Orrery

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(the heliocentric ecliptic longitudes are correct).
(the heliocentric ecliptic longitudes are correct).
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[[Image:OrrerySchematicSolarSystem.png|200px|Solar System View (schematic)]]
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[[Image:OrrerySchematicSolSys.png|200px|Solar System View (schematic)]]
Several buttons are drawn at the bottom of the display, which allow you to
Several buttons are drawn at the bottom of the display, which allow you to
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Mars.  Note that the orbits of Mercury and Mars are quite noncircular:
Mars.  Note that the orbits of Mercury and Mars are quite noncircular:
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[[Image:OrreryToScaleSolarSystem.png|200px|Solar System View (to-scale)]]
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[[Image:OrreryToScaleSolSys.png|200px|Solar System View (to-scale)]]

Revision as of 01:14, 11 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.

Main Menu

File:OrreryMainMenu.png

The main dropdown menu for the orrery has four options:

Other Pages

The "Other Pages" option takes you to a menu that allows you to display things other than the night sky.

File:OrreryOtherPagesMenu.png

Flashlight Modes

The "White Flashlight" button paints the entire screen white, and the "Red Flashlight" button paints the entire screen red. Both may be used as a flashlight, but the red one is best if you are trying to preserve your night vision.

Sunrise and Moon

The "Sun and Moon" button displays a page giving the sun and moon positions, rise and set times, phase etc for five days, centered on today:

File:OrrerySunAndMoon.png

The highlighted date is the current UT date (unless the time menu has been used to select a different time). The highlighted rise and set times are the next ones which will occur at your location. Of course, that may be on a different UT date (for example, during daylight hours, the next sunrise will be tomorrow). The times are shown as Universal Times (UT), which may annoy people, UT is used because the orrery allows you to select any location on the earth, and any time from 3000 BC until 3000 AD. It would be very difficult to keep track of all the timezone, daylight savings change dates, etc for that time range! So I have wimped out, and displayed UT, which is always reasonably well defined.

The current sun and moon positions shown are geocentric - no topocentric correction has been applied. However topocentric corrections are applied when rise and set times are calculated, so they should be reasonably accurate for a specific location on earth.

The moon illumination percentage shown on the line with the rising and setting times is the value at transit. The illumination shown on the line that has the drawing of the moon phase is the illumination percentage at this moment.

This page also shows the UT date and time of the four major moon phases (New, First Quarter, Full and Last Quarter) for a time span of 17 lunar months, centered on the current time (which of course can be changed with the time menu). The next time and date when each phase will occur is highlighted.

Big Moon Calendar

The Big Moon Calendar is a graphic display of the phase of the moon for each day from 10 months before the current month, until 10 months after the current month (roughly 640 days), as shown below:

File:OrreryBigMoonCalendar.png

Each column shows one month, and the day number is shown for each row. At the top and bottom of each column is shown the first letter of the name of the month shown in the column. Two vertical green lines separate years. The current Universal Time date is outlined in a red box.

This Month's Moons

The This Moon's Moons page shows the phase of the moon for each day of a particular month. By default, the current month is shown, but one may use the arrow buttons to pan through the months.

File:OrreryThisMonthsMoons.png

Both moon calendars show "Blue Moons", which are (according to the most commonly used definition) the second full moon in a calendar month which has two full moons. Both images above show the Blue Moon which falls on New Year's Eve, 2009.

To exit the Moon Calendar displays, just tap the screen anywhere.

Meteor Showers

This display shows information for all the meteors showers which will occur during the current year. Remember that to see a meteor shower, you must travel to a site that has a very dark sky. In a city, few meteors will be seen even during a shower.

File:OrreryMeteorShowers.png

The items shown on the Meteor Showers page are 1) The name of the meteor shower 2) the abbreviation used for this shower when its radiant is plotted on the sky display 3) the expected number of meteors per hour - this is the maximum you will see under the best circumstances with a very dark sky and the shower radiant directly overhead on the date of the shower maximum. If the rate entry says "Var", that means the rate is variable; such shower are not apt to show a large number of meteors per hour. 4) the range of dates during which the shower will occur 5) the date at which the shower shows the maximum number of meteors 6) the typical velocity of the showers meteors in km/sec - larger numbers lead to brighter, bluer meteors 7) the percentage of the moon's disk which is illuminated on the date of shower maximum - full moon = 100%. A minus sign following the percentage means the moon is waning, a plus sign means it is waxing. Moonlight greatly interferes with seeing meteors 8) the number of dark hours on the date of the shower maximum - defined as the time during which the sun is more than 12 degrees below the horizon, and the moon is far enough below the horizon to not appreciably brighten the sky.

