Star Points for October, 2004; by Curtis Roelle ClearSky Clock Helps Plan Your Observing Session In the previous two installments we have been discussing sky conditions that affect astronomical observing. These have included the weather (clouds, air transparency and steadiness) as well as the effects of light pollution caused by wasted artificial outdoor lighting. This month we'll discover an on-line web tool that is useful in predicting upcoming observing conditions over the next 48 hours or so. It can also be used to discover alternative observing locations that could offer improved results for more successful observing sessions for those who want to observe fainter objects or see more subtle details in more familiar ones. The ClearSky Clock is not a single web site. It is a collection of sites for various observing locations around the U.S. and Canada. Westminster has its own ClearSky clock for the Bear Branch Nature Center (BBNC). With your web browser visit the BBNC ClearSky clock (http://cleardarksky.com//c/BearBraMDkey.html). The first thing you will notice is four rows of colored squares. Each column of squares represents one hour's time. The bottom row of squares indicates when it will be daytime and nighttime. The top three rows indicate weather related sky conditions. The top row shows the predicted hour by hour cloud cover. The second row indicates the sky clarity or transparency. The third row represents the steadiness of the air or "seeing." Dark blue squares mean good weather -- clear, transparent, or steady skies. White means bad. In between are various shades of blue. Use the BBNC ClearSky clock to anticipate when good weather will provide ideal seeing conditions. The weather data used by ClearSky clock is furnished by the Canadian Meteorological Center. On the ClearSky clock page there are links to other sites helpful in planning an observing session. You can obtain daily sun and moon rise and set information from the U.S. Naval Observatory using the "Sun & Moon Data" link. You can generate an on-line star map from Your Sky for any time of night with the "Star Map" link. ClearSky Alarm-Clock issues you alerts via either e-mail, pager, or cell phone when observing conditions are expected to be favorable. The user configuration is fully customizable letting you set the specific criteria for your alerts. Interactive terrestrial maps are also available. You can request either road maps from MSN or topographic maps from topozone.com. One of the most unique features of the ClearSky Clock is the light pollution map. It presents a map with colored contours based on the Bortle light pollution scale discussed last month. The "Light Pollution map" link is also color coded so that you can instantly see the site's Bortle Ranking (as detailed last month). For BBNC the link is orange, which corresponds to Bortle scale 5, i.e. suburban sky. The light pollution map is also interactive. Radio buttons are used to select what happens when you click on the map. Display choices include showing the click point's Latitude & Longitude, an interactive road map, or an interactive topographic map. If you see a location on the light pollution map that looks promising because it is darker, then selecting Road Map and clicking on the point will produce an interactive map that you may use to obtain driving directions to the site. But how does one find other good observing sites beyond the borders of the local light pollution map? The ClearSky Clock page has links for obtaining lists of other ClearSky Clock locations with radii of 60 and 120 miles. These lists are also color coded by Bortle scale so that you can immediately tell which sites are darker than others. For example, say that you want to find the nearest sites to BBNC with a Bortle 4 (rural transition) ranking. Clicking on the BBNC's "Within 60 miles" link produces a list of around 30 ClearSky Clock locations in Maryland and Pennsylvania. The list is sorted by increasing distance. To find the nearest Bortle 4 site, scroll down looking for a green square in the "light pollution" column. The closest Bortle 4 ClearSky Clock is the Tuscarora Shale Pit in Pennsylvania located 54 miles away as the crow flies. Where is the Tuscarora Shale Pit located? Click on the link and you are taken to its ClearSky Clock page. Simply click on the "Road Map" link to see the location and get driving directions. Use the ClearSky Clocks for planning your next observing session at home and to discover new places to observe.