By Alain Dussault
The following information are based
on the Planisphere, which I think is the best available on the
market, in as much that you opt for the big format. The last
edition is improved compare to the one I own, since the zone of
the Hour circle, which does not represent the visible sky is
smoke. It is the only one that I know that has a small ruler.
this ruler facilitate the location of objects and given in bonus
their coordinates.
A Planisphere, allow the amateur astronomer to follow the reel movement of the visible sky, and by the same occasion be able to localize the principal constellations and the most brilliant stars, and from there the deep sky objects.
On the Planisphere, the low constellations, those that are locate on the external perimeter of the Great Oval, are deformed. This is due to the fact that the Planisphere represent half a sphere on a plane.
To be able to find objects in the sky, astronomers used a coordinate system which is based on Right Ascension and Declinaison.
This coordinate is in hours, minutes and seconds (h,m,s). the ours are form 0 to 24 hr., that is from 0h 0m 0s until 23h 59m 59s.
Declinaison (Dec. or D.), is located on the slide ruler and is defined n degrees ranging from +90 to -45o, for this particular Planisphere, which is for a 45o latitude North.
The names of the different constellations are identified
par their French names, on this Planisphere. On other it is
usually their Latin names.
The starts
The stars on the sky circle are either represented by their name or by a Greek alphabet letter, like Alpha, Beta, Gamma, Delta, and so on by order of their decreasing magnitudes.
A star identified by Alphamis more brilliant than the one identified by Beta. The most brilliant stars of our sky are generally identified by their names.
Deep sky objects are either represented by their Messier
numbers, like M45, or by a number, like 884, for NGC884. On
this Planisphere they are all represented by a small ellipse.
Ecliptic
The ecliptic, this imaginary line on which or near which all planets of our solar system are located, is represented by a circle on the sky map.
The celestial equator is represented by a dashed circle, on the Planisphere.
Immediately inside the Right Ascension circle, is represented another graduated in months circle. Each graduation on this circle represent a month.
On the Hourly circle, we generally found the following information:
1 - The inferior Meridian.
2 - The superior Meridian.
3 - The position of North Pole.
4 - The Zenith.
5 - The first vertical. Which divide the celestial sky in
direction EAST - WEST.
6 - The Great Oval or Horizon. The inferior limit of the
visible sky.
7 - Cardinal Points, NORTH, SOUTH , EAST and WEST.
8 - The crepuscular indicators.
On the periphery of the Hourly Circle, there is a graduated scale, which represent each hour of a day. Each small division mean two minutes.
On the Hourly Circle is drawn, an oval form or ellipse, which is call the Great Oval. Within this zone, is the visible window of the night sky, for a given date at a given hour. the perimeter of the ellipse represent the horizon line. The arc circle that pass through the cardinal points EAST and WEST, pass by the Zenith. It is call the first vertical. the straight line represent the NORTH and SOUTH axis. This is the superior meridian.
How to use its Planisphere?
The first thing to do, consist in positioning on the sky map, the Great Oval, which represent the visible sky. You will remark that on this Planisphere there is two moving parts. First the slider rule, which look like smoke on the picture above, Second is the Hourly Circle.
We will suppose that we are September 22nd., and that the time is one minute pass twelve. The year is of no importance here. I have chosen that time, which is not a logical hour to see the stars, being in daylight, for the simple reason that allow me to show the complete observing window and the slider rule, the extremity of which cut through the date.
In the first image, the slider rule is positioned at a logical hour of the day, that is at 21hr. But in that case the date is May 10th.,
As a guide you place the center line of the slider rule on the date September 22nd., which is found on the external graduated circle graduated in months.
After we rotate the Hourly Circle so that the division 12hr 01min, is positioned on the graduated center line of the slider rule.
Great Oval on September 22nd., at 12hr 01min
or at May 10th., at 21hr.
We just happen to position the visible window of the sky for this date and hour. From now on we can locate ourself on the night sky with the Planisphere.
Naturally we need first to be able to locate the direction of the NORTH, the SOUTH, the EAST or the WEST when we are outdoor. At first, we need to locate Polaris, in using Ursa Major as a pointer.
We have just position the observing window of the Planisphere for this date and this time of the year. From now on, you can locate yourself, with the Planisphere. Naturally from this point, you need to be able to find outside, the North, the South, the East or the West cardinal points. At first if you are not sure, locate the star Polaris, in using the end stars (pot end) of Ursa major. When you have localize the star Polaris, you are looking toward the North. Turn around 180 degrees, and you face South.
On the Planisphere, the cardinal points are identified. Now hold your Planisphere so that the South point on it, is at the bottom. In our example above, the South point is locate at the bottom of the photo.
Now let us suppose that we are on May 10 th., and the time is 21h00 (9 o'clock in the evening). The position of the Great Oval or the observing visible window of the sky, on the Planisphere is as it is shown above. Now hold the Planishpere in one hand and raised it before your eyes and try to identify stars group identical to those found on the sky of the Planisphere.
For example, the constellation, Ursa Major, is locate near the zenith et is easily recognizable. A little bit lower you will see the constellation Leo. To the right of Ursa Major, the constellation Gemini and Bootes. And at the left of Ursa Major, Hercules.
In the constellation of hercules, try to find the Messier object M13, a very nice globular cluster, which is visible to the naked eyes and to binoculars.
Now try to locate the very nice open cluster, M44, which is named the Beehive and is locate in the Cancer constellation, which is at the right of Leo.
How to find the coordinates of an object?
To be able to find the coordinates of an object, you need only to move the slider rule so that its center line is over the object. For example, let us find the coordinates of the star Capella, which belong to the constellation Auriga. On the external circle, we read 5hr 19min, as the Right Ascension and 44 degrees as declinaison.
How to locate the Planets?
Planets are locate on or near the ecliptic. So in
following mentally the trace of the ecliptic, as shown on the
Planisphere, you can find out the most brilliant planets, like
Venus, Jupiter, Saturn and Mars by its distinctive red color.
Mars is visible about every two years. Saturn, less
brilliant, because more far from us, will be at your first
attempts difficult to locate.
The local time is the time of your watch. You must
though remember that to be able to properly orient the Great
Oval, you must always use the Solar time, that is, always use the
Standard time of your zone, and not the advance time. In
winter it is the time shown on our watches, but during summer, we
must subtract one our, before porting it on our Hour
Circle.
The Star Hopping method, will allow you to find in the sky all deep sky objects,like open clusters, galaxies or nebulae.
Naturally, a good set of celestial maps is required, for using this method correctly.
As binoculars or viewfinder of a telescope have generally a wide field of view of at least 5 degrees, we make with a small electric wire a small loop, like in the photo on the right, in such a way that the diameter of the round or loop, represent on the maps scale the field of view of our instrument. In our example it is 5 degrees. Once it is realized, our guide should have a form like a magnifying glass (MG).
To find, for example, the Messier object M11, starting from the star Altair, we will follow the way as per the sky map below.
On our sky map, we localize the most
brilliant star situated the nearest from the object that we want
to find.
We first center our MG on the star Altair and after that we more our MG on the map in such a way that we always keep in its field reference stars. We move the Mg step by step until we arrived at M11, always keeping reference stars in our filed of view.
In our example, we want to find the open cluster M11. the most brilliant star near the object is Altair. We center Altair in our MG and move it slowly toward the object M11, memorising the stars that we see in the field of view of the MG, until we reach M11. In this case five displacements are necessary.
We practice these manoeuvrering, so that we can memorize
the way to follow in the sky with our binoculars or our
viewfinder.
Next chapter, Celestial Coordinates and Universal Time, Star magnitude...
Last update August 1st., 1999.