetary precession as early as 1863, while the dominant component was named lunisolar precession. Their combination was named general precession instead of precession of the equinoxes. Lunisolar precession is caused by the gravitational forces of the Moon and Sun on Earth's equatorial bulge, causing Earth's axis to move with respect to inertial space. Planetary precession (actually an advance) is due to the small angle between the gravitational force of the other planets on Earth and its orbital plane (the ecliptic), causing the plane of the ecliptic to shift slightly relative to inertial space. Lunisolar precession is about 500 times larger than planetary precession. In addition to the Moon and Sun, the other planets also cause a small movement of Earth's axis in inertial space, making the contrast in the terms lunisolar versus planetary misleading, so in 2006 the International Astronomical Union recommended that the dominant component be renamed the precession of the equator and the minor component be renamed precession of the ecliptic, but their combination is still named general precession.


The precession of the Earth's axis has a number of observable effects. First, the positions of the south and north celestial poles appear to move in circles against the space-fixed backdrop of stars, completing one circuit in 25,771.5 years (2000 rate). Thus, while today the star Polaris lies approximately at the north celestial pole, this will change over time, and other stars will become the "north star". As the celestial poles shift, there is a corresponding gradual shift in the apparent orientation of the whole star field, as viewed from a particular position on Earth.

Secondly, the position of the Earth in its orbit around the Sun at the solstices, equinoxes, or other time defined relative to the seasons, slowly changes. For example, suppose that the Earth's orbital position is marked at the summer solstice, when the Earth's axial tilt is pointing directly towards the Sun. One full orbit later, when the Sun has returned to the same apparent position relative to the background stars, the Earth's axial tilt is not now directly towards the Sun: because of the effects of precession, it is a little way "beyond" this. In other words, the solstice occurred a little earlier in the orbit. Thus, the tropical year, measuring the cycle of seasons (for example, the time from solstice to solstice, or equinox to equinox), is about 20 minutes shorter than the sidereal year, which is measured by the Sun's apparent position relative to the stars. Note that 20 minutes per year is approximately equivalent to one year per 25,771.5 years, so after one full cycle of 25,771.5 years the positions of the seasons relative to the orbit are "back where they started". (In actuality, other effects also slowly change the shape and orientation of the Earth's orbit, and these, in combination with precession, create various cycles of differing periods; see also Milankovitch cycles. The magnitude of the Earth's tilt, as opposed to merely its orientation, also changes slowly over time, but this effect is not attributed directly to precession.)

For identical reasons, the apparent position of the Sun relative to the backdrop of the stars at some seasonally fixed time, say the vernal equinox, slowly regresses a full 360° through all twelve traditional constellations of the zodiac, at the rate of about 50.3 seconds of arc per year (approximately 360 degrees divided by 25,771.5), or 1 degree every 71.6 years.

13 Ocak 2011 Perşembe


Android is a mobile operating system initially developed by Android Inc. Android was bought by Google in 2005. Android is based upon a modified version of the Linux kernel. Google and other members of the Open Handset Alliance collaborated on Android's development and release. The Android Open Source Project (AOSP) is tasked with the maintenance and further development of Android. Unit sales for Android OS smartphones ranked first among all smartphone OS handsets sold in the U.S. in the second and third quarters of 2010, with a third quarter market share of 43.6%.

Android has a large community of developers writing application programs ("apps") that extend the functionality of the devices. There are currently over 200,000 apps available for Android. Android Market is the online app store run by Google, though apps can be downloaded from third party sites (AT&T only permits third party apps on their Aria phone). Developers write primarily in the Java language, controlling the device via Google-developed Java libraries.] Python, Ruby and other languages are also available for Android development via the Android Scripting Environment.

The unveiling of the Android distribution on 5 November 2007 was announced with the founding of the Open Handset Alliance, a consortium of 79 hardware, software, and telecom companies devoted to advancing open standards for mobile devices. Google released most of the Android code under the Apache License, a free software and open source license.

The Android operating system software stack consists of Java applications running on a Java based object oriented application framework on top of Java core libraries running on a Dalvik virtual machine featuring JIT compilation. Libraries written in C include the surface manager, OpenCore media framework, SQLite relational database management system, OpenGL ES 2.0 3D graphics API, WebKit layout engine, SGL graphics engine, SSL, and Bionic libc. The Android operating system consists of 12 million lines of code including 3 million lines of XML, 2.8 million lines of C, 2.1 million lines of Java, and 1.75 million lines of C++.

