How to Use a Planisphere
Introduction
A planisphere is a type of star chart that can be set
to show the location of objects in the sky for any given date and time. It is
small, portable and 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 accurate for a specific observer
latitude, but are useful over a range of latitudes. The planisphere in
Figure 1 is designed to be used for latitudes from 30º to 40º North.
Figure 1
Basic
Features
Figure 1 shows the front side of a planisphere. Bright
stars, constellation outlines, and some deep sky objects are shown on the white
“sky” portion of the planisphere. This “sky” can be rotated about the center of
the planisphere, corresponding to 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 which covers a portion of the bottom 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 right side of
the horizon is the eastern horizon
(90º azimuth), and the left side is the western
horizon (270º azimuth). To use the planisphere, it must be oriented so that
the planisphere horizon corresponding to the direction you are facing is at the
bottom. For example, the planisphere in Figure 1 is correctly oriented for a
person facing North.
Figure 2 indicates the location of several additional
reference points and lines on the planisphere.
Figure 2
The meridian
is an imaginary line that runs from the North Celestial Pole, through the zenith,
to the South Celestial Pole. 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.
Additional reference lines on the planisphere include
the celestial equator, and the ecliptic. The celestial equator is the
solid circular line that near the edge of the “sky”. The ecliptic is the curved dashed line that
intersects the celestial equator at two points (vernal and fall equinoxes).
Motion
of the Sky
The “sky” portion of the planisphere can be rotated
either clockwise or counterclockwise.
Figure 3
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 declinations greater than (90º minus
observer’s latitude) and never go below the horizon and therefore never “set”. The
direction of rotation of the “sky” is easily determined by remembering that
objects rise in the east and set in the west (see Figure 3). To rotate the (facing
north side) sky portion of the planisphere to correspond to the rotation 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 throughout the year are marked on the outer
edge of the moveable portion of the planisphere (the “sky”) as shown in Figure
4. Standard times are indicated on the outside edge of the fixed portion of the
planisphere below the horizon (add one hour to the printed times for daylight
savings time). To set the planisphere to display the sky for a specific date
and time, rotate the “sky” until the desired date and time are aligned. In
Figure 4, one date and time at which the sky will appear as shown on the
planisphere is April 20 at
Figure 4
There are many
combinations of times and dates shown on the planisphere in Figure 4 (For
example: March 5 at
With the planisphere set to a specific date and time of interest, orient it with the azimuth direction (N, E, or W) that you are facing at the bottom when viewing the sky. The “sky” on the planisphere will then correspond to the night sky you are facing. To view the southern sky, the back side of the planisphere is used (see Figure 5).
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 determine when
an object will rise on a specific date, rotate the planisphere “sky” until
the object is aligned with the eastern horizon. Then read the time of day
corresponding to the specified date. Figure 6 illustrates the star Vega rising
at
To determine when
an object will set on a specific date, rotate the planisphere “sky” until
the object is aligned with the western horizon, and read the corresponding time
for the specified date. Figure 6 illustrates that the star Rigel and the
constellation Orion are setting at
An object is said to “transit” when 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
Figure 6
Additional
Planisphere Features
Figure 7 illustrates several additional features of
the planisphere. For example, the celestial coordinate system coordinates of
Right Ascension (RA) and Declination (Dec) are marked on the planisphere.
Right
Ascension values are marked at one
hour intervals by “tick marks” on the celestial equator. Radial lines from the
North Celestial Pole mark RA values at three (3) hour intervals (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 on Figure 7. RA values increase in a clockwise
direction on the celestial equator.
Figure 7
Declination
values are marked at 10º intervals
along the radial lines of RA. 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.
As an example, using the planisphere in Figure 7, the
celestial coordinates of the star
Arcturus can be estimated to be approximately RA = 14 hrs 20 minutes and Dec =
20º.
Apparent (visible) magnitude, or brightness of stars
shown on the planisphere correspond 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 across the sky” represents the Milky Way.