Introduction

A planisphere is a kind of star chart that will be set to indicate the placement of objects within the sky for any given date and time. It is small, portable and very easy to use, providing a very convenient tool for identifying objects seen in the sky or locating specific objects of interest. Planispheres are designed to be correct for a particular observer latitude but are helpful over a spread of latitudes. The planisphere in Figure 1 is designed to be used for latitudes from 30º to 40º North.

Basic Features

Figure 1 shows the front side of a planisphere. Bright stars, constellation outlines, and a few deep sky objects are shown on the white “sky” portion of the planisphere. This “sky”are often revolved around the middle of the planisphere, adore the apparent rotation of the night sky about the North Celestial Pole. The star Polaris (the North Star) is located in the sky very near the North Celestial Pole and can be found at the “center” of the planisphere. The solid curved portion of the planisphere that covers some of the bottoms of the sky represents the horizon (where the earth meets the sky). On the front of the planisphere, the center of the horizon corresponds to the direction North (0º azimuth). The left facet of the western horizon (270º azimuth), and the right facet is the eastern horizon (90º azimuth), To utilized the planisphere, it should be oriented so that the planisphere horizon such as the direction you’re facing is at the lowest. as an example, the planisphere in Figure one is properly orienting for someone facing North.

Figure two indicates the location of many additional reference points and features on the planisphere.

The meridian is an imaginary line that keeps running from the North Celestial Point, through the zenith, to the South Celestial Point. It corresponds to a line connecting the centers of the two “grommets” or fasteners on the planisphere. The zenith (the spot directly overhead in the sky, altitude = +90º) lies on the meridian approximately in the center of the “sky”. More precisely, it is located where the declination equals the observer’s latitude.

Extra reference lines on the planisphere incorporate the divine equator and the ecliptic. The divine equator is the strong round line that closes to the edge of the “sky”. The ecliptic is the curved dashed line that intersects the celestial equator at two points (vernal and fall equinoxes).

The Motion of the Sky

The “sky” part of the planisphere can be pivoted either clockwise or counterclockwise.

As the night sky appears to rotate around the north celestial pole due to the rotation of the earth, objects in the sky that are not circumpolar rise above and set below the horizon throughout the night. Circumpolar objects have declination greater than (90º minus observer’s latitude) and never go below the horizon and therefore never “set”. “The bearing of revolution of the “sky” is effectively dictated by recalling that articles ascend in the east and set in the west (see Figure 3). To turn the (confronting north side) sky segment of the planisphere to compare to the revolution of the night sky, it must be turned counterclockwise.

Using the Planisphere

To use the planisphere, it must be set to a specific date and time. The months and days during the time are set apart on the external edge of the moveable segment of the planisphere (the “sky”) as appeared in Figure 4.” Standard times ar indicated on the surface fringe of the mounting portion of the planisphere below the horizon (add one hour to the written times for daylight savings time). To set the planisphere to show the sky for a particular date and time, pivot the “sky” until the ideal date and time are adjusted. In Figure 4, one date and time at which the sky will show up as appeared on the planisphere is April 20 at 9:00 PM (neighborhood standard time; 10 PM).

There are numerous mixes of times and dates that appeared on the planisphere in Figure 4 (For instance: March 5 at 12 PM, and December 18 at 5 AM. As the days progress consistently, a similar setup of the sky will show up at earlier times.

With the planisphere set to a particular date and time of intrigue, situate it with the azimuth heading (N, E, or W) that you are looking at the base when survey the sky. The “sky” on the planisphere will then correspond to the night sky you are facing. To view the southern sky, the backside of the planisphere is used (see Figure 5).

The back side of the planisphere provides a view of the southern sky which has less distortion than the front side. Note that when facing south, the eastern horizon is on the left side, and the western horizon is on the right side of the planisphere.

Determining Rise, Transit, and Set Times

To decide when an item will ascend on a particular date, turn the planisphere “sky” until the article is lined up with the eastern horizon. Then read the time of day corresponding to the specified date. Figure 6 illustrates the star Vega rising at 9 PM on April 20. It also rises at Midnight on March 5.

To decide when an object will set on a particular date, pivot the planisphere “sky” until the object is lined up with the western horizon, and read the relating time for the predefined date. Figure 6 illustrates that the star Rigel and the constellation Orion are setting at 9 PM on April 20.

An object is said to “transit” once it crosses the meridian. To determine when an object transits, set the object on the meridian (on the planisphere, the meridian is on a line connecting the two grommets) and read the corresponding date and time. In Figure 6, the constellation Leo is on the meridian, and the star Regulus has just crossed the meridian at 9 PM on April 20.

Additional Planisphere Features

Figure 7 illustrates several additional features of the planisphere. For instance, the heavenly organize framework directions of Right Ascension (RA) and Declination (Dec) are set apart on the planisphere.

Right Ascension values are marked at one-hour intervals by “tick marks” on the celestial equator. Outspread lines from the North Celestial Pole mark RA esteems at three (3) hour interims (0, 3, 6, 9, 12, 15, 18, and 21 hrs RA) Although the values of RA are not labeled on small planispheres, the line corresponding to 0 hrs RA can be identified by locating the vernal (spring) equinox. The vernal equinox is located at the point where the ecliptic crosses the celestial equator in March. The line corresponding to a value of RA = 12 hrs corresponds to the fall equinox. The fall equinox is indicated in Figure 7. RA values increase in a clockwise direction on the celestial equator.

Declination esteems are set apart at 10º interims along the spiral lines of RA. The Values of declination can be determined by remembering that the declination at the North Celestial Pole is = + 90º. Declination values decrease as you move away from the North Celestial Pole and toward the Celestial Equator.

For instance, utilizing the planisphere in Figure 7, the heavenly arranges of the star Arcturus can be evaluated to be roughly RA = 14 hrs 20 minutes and Dec = 20. The apparent (visible) magnitude, or brightness of stars shown on the planisphere corresponds to the size of the dot indicating the star. The larger the dot, the brighter the star will be in the night sky.

Finally, notice the gray “smudging” that appears in each of the figures. This “band over the sky” speaks to the Milky Way.