Date and Time
Start by examining the area around the perimeter of the chart. You will find the time printed for each hour of the day, with small arrows shown in-between marking the half-hours. This ‘clock dial’ serves two purposes. The primary purpose is to be used in setting the planisphere to display the portion of the sky that is above the horizon at a selected date and time. The clock dial can also be used to quickly locate right-ascension (RA) grid lines on the star chart. See Celestial Coordinates to learn how to use the grid lines to locate objects on the chart.Date and Time Setting
Moving slightly inward from the clock dial, you will find a calendar. Each month of the year is printed in purple text with they days of that month shown below it. This calendar is used in conjunction with the clock dial to set the planisphere for a chosen date and time. Simply rotate the top dial of the planisphere until the selected date aligns with the desired time on the clock dial. This will result in the transparent window on the top dial being positioned over the portion of the star chart that is visible above the horizon at that date and time. Alternatively, you can rotate the top dial until a selected star or constellation rises above the horizon (moving into the window from the East) or sets below the horizon (moving just out of the window to the West), and use the calendar and clock dial to see what time the object will rise or set in the sky on any particular date. After rotating the dial to cause your selected star to ‘rise’, find the date in question on the calendar. The time that aligns with your selected date will be the rise-time for that object on that date. Note: You may have to account for Daylight Saving Time if it is observed at your location
One final note about the calendar, you may notice that there are ‘shooting-star’ symbols located at certain points around the dial. These symbols indicate that a meteor shower peaks around that date each year. Turn the planisphere over to consult the Meteor Shower table on the back to see the name and relative intensity of any particular shower. Meteor showers are the result of the dusty trail of debris left by a comet as it passes through the solar system. As the Earth moves around the sun in its annual orbit, it moves through these dusty trails causing individual grains to burn up brightly when they hit the atmosphere. These shooting stars will appear to radiate outward from the point in the sky towards which the Earth is moving. Shooting stars can be seen on any given night during the year, but you will notice them more frequently during meteor showers.
Moving further inward from the edge of the chart, the most obvious element of the planisphere is the transparent window through which you can see the star map. The black line around the edge of the window represents the Horizon, the place where the sky meets the ground. Just as you can face any direction when looking at the real horizon in front of you, you can also look towards any direction on the chart by rotating the entire planisphere until the chosen direction printed along the horizon line is oriented towards the ground.
Take a moment to look at the actual sky. This may seem obvious, but notice how only half of it is visible to you at once, starting near the ground at the horizon and rising up to the zenith directly overhead. You would have to literally bend over backwards to see the rest of the sky. Though, turning around and facing the opposite direction might be easier on your neck! Let’s apply this same concept to the horizon window on the planisphere. Imagine a line cutting the horizon window in half horizontally. You should be focused on the portion of the chart below this imaginary line. Stars on the chart near the lower edge of the window will be found low in the sky near the horizon. Stars on the chart rising higher up towards the imaginary half-way line will be found higher up in the sky near the zenith directly above your head. Stars on the chart that are above this imaginary line will be past the zenith - behind you. Don’t bend over backwards! Turn completely around and rotate the planisphere 180 degrees to view these objects.
Above the Horizon Window on the planisphere, you will see the numbers 1, 2, and 3 printed in large type with smaller adjacent text. These represent simple step-by-step instructions for those who may need a reminder about how to operate the planisphere. If you have read this far, the instructions printed there will already be familiar to you.
Take a look at the planisphere just to the left of the Horizon Window, near the word ‘East’. There, you will find a legend that will help you understand the symbols printed on the star map. The first item in the legend is the Star Magnitude chart. This shows the relative sizes of the black dots representing stars on the map and their corresponding stellar magnitudes. If you are unfamiliar with stellar magnitude, lower numbers are brighter and larger numbers are dimmer. Early astronomers would refer to the brightest stars in the sky as stars of the ‘first magnitude’. Slightly dimmer stars were ‘second magnitude’ stars, and so on. The modern definition defines a star of magnitude 1 to be 2.5 times brighter than a magnitude 2 star. Likewise, that second magnitude star is 2.5 times brighter than one of magnitude 3. And so on. On this scale, some bright stars can have magnitudes that are even smaller than 1. Sirius, the brightest star in the sky even has a negative magnitude! For our purposes, just understand that bigger dots on the map represent brighter stars and smaller dots represent dimmer ones. Depending on the light pollution at your observing site, you may not be able to see all the stars on the map with the naked eye. That’s OK, they will become easily visible in your finder scope or in a pair of binoculars. Of course, there are dimmer stars in the sky that will be visible in the telescope that are not printed on the chart.
