Orrery

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


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.