Twin Help 2

Visualization of the stellar sky


Getting started

Be aware that the applet needs a lot of memory and it is therefor recommended to download it and execute it in a command prompt with

java -jar -Xmx1024m RelativisticSky.jar

If you work with a Windows OS, you can start the RelativisticSky application with a double click on the jar-file.

If you want to see the detailed star information (see GetInfo) you should be online while working with the applet.

Main selection

The main selection can be found in the upper left edge of the applet. It is possible to choose between three views:

  • Stars

  • Star Collections

  • Microwaves

This selection determines which objects can be observed inside the sky views and limits the visible input fields.


The default setting for coordinates is "Equatorial Coordinates". In this setting the position of the observer is the earth and Right Ascension and Declination correspond to the usual conventions for equatorial coordinates.

In "Stars"-mode it is possible to change the coordinates to "Galactic Coordinates". In this mode you have to choose first a position for the observer in galactic coordinates (zero corresponds to the position of the sun). Bear in mind that choosing galactic latitude and longitude doesn't change the view without choosing a distance!= 0 too, because the line of sight is to be choosen beneath.

Having choosen "Galactic Coordinates", "Right Ascension" and "Declination" correspond no longer to their equatorial meaning. In this case "Right Ascension" corresponds to the galactic longitude (or having choosen a position!=0 to a parallel to the galactic longitude with an origin in the actual position). "Declination" corresponds to the galactic latitude.


The field "Velocity" can hold a value between 0 and 1. It specifies the velocity of the observer relativ to the speed of light.


The field "Magnitude" holds a limes for the viewable magnitude of the stars. ( The lower the value of the magnitude of a star the brighter it shines). To humans naked eye, stars are only visible up to the 6. magnitude.

Show Grid

By activating the checkbox "Show Grid" a grid is plotted in the sky view to indicate the course of right ascension and declination.


By activating the checkbox "Animate" and clicking the button "Play" the velocity is increased stepwise to the velocity in field "Velocity". Additionally the checkbox "Tails" can be selected to observe the trace of the stars at different velocities.


Load or relaod the current view. Depending on the main selection the relevant datas are loaded


Opens a frame with more information (Hipparcos dataset, some relativistic values) about the last selected star.


Opens a list of all viewable stars.

Magnitude Scale

Shows the displayed size of the stars according to their magnitudes. The blow-up ratio can be set manually.

Color Scale

There are four modes for Color Scale:

  • Visual: The colours are adjusted to the human eye

  • All: Similar to "Visual" but also stars with very high and very low temperatures are displayed

  • Microwave: The temperature range of the microwaves is made visible

  • Manual: It is possible to set the color scale to any values to observe a certain temperature range

Sky Views

You can choose between two sky views:

  • A 4-pi projection (complete sky)

  • An hemispherical view (semi sky)

The line of sight points to the middle of the sky view.

A short example for the Relativistic Sky Applet

For a first example, we choose to look towards Sirius. Sirius's coordinates in Equatorial Coordinates are 101.3deg right ascension and -16.7deg declination.
You can choose a higher magnitude, if you like, for example 6.0 (limit of humans naked eye)
Press "Load" to see a first view of the unrelativistic sky. The big blue star in the middle of the screen is Sirius.
If you click on the star, you will see it's HIP Identifier (for Sirius: 32349), and if you have established a network connection, also it's name (SIRIUS A) after a short while.
Now enter a velocity, for example 0.3, which means 30% of the speed of light.
Pressing "Load" you see most of the visible stars clumping in the middle of the screen. Sirius doesn't change position, as it is already in the middle of the screen.
Activating the checkbox of "Show Grid" shows a coordinate grid of equatorial coordinates.
Activating the checkbox of "Animate" and pressing the "Play" button lets you see the stars changing their apparent position with increasing velocity. The actual velocity is displayed beneath the "Animate" checkbox. Looking at the animation with activated coordinate grid helps you to understand the apparent deformation of the sky.
Activating the "Tails" checkbox allows you to see the "trace" of the stars from their origin position to their apparent position with increasing velocity. Try to press "Play" once more after activating the checkbox.

Feel free to change any parameter and see what happens. 
For more possibilities, please have a look at the help instructions above. 

All contents copyright © 2006, Thomas Müller, Andreas King, Daria Adis
All rights reserved. 

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