Below the Star Magnitude chart is the list of Symbols used on the star map. Galaxies in the telescope look very different than globular clusters. And those look different to open clusters and nebulae. Before you start searching for a new object in your telescope, look at the symbol on the chart to make sure you know what type of object you are expecting to see in the eyepiece.
Take a moment to look at the upper portion of the planisphere, above the Horizon Window. There you will find six small circular star charts. These locator charts, and three more on the back of the planisphere are designed to aid you in finding and identifying some more challenging Messier objects. If you have already got some experience observing the Messier objects in a telescope, you may realize that most of them look very different to the surrounding background of stars. As a result, you will find that after pointing your telescope to the correct part of the sky you are easily able to pan around and identify the object even if it did not land perfectly in your telescope’s field of view. For example, if you are looking for the Ring Nebula, there will be no mistaking when you see it, even if you have to scan back and forth across the area a bit. Some of the Messiers present more of a challenge, either because they do not stand out conspicuously from the background of stars (e.g. M40), or because there are several objects of that type within the same area of the sky (e.g. Virgo galaxies). In either case, it can be difficult to know if you are actually observing the correct object in the eyepiece. This is especially critical if you are logging your observations, such as for the AL’s Messier certificate. Each of the included locator charts represents an area of sky about the same size as what is visible in a typical finder scope. Point your telescope to the area of the sky shown on the main star map, then use your telescope’s finder to match the star pattern shown in the locator chart for one of these challenging objects. This method will help you to positively identify your target in the eyepiece.
M31, the Andromeda galaxy is a huge object on the night sky, covering an area larger than the full moon. Chances are, your telescope’s field of view is not large enough to see the entire galaxy! You will most likely observe the bright core of M31, while the tenuous and wispy arms of the galaxy extend well beyond your view in the eyepiece. The Andromeda galaxy also has two satellite galaxies that have Messier designations, M32 and M110. M32 is often visible in the same field of view while observing M31. M110 sometimes requires searching a bit further from the larger galaxy’s bright core. Use the locator chart on the front of the planisphere to help you properly identify each object.
This globular cluster is located in a part of the sky with few bright naked-eye stars. As a result, it can be difficult to see exactly where to point your telescope when looking at that part of the sky. Use your finder to locate the star patterns in this locator chart after pointing the scope in the general direction.
Like M75 above, M55 is found in an area of the sky with few bright stars visible to the naked eye. Use the locator chart and your telescopes finder to tweak your telescope’s position before moving to the main eyepiece.
M95, M96, M105
These galaxies in Leo share the view with some friends. Once you get your telescope pointed to the correct position on the sky, several galaxies will compete for your attention in the eyepiece. Which are your targets and which are just along for the ride? This locator chart will help you to be sure you are observing the correct target.
Another pair of Messier galaxies in Leo, with the beautiful galaxy NGC 3628 sharing the field of view. Use the locator chart to make sure you are describing the correct object in your observing logs.
M17, M18, M24
These three objects are located in Sagittarius near the center of our Milky Way galaxy. There is a lot to see in this part of the sky. Let’s start with M24, the Sagittarius Star Cloud. This object is unlike any other in the Messier catalog. It is a huge accumulation of stars drawn out into a long bar. If you are fortunate enough to observe from a location with very little light pollution, you may even be able to locate it with the naked eye! Like the Andromeda galaxy, it is easy to zoom in too far with the telescope so that the entire object is not visible in the field of view. If you fall for this trap, you may mistakenly take the small open cluster NGC-6603 to be your target. You are looking for a forest, but seeing a close-up of the bark on one of the trees. Use your lowest power eyepiece to get started with your observation. If that is still too much magnification, make your observations through your scope’s finder instead of the main eyepience!
M17, the Swan Nebula is easily identifiable in the eyepiece, and often in the finder. Depending on the aperture of your scope, it can really look like a swan floating on a lake. It also resembles a check-mark or the Greek letter Omega in some scopes. After locating M24, M17 is easily found nearby. The open cluster M18 lies neatly between them, making for easy identification using this locator chart.