Planispheres can help you plan the best times to observe any celestial object of your choosing. A planisphere is a disk which shows you which stars should be visible from your location at any date and time you select. They sometimes go by the name of "Star and Planet Locator". A planisphere is simply a disk with bright stars and constellations drawn on it and a cover which has a window showing what stars are visible from your location.
One sunny summer afternoon, many years ago, I was spending my summer vacation pondering all those thoughts a young man ought not think about (okay now, keep your mind on track. This is a science lesson). Those thoughts that confuse even the most respected scientist; the thoughts that make them stutter for an answer in such technical jargon lingo-jingo as to make them sound as if they have known all along. My main thought of the day came to me while picking up the postal mail from our mail-box on the farm. We were living away from civilization, out in the "boonies", a place that would make the town in "The Shining" or "Fargo" seem like New York City! I thought, "Out of the millions of people on this earth, how did this letter end up in our mail-box; founded in the middle of know-where land?". Being just a young man back then, my train of thought was quickly lost when mom asked if I wanted a freshly baked chocolate chip cookie! Those were certainly, the days!
So the other day I seemed to finally gain my conscious thought train and picked-up where I left off with my question from so many years back. But this time it came to me while looking up at the dumb-bell galaxy that night. It's analogous I thought, "The postal service knows how to get that Publishers Clearinghouse million dollar letter to my door, just like I know how to find the dumb-bell galaxy through the millions of stars out there." Ah-ha! I finally figured it out.
Beginners and old-pros alike need to have an easy way to find the stars, galaxies, and planets they choose to look at each night. It's very much like the address of our house, in a sense. First let's start out by describing how we address a letter.
A postal letter is addressed with several key pieces of information. Especially if you are sending a letter to a different country. For a letter making its way to the United States for example:
As you can see, I listed the order in progression of "size", if you will. Specifying the Country first narrows the number of prospective delivery addresses alone. The State narrows it even more-so. And the City, Zip, Street and House number respectively narrow the address down to a single abode. Oh yeah, and I forgot your Name! That would be pretty important huh? (Wouldn't want your dog Delmar to open up your income tax refund would you?) Pretty interesting, don't you think? Just be glad your brother doesn't really live on Mars, or we would need to start putting the name of the Planet on the address as well! So just for fun, (admit it, you've done this before), here is our solar address from outside our Galaxy:
John Smith
123 West 45Th Street
Kansas City, MO 64114
United States of America
Earth (Third rock from the Sun)
The Inner Solar System
The Milky Way
In all fairness here, I must say that I believe finding a celestial body is a lot less complicated than what a postman needs to know to deliver your letter. So now that you are "postal", lets see how this is analogous to bodies outside the Earth.
Constellations
There is a word for a pattern of stars in the sky which appears to be so distinctive that it is easily identifiable and remembered. That word is "asterism". In ancient times, people saw asterisms and made up all kinds of stories about mythological creatures and characters which they associated with the star patterns. As astronomers subsequently began to make maps of the stars, the named asterisms were included in the maps and called constellations.
As time passed, the sky became filled with constellations, many of which included the same stars. The International Astronomical Union stepped in to gain control and make sense of things in 1925. They adopted 88 official constellations and assigned areas of the sky to specific constellation names. It should be noted, however, that they made no requirement that the constellation be easily seen nor that the legends associated with the constellation make any sense. In fact, most constellations don't really resemble the creatures or characters they are named after. So don't worry if you can't make out the shapes!
Constellations can be a useful way to help identify positions of stars in the sky. Constellations have imaginary boundaries formed by "connecting the dots" and all the stars within those boundaries are labeled with the name of that constellation. However, keep in mind that constellations are not real objects; they are just patterns as seen from our observation point on Earth. The patterns we see are for the most part just by chance. The individual stars in a constellation may appear to be very close to each other, but in fact they can be separated by huge distances in space and have no real connection to each other at all. For example, look at the image below of the stars which make up the constellation Orion. The stars in this easily-observed constellation are at VERY different distances from Earth!
At different times of year, different constellations can be seen in the sky. Different constellations can also be seen depending on where you are on Earth. Thank goodness for the Planisphere!
Planisphere (Star Finder and Planet Locator)
Planispheres can help you plan the best times to observe any celestial object of your choosing. A planisphere is a disk which shows you which stars should be visible from your location at any date and time you select. Planispheres sometimes go by the name of "Star and Planet Locator". A planisphere is simply a disk with bright stars and constellations drawn on it and a cover which has a window showing what stars are visible from your location. You need to be sure to get a planisphere appropriate for your latitude, otherwise the window will not show the sky accurately. Around the edge of the disk is a scale with each of the months of the year usually divided into days. The edge of the cover is marked off with the hours of the day. By turning the disk (which is attached to the cover through the center) so that the day and month line up with the hour you are going to observe the window on the cover shows the stars that are up at that time of night.
Taking your planisphere observing with you
The edge of the window on the cover of the planisphere corresponds to your horizon and should indicate which side is north, south, east and west. This will help orient your planisphere to the sky above. Another thing to notice is where the axis of the disk is located. Just like the sky overhead the planisphere rotates around a point very close to the star Polaris. The point in the middle of the window corresponds to your zenith and if you run a line from Polaris through your zenith and extend it all the way to the edge of the window you have drawn your local meridian. Note that the window represents your entire sky not just a small part of it. You have to imagine that all of the stars that appear in the small window of your planisphere are actually spread out across the whole sky.