English

Noun

1. Any representation of part of a sphere on a plane surface
2. Any of several charts of the celestial sphere having an overlay or window that may be adjusted to show the stars visible at a particular time, or from a particular place

A planisphere is a star chart analog computer in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date. It is an instrument to assist in learning how to recognize stars and constellations. The astrolabe is a predecessor of the modern planisphere. That instrument was known to the ancient Greeks.

Summary description

A planisphere consists of a circular star chart attached at its center to an opaque circular overlay that has a clear eliptical window or hole so that only a portion of the sky will be visible in the window or hole area at any given time. The chart and overlay are mounted so that they are free to rotate about a common pivot point at their centers. The star chart contains the brightest stars, constellations and (possibly) deep-sky objects visible from certain locations on Earth. That location is a band around the Earth centered on a certain (designed for) latitude within the northern or southern hemispheres. Since the night sky that one sees from the Earth depends on the observer's latitude, planisphere windows are designed for a range of particular latitudes and one should choose a planisphere for which the stated design latitude is the closest match to the latitude and hemisphere of the observer.

A complete twenty-four hour time cycle is marked on the rim of the overlay. A full twelve months of calendar dates are marked on the rim of the starchart.

The window is marked to show the direction of the eastern and western horizons.

The disk and overlay are adjusted so that the observer's local time of day on the overlay corresponds to that day's date on the star chart disc. The portion of the star chart visible in the window then represents (with a distortion because it is a flat surface representing a spherical surface) the distribution of stars in the sky at that moment for the Planisphere's designed location.

To easily translate the Planisphere's chart to actual star positions, hold it above your head with the eastern and western horizons correctly aligned.

History

The word planisphere (or "planisferium") means "celestial plane", defined as the flat plane representation of the star-filled sky. The first star chart to have the name "planisphere" was made in 1624 by the son-in-law of Johannes Kepler, Jacob Bartsch. It was Kepler who discovered those orbit laws 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 the zone for which is was designed, the user will either see stars that are not in the planisphere, or the planisphere will show stars that are not visible in that latitude zone's 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 will not show many 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) except under the very best conditions. Therefore, you can still very accurately use a planisphere from +5° to -5° of the design latitude. For example, a planisphere for 40° north can be used perfectly between 35° and 45° north.

Coordinates

Accurate planispheres somehow represent the celestial coordinates: right ascension and declination. The changing positions of planets, asteroids or comets in terms of these coordinates can be looked up in annual astronomical guides, and these enable planisphere users 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 illustration 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.

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
• Edmund Scientific
• Miller Planisphere
• The Night Sky
• Rob Walrecht Productions
• Philips' Planisphere
• Firefly
• Drehbare Himmelskarte - with 250 deep sky objects