Information about Planisphere

A planisphere is a star chart analog computer that can be adjusted to display the stars for any time and date. It is an instrument to learn how to recognize stars and constellations. The astrolabe is a predecessor of the modern planisphere. That instrument was known to the ancient Greeks.

It consists of a circular chart attached at its center to an opaque overlay that has a clear roundish window or hole. The chart is mounted so that it is free to rotate about the pivot point at the center. The star chart contains all stars, constellations and (possibly) deep-sky objects that are visible from a certain area on Earth. That area is a band around the Earth centered on a certain (design) latitude. Since the night sky that one sees from the Earth depends on the observer's latitude, planisphere windows are designed for particular latitudes and one should choose the latitude that is the closest match. Times are marked on the rims of the overlay (e.g., 10pm or 5am) and dates are marked rim of the starchart. Because most stars rise and set and also the starry sky changes from season to season (i.e. the visibility and the positions of the stars with respect to the horizon), a second disc (with the window mentioned above) is mounted on the star chart, showing that part of the starry sky that is visible at a certain moment and at the latitude in question. You can set the planisphere for that moment (date and time) by adjusting the discs. The inside border of the window (or cutout) of the upper disc represents the horizon.

History

The word planisphere (or "planisferium") literally means "celestial plain": the representation of the starry sky in in a flat plane. The first star chart to have the name "planisphere" was one from 1624, made by Jacob Bartsch. Bartsch was the son in law of Johannes Kepler, the man who came up with the famous laws that explain the orbits of the planets.

The star chart

Since the planisphere shows the celestial sphere in a printed flat, there is always considerable distortion. Planispheres, like all charts, are made using a certain projection method. For planispheres there are two major methods in use, leaving the choice with the designer. One such method is the polar azimuthal equidistant projection. Using this projection the sky is charted centered on one of the celestial poles (polar), while circles of equal declination (for instance 60°, 30°, 0° - the celestial equator - -30° and -60°) lie equidistant from each other and from the poles (equidistant). The shapes of the constellations are proportionally correct in a straight line from the centre outwards, but at right angles to this direction (parallel to the declination circles) there is considerable distortion. That distortion will be worse as the distance to the pole gets greater. If we study the famous constellation of Orion in this projection and compare this to the real Orion, we can clearly see this distortion. It is the only disadvantage of this projection.

The stereographic projection solves this problem. Using this projection the distances between the declination circles are enlarged in such a way that the shapes of the constellations remain correct. Naturally in this projection the constellations on the edge become too large in comparison to constellations near the celestial pole: Orion will be twice as high as it should be! It is the same effect that makes Greenland so huge in Mercator charts. Another disadvantage is that, with more space for constellations near the edge of the planisphere, the space for the constellations around the celestial pole in question will be less than they deserve. For observers at moderate latitudes, who can see the sky near the celestial pole of their hemisphere better than that nearer the horizon, this may be a good reason to prefer a planisphere made with the polar azimuthal equidistant projection method.

The upper disc

The upper disc contains a "horizon", that defines the visible part of the sky at any given moment, which naturally half of the total starry sky. That horizon line is most of the time also distorted, because of the same reason as the constellations are distorted. It has become a kind of "collapsed" oval. The horizon is designed for a particular latitude and thus determines the area for which a planisphere is meant. Some more expensive planispheres have several upper discs that can be exchanged, or have an upper disc with more horizon-lines, for different latitudes.

When a planisphere is used in a latitude zone other than for which is was designed, than you can see either stars that are not in the planisphere or your planisphere shows stars that are not visible in the real sky. To study the starry sky thoroughly it may be necessary to buy a planisphere particularly for the area in question.

However, most of the time the part of the sky directly above the horizon doesn't show too much stars, due to hills, woods, buildings or just because of the thickness of the atmosphere we look through. The lower 5° above the horizon in particular hardly shows any stars (let alone objects) unless under the very best conditions. Therefore you can still very accurately use your planisphere from +5° to -5° of the design latitude. So a planisphere for 40° north can be used perfectly between 45° and 55° north.

Coordinates

Accurate planispheres somehow represent the celestial coordinates: right ascension and declination. Those coordinates can be looked up in annual astronomical guides, enabling users to pinpoint the positions of planets, asteroids or comets and thereby find them in the sky. Some planispheres use a separate pointer for the declination, using the same pivot point as the upper disc. Some planispheres have a declination feature printed on the upper disc, along the line connecting north and south on the horizon. Right ascension is represented on the edge, where you can also find the dates with which to set the planisphere.

Using a planisphere

To use the planisphere, the overlay is rotated to match the time to the desired date. Any daylight saving time must be taken into account when setting the time. The stars currently visible in the sky that night at that time are then visible on the star chart through the window in the overlay. The overlay will also have a north indicator or even better all the points of the compass. Many users find it useful to hold the planisphere above their head with the north-indicator pointing towards true north or south. In this position, it is possible to imagine projecting the stars out onto the night sky. This allows quick identification of constellations and stars that are currently visible. The stars in the opposite hemisphere of which, in absolute value, the declination is more than the colatitude for which the planisphere applies, are not on the chart, because they are always below the horizon (see culmination). When we have set the planisphere we will notice that the same starry sky is visible during many days of the year, but at varying times. The illustratie shows that clearly. Here a planisphere is set for 25 August, 5.00 h DST. The same sky will be visible on a clear night on 17 September, at 3.30 h DST; on 28 November 21.45 h; on 25 December at 20.00 h, and on 9 January at 19.00 h. And on every day in between!

List of Planispheres

• Planisphere - high quality PDF format planisphere for the Northern Hemisphere. For best results, print with the scale set to 100% and the image rotated to fit a single page.
• http://www.asahi-net.or.jp/~zs3t-tk/planisphere/planisphere.htm - free planisphere for the Northern and Southern Hemisphere by Toshimi Taki
• http://members.ozemail.com.au/~starrylady/Planis1.htm - free planisphere for the Southern Hemisphere (30ºS to 35ºS latitude) by StarryLady Astronomy Services
• Edmund Scientific
• Miller Planisphere
• The Night Sky
• Rob Walrecht Productions
• Firefly
• Drehbare Himmelskarte - with 250 deep sky objects
• Download a 'built it yourself' planisphere (in Hebrew)