How Dark are your Skies? Introduction: Estimating Limiting Magnitude Limiting magnitude is used to evaluate the quality of observing conditions. You can be in the best and darkest sight, but you still may be getting some reflected light from nearby houses or strip malls or whatever. So this time we are going to look at the best way to measure your skies: finding the magnitude of the faintest star visible to the unaided eye. This is useful for figuring out what is most easily visible at your observing site and whether you should be able to see the "faint fuzzies" like nebulas and galaxies. Before electric lights became common, it was routine that people could see all 7 stars in the Pleiades and the Andromeda Galaxy (M101 in Andromeda). Without binoculars or a telescope! But, not today. The simplest way to evaluate limiting magnitude is to find suitable stars with known magnitudes from star map and check which of them are visible. A more clever way is to count visible stars inside known star squares and triangles. This method was originally invented by meteor observers. To identify new constellations, you can either use a star chart OR link to the constellations below. Some of these will not be visible during this season or
because of trees, buildings, or light pollution. Spend some time figuring out what constellations you should be able to see before you go outside. Part 1: Viewing your Skies Method 1. Wait for your eyes to dark adapt (at least 15 minutes). Spend the time practicing finding your known constellations and planets. 2. Choose four of the 10 estimating areas from the image map above (or the larger version here) or one of the links below; the resulting page has a larger sky chart and a limiting magnitude table. 3. Count the total number of stars you see in the area including the corners. 4. Look up that number in the table and read off your limiting magnitude. You may see more (or fewer) stars than are plotted on the charts. Different people may get different results. The areas used in limiting magnitude estimation: In observations the area should be chosen so that it is either near the view direction or the zenith, depending on observations and the situation. Meteor observers use the viewing direction. Area Corner stars Constellation 1 Chi-Zeta-Delta-Xi Dra Draco 2 Beta-Delta-Zeta Per Perseus 3 23-Theta-Beta UMa Ursa Major 4 Alpha-Epsilon-Beta Gem Gemini 8 Alpha-Beta-Zeta Tau Taurus 9 16 Alpha-Beta-Gamma-Delta Leo Alpha CVn--Epsilon-Eta UMa Leo Canes Venatici-Ursa Major 17 Epsilon-Theta-Delta Aur Auriga 18 Mu-Gamma-Phi And Andromeda 19 Kappa-Alpha Dra--Beta UMi Draco-Ursa Minor 29 Gamma-Alpha-Beta Hyd Hydra
Questions (1 is the table, 2 through 5 follows) Constellation Date Area Limiting Magnitude Notes 1. What is your estimation of the overall limiting magnitude? You may want to average your value or take values for each separate quarter of your sky. 2. Are there areas that are much higher or lower than the average? How different? Average the stars in that area. 3. Are those differences in #2 due to different overall average magnitudes of stars in those areas? 4. What local factors cause that (skyglow from a particular area, etc)? 5. Look back at the stars - are fainter stars harder to see in bright areas or are fainter stars just faint in general? Part 2: Light Pollution in the United States The rest of the observation is from the International Dark Sky Association webpage (http://www.darksky.org/ida/darksky/index.html). Please complete the four questions at the end of this exercise.
Introduction to the Dark Sky Finder For the past several decades, it has become increasingly more difficult to observe the night sky. Many of our generation have not witnessed the beauty of the summer Milky Way or the mystical dance of the Aurora Borealis. The vast majority of this problem, known as light pollution, is caused by the proliferation of poorly designed lighting fixtures and/or installations. Much information on how you can help bring back the stars is available in our Information Sheets. This page was set up to help stargazers, that reside in the continental United States, find the most suitable sites for enjoying the nighttime sky. The GIF image at the top of this page is a model of light pollution based upon the 1990 US Census and Walker's Law. How DarkSky Works First select a location from a larger version of the GIF image displayed at the top of this page. The point that you click on is then used to populate an array, which basically contains all of the pixels in the image that are within a 60 mile radius of the point selected. These pixels are then ranked according to distance and naked eye limiting magnitude. Only the darkest sites within the 60 mile radius are selected and displayed for you. No greater than 25 sites will be listed at any one time. The site listing gives approximate zenithal naked eye limiting magnitude, distance, longitude, and latitude. The limiting magnitude is only approximate and assumes perfect conditions. Steve would like to calibrate his model with actual observations. Your data here will do that. Of course the sites selected near the border with Canada and Mexico may be in error since data for those countries is not taken into consideration. In addition DarkSky is not smart enough to select sites that are inaccessible. For instance, if you click on Chicago, many of the darkest sites are of course in Lake Michigan. You will have to adjust your clicking position appropriately to find a dry site.
Once the listing of dark sites appear, you need to select a dark site number. The first entry in the table is the site on the map that you selected and it is listed as "Home". This is given for you as a point of reference. Once a site number is selected, a map of the area will be generated by a link to the US Census Bureau. You may wish to play with the map given at that site by zooming in and out. The closer you zoom in, the more detailed the map. The default map requested by the DarkSky code is ~35 miles on a side. Often you will need to zoom out to get your bearings. Select a method for finding a dark site near you: Clickable Imagemap Note that this tool requires the downloading of a 270 kb GIF file and it has limited options available. The DarkSky Java Applet You must have a Java 1.2 plugin loaded into your browser. It may take up to a minute or so to load in the entire applet, please be patient. QUESTIONS 1. What is the dark sky rating of your observation area? 2. Are there any pockets of darker skies within 30 minutes of your house (not on water!)? 3. Using the Milky Way magnitude guide below, what is the limiting magnitude of your site? 4. Compare your constellation and Milky Way data which allows you to see more stars? 5. Does light pollution make a difference in where you live? Would it if you had more time to look at the night sky?
A Brief Discussion on Limiting Magnitudes So now that you have found a site to observe from, what will you be able to see once you get there. Below we try to give a feel for what the nighttime sky looks like to the naked-eye as the limiting magnitude increases. Note that the descriptions deviate for high and dry locations. Here we use the Milky Way to refer to that part of the Milky Way from Cygnus through Sagittarius unless otherwise noted. Further information on wat the night sky looks like in the absence of light pollution can be found in IDSA Information Sheet 111. Zenithal Sky Features Limiting Magnitude 4.5-5.0 Milky Way and Zodiacal light invisible. Typical conditions found in suburbs of major cities. Passing clouds are easily seen due to being lighted up from surrounding lights. 5.1-5.5 The indistinct Milky Way faintly visible only near the zenith. Zodiacal light invisible. M31, the Andromeda Galaxy, is barely discernible. 5.6-6.0 The Milky Way is now more easily seen, but lacks detail. M13, the Great Hercules globular star cluster can now be just glimpsed when near the zenith. The Zodiacal light is still invisible. The Milky Way from Auriga through Orion still invisible. 6.1-6.5 The Milky Way is now obvious and some detail can be glimpsed. The Zodiacal light is now barely visible, but not obvious. The Milky Way from Auriga through Orion is faintly visible. There is still noticeable skyglow along the horizon due to distant towns and cities. 6.6-7.0 Much structure is visible in the Milky Way. The Zodiacal light is an obvious cone of light. The major constellations are less obvious due to "noise" caused by the large number of faint stars now visible. Passing clouds appear as dark moving masses as they block the natural skyglow or the Milky Way. A few sources of skyglow are still visible. 7.1 + Incredible! The Milky Way contains an enormous amount of structure all the way to the horizon and you can easily see your way around by it's light. The Zodiacal light now encircles the entire ecliptic. There are no sources of skyglow along any part of the horizon. Many meteors are visible.