The Meteor Showers page shows information for all showers, the large majority of which are really minor events. Even when a meteor shower is not occurring, you can expect to see 4 to 10 meteors per hour. Many of the showers listed do not even double that background rate, and are of interest only to serious meteor enthusiasts. If you just want to go out and see some meteors for fun, it's best to select a shower with a rate of at least 50 meteors per hour, and to watch it on or very near the date of the shower's maximum.

Each line of the display describes a separate shower. If the shower currently in progress, the line is shown in green - bright green if there will be 2 or more hours of dark sky tonight, and dark green otherwise. If the shower is not currently underway, it will be shown in a cream color if on the night of the shower's maximum, the sky will be dark for at least 2 hours. Otherwise the shower information will be shown in grey, indicating that this year is not a good one to observe that particular shower.

At the bottom of the Meteor Showers page is shown the number of dark sky hours to expect tonight. In this context, "tonight" means the upcoming night if the sun is currently above the horizon, or the current night, if the sun is below the horizon.

If you wish to get meteor shower information for a different year, simply change in year using the time menu.

Many thanks to the International Meteor Organization (http://www.imo.net/imo/intro) for permission to use their data to produce the meteor shower related displays.

Planet Compass

The Planet Compass page graphically shows where each of the planets is at the current time.

File:OrreryPlanetCompass.png

The image has one complete circle, which shows the planet azimuth, and one semicircle, which shows the elevation.

The azimuth circle has blue triangles marking the four cardinal points: North, South, East and West. There are small tick lines at the NE, SE, SW and NW directions, as well as a small dot every 10 degrees. The planet symbols are shown with lines projecting to their current azimuth. The line is white if the planet is above the horizon, and red if it is below the horizon.

The elevation semicircle has blue triangles marking the directions of the Zenith, Horizon and Nadir. Small dots are shown every 10 degrees, and there are short lines at +-45 degrees. As with the azimuth circle, lines connect the planet symbols to their current elevations.

Below the compass graphic, the name, Hour Angle (HA) rising time and azimuth, transit time and elevation, and setting time and azimuth is shown for each planet. The name and HA of the planet is highlighted if the planet is above the horizon. The time of the next event - rising, transiting or setting, is also highlighted for each planet.

Solar System

The program can display the relative planet positions in two different ways. In both cases, the view is from the north looking down, with the direction of the Vernal Equinox to the left of the screen (marked with the vernal equinox symbol, and an arrow). At the time of the vernal equinox, the earth will be at the 3 o'clock position in its orbit, so that the Sun will appear at the vernal equinox position.

Schematic Solar System View

This button presents a schematic view of the Solar System, showing the locations of the earth, the earth's moon and the other planets in their orbits around the sun. The view is schematic in the sense that the orbit radii and planet sizes are not shown to scale. However the positions of the objects within their orbits are correct (the heliocentric ecliptic longitudes are correct).

File:OrrerySchematicSolSys.png

Several buttons are drawn at the bottom of the display, which allow you to show an animation of the motion of the Solar System objects for several time increments. Showing the motion of planets in this way is what mechanical orreries do.

To-Scale Solar System View

This button displays the Solar System with the planet orbits shown with the proper relative sizes. The orbits are also plotted as ellipses in this display, with the correct eccentricity and orientation. The moon is not plotted, because it would be too close to the earth to display if any of the planet orbits were completely shown. Because Neptune's orbit is nearly 100 times larger than Mercury's, it is not possible to display all of the planet orbits simultaneously. There are "Zoom In" and "Zoom Out" buttons which allow you to select which proper subset of planet orbits is shown. The current position of each planet in its orbit is shown. Also plotted are grey lines connecting the Sun to each orbit, terminating with a white dot at the position of the planet's perihelion (where it is closest to the sun in its orbit). The image below shows the display when the outermost plotted planet is Mars. Note that the orbits of Mercury and Mars are quite noncircular:

File:OrreryToScaleSolSys.png