2.3 (Gingerbread) Based on Linux Kernel

Updated user interface design
Support for extra-large screen sizes and resolutions (WXGA and higher)
Native support for SIP VoIP telephony
Support for WebM/VP8 video playback, and AAC audio encoding
New audio effects such as reverb, equalization, headphone virtualization, and bass boost
Support for Near Field Communication
System-wide copy–paste functionalities
Redesigned multi-touch software keyboard
Enhanced support for native code development
Audio, graphical, and input enhancements for game developers
Concurrent garbage collection for increased performance
Native support for more sensors (such as gyroscopes and barometers)
A download manager for long running downloads
Improved power management and application control
Native support for multiple cameras
Switched from YAFFS to the ext4 filesystem

Community-based firmware

There is a community of open-source enthusiasts that build and share Android-based firmware with a number of customizations and additional features, such as FLAC lossless audio support and the ability to store downloaded applications on the microSD card. This usually involves rooting the device. Rooting allows users root access to the operating system, giving more control over their environment variables. In order to use custom firmwares the devices bootloader must be unlocked. Rooting alone does not allow the flashing of custom firmware. Modified firmwares allow users of older phones to use applications available only on newer releases.

Those firmware packages are updated frequently, incorporate elements of Android functionality that haven't yet been officially released within a carrier-sanctioned firmware, and tend to have fewer limitations. CyanogenMod and VillainROM are two examples of such firmware.

On 24 September 2009, Google issued a cease and desist letter to the modder Cyanogen, citing issues with the re-distribution of Google's closed-source applications within the custom firmware. Even though most of Android OS is open source, phones come packaged with closed-source Google applications for functionality such as the application store and GPS navigation. Google has asserted that these applications can only be provided through approved distribution channels by licensed distributors. Cyanogen has complied with Google's wishes and is continuing to distribute this mod without the proprietary software. He has provided a method to back up licensed Google applications during the mod's install process and restore them when it is complete.

1 Ocak 2011 Cumartesi


Plex (also known as "Plexapp" or "Plex Media Center") is a partially open-source freeware media player for Intel-based Apple Macintosh computers. It has a 10-foot user interface design for the living-room TV. Its source code was initially forked from XBMC Media Center on May 21, 2008 which Plex today uses as an application framework platform for its GUI (Graphical User Interface) and media player part of their software. Similar to XBMC and Boxee, it is an alternative to Apple's Front Row for Mac, with skinnable and user-configurable interface.

Plex integrates content from iTunes and iPhoto (from the iLife software suite) as well as allows the user to manage all video, photos, music, and podcasts from a computer, optical disk, local network, and the Internet using an Apple or Harmony remote control. In 2009 the developers added their own 'app store' digital distribution platform called "Plex Online" with a growing list of community driven plugins for online content like Hulu, Netflix, and CNN video that are being distributed via "Plex Media Server" application which runs as a standa-alone software and media management interface.

Plex began as a free software hobby project but since 2010 has evolved into a (freeware) project that is owned and developed by a single for-profit startup company, (Plex, Inc.). It is a high tech company based in the United States that is responsible for the development of the Plex front-ends and back-end, its client–server model, and all accompanying software under the "Plex" trademark, as well as the exclusive copyright of the closed source software/code parts for both commercial and non-commercial use.

Plex supports a wide range of multimedia formats and includes features such as playlists, audio visualizations, slideshows, weather forecasts reporting, and an expanding array of third-party plugins. As a media center, Plex can play most audio and video file formats, as well as display images from many sources, including CD/DVD-ROM drive, USB flash drives, the Internet, and local area network shares. DVD playback is not yet fully integrated and requires the use of helper applications like Apple's DVD Player.

Through the processing power of modern Mac computer hardware, Plex is able to decode high-definition video up to 1080p. For older Macintosh computers, the software does not however support any hardware accelerated video decoding which means that users require a 2 GHz Intel Core 2 processor to decode the majority of 1080p videos encoded with the H.264 codec. Newer Apple models using Nvidia 9400M/GT320M/GT330M chipsets and Snow Leopard OS 10.6.3 or later does however benefit from H.264 hardware accelerated video decoding meaning that most of the decoding process is offloaded to the GPU.

Through its plugin system, Plex includes features such as YouTube and Apple movie trailer support, SHOUTcast, and more. Most plugin content (such as the Hulu and Netflix) is provided via a separate helper program called Plex Media Server, while some use an integrated Python runtime engine and plugin framework.

Plex Media Server is from closed source (contains proprietary code), however the other parts of Plex media center software are open-sourced and distributed under the GNU General Public License. Plex's open source code is hosted on GitHub. Plex media center and media player source code was initially based upon XBMC Media Center, which it uses as its application framework. The founder of Plex, Elan Feingold, was actually part of the official XBMC development team for a short while, but tension over the rest of XBMC's developers' strict adherence to the GPL and their open-source software mindset was one of the factors that led Elan (Plex founder) to leave the XBMC project and create the Plex fork.