Orrery

(Added section on asterisms)
(Added section on panning)
Line 61: Line 61:
There are two sky display screens defined.  By default, one of them shows
There are two sky display screens defined.  By default, one of them shows
the display shown above, and the other shows the constellations, and
the display shown above, and the other shows the constellations, and
-
symbolic representations of the solar system objects.
+
symbolic representations of the solar system objects.  Tapping the screen
 +
within the upper 4/5 of the display toggles in and out of the constellations
 +
page.
[[Image:orreryConstellations.png|200px|Orrery Constellation Screen]]
[[Image:orreryConstellations.png|200px|Orrery Constellation Screen]]
Line 87: Line 89:
[[Image:OrreryAsterisms.png|200px|Orrery Northern Asterisms]]
[[Image:OrreryAsterisms.png|200px|Orrery Northern Asterisms]]
 +
 +
== Panning ==
 +
The image can be panned by tapping your finger in
 +
the bottom 1/5 of the display (excluding menus).  The size of
 +
the panning step is controlled by how close your finger is
 +
to the edge of the display.  The program tries to center
 +
the azimuth value near where you tap.  If you tap near the
 +
left or right edges, it
 +
pans by the maximum allowed amount, +-40 degrees.  Pan steps are
 +
quantized in increments of 5 degrees.
 +
 +
=== Selecting a New Center Azimuth ===
 +
Panning to a new central azimuth by panning 40 degrees at a time
 +
can be tedious.  To make a large change in the display's center
 +
azimuth you may use the azimuth compass.  To call up the azimuth
 +
compass, tap near the center of the finger-pan area (bottom 1/5 of
 +
the display)  The compass shown below will appear:
 +
 +
[[Image:OrreryAzCompass.png|200px|Orrery Azimuth Compass]]
 +
 +
You may use your finger or stylus to select another center
 +
azimuth.  Keep you finger pressed to the display as you select
 +
your new center azimuth - the selected azimuth will be shown by
 +
a white pointer on the inside of the compass circle.  Once you release pressure on the screen,
 +
the compass will disappear, and the display will be redrawn.
 +
 +
The azimuth compass cannot be accessed if the display is zoomed.

Revision as of 19:55, 10 January 2010

Contents

Overview

The orrery is a simple open-source application for Maemo 5 which displays the night (and day!) sky. It is nowhere near as elaborate as, for example, kstars or xephem. It is intended to be a small, finger-friendly application, requiring no network connection. The most recent version is 3.0.x (the x just increments when bugs are fixed). The program version is shown on the "Symbol Key" page.

The most recent version of the program may be downloaded at https://garage.maemo.org/projects/orrery/ .

The star database was extracted from the Hipparcos catalog. To ease the computational load, the coordinates are not precessed or nutated before being displayed. Orbital elements are used, rather than ephemerides, to calculate planet positions. Although this is less accurate, it dramatically reduces the memory footprint. The planet positions are accurate to a few arc minutes, from 3000 BC to 3000 AD. Since the scale on the default display is approximately 8 arc minutes per pixel, these small errors are imperceptible, unless a very large zoom factor is used.

The Display

File:OrreryDefaultPage.png

The image above shows the default display - all stars visible to the unaided eye, with colors for the brightest ones. It's displayed with a Transverse Mercator projection (similar to Norton's Star Atlas) which works well with the N900 in portrait mode. It's a conformal transformation, so the constellations have about the right shapes. The Sun, Moon and planets are plotted, and the Moon is shown with the proper phase. The Sun and Moon are plotted with a size about 3.5 times larger than their true angular size on the sky.

The user may chose to have the program display the stars visible at the user's location and at the current time, or at any other position on the earth, or any other time between 3000 BC and 3000 AD.

The blue line near the bottom of the display is the horizon. Nothing below that line is visible, but the program plots objects there anyway, because it is often useful to know which objects are just about to rise, and which ones have recently set.

The orrery as a compass

The green numbers at the bottom are the azimuth values. The azimuth is measured along the horizon, from north through east. So the azimuth of due north is 0, east is 90, south is 180 and west is 270 degrees. By using these azimuth values, the orrery can be used as a celestial compass, if the sky above you is at least partially clear. Notice that the azimuth values are not exactly evenly spaced. This is caused by the distortion introduced by the Transverse Mercator projection.

If either the sun or moon is visible, you can use the planet compass (under the opts menu) to quickly orient yourself with an accuracy of about 1 degree,

Constellations Display

There are two sky display screens defined. By default, one of them shows the display shown above, and the other shows the constellations, and symbolic representations of the solar system objects. Tapping the screen within the upper 4/5 of the display toggles in and out of the constellations page.

File:OrreryConstellations.png

The constellations are plotted in three colors. The twelve zodiac constellations are plotted in hot pink. The constellations plotted in gold are the classical Greek constellations, as listed in Ptolomy's Almagest (apart from the zodiac constellations, which are also in the Almagest). The only constellation from the Almagest which is not plotted is Argo Navis, which is not one of the official modern constellations (Argo Navis was broken up into the modern constellations Carina, Puppis and Vela). The remaining constellations, plotted in blue, where added during the last few hundred years.

In addition to constellation names and figures, three great circles, the Celestial Equator (light yellow), Ecliptic (red) and Galactic Plane (blue-green), are shown. Solar system objects are shown symbolically (see Mercury, Venus, the Moon and the Sun in the above image).

Displaying Asterisms

There are many informal constellations which are widely known, such as the Big Dipper, The Summer Triangle, etc. The user can choose to display some of these unofficial constellations by selecting "Use Asterisms" from the "items" menu. If you know of some nice asterisms that the orrery does not yet display, please send them to orrery.moko@gmail.com, and I'll include them in a future release if I can. The image below shows the Big Dipper and Little Dipper asterisms.

File:OrreryAsterisms.png

Panning

The image can be panned by tapping your finger in the bottom 1/5 of the display (excluding menus). The size of the panning step is controlled by how close your finger is to the edge of the display. The program tries to center the azimuth value near where you tap. If you tap near the left or right edges, it pans by the maximum allowed amount, +-40 degrees. Pan steps are quantized in increments of 5 degrees.

Selecting a New Center Azimuth

Panning to a new central azimuth by panning 40 degrees at a time can be tedious. To make a large change in the display's center azimuth you may use the azimuth compass. To call up the azimuth compass, tap near the center of the finger-pan area (bottom 1/5 of the display) The compass shown below will appear:

File:OrreryAzCompass.png

You may use your finger or stylus to select another center azimuth. Keep you finger pressed to the display as you select your new center azimuth - the selected azimuth will be shown by a white pointer on the inside of the compass circle. Once you release pressure on the screen, the compass will disappear, and the display will be redrawn.

The azimuth compass cannot be accessed if the display is zoomed.