Hey Everyone, The 2007 Autumnal Geosat Flare season has already started (actually, it is more than half over!) and now it is a purely Northern Hemisphere event. Over the next few nights you will have a chance to see spectacular flaring of geostationary satellites (geosats) at certain points of the night. These flares will rival the brighter stars in the sky and last for several minutes (not seconds like Iridium flares), lighting up and then dimming down from the east to the west along a thin line near the celestial equator – like a string of diamonds slowly lit by flashlight. And occasionally, you may catch a stunning set of 2 or more very compact geosats flaring all at once (like the XM satellites for my location). And at nearly 6 earth radii away you will know that you are viewing the most distant manmade objects possible without the need for binoculars or a telescope (although these instruments will help you see the dimmer geosats of which there are many more). The last couple of years I provided star charts; this year it is tables! These tables will tell an observer, based on their latitude and date where and when to look in the night sky to maximize their chance of seeing a flaring geosat. In particular, the table will, for each latitude and date, define two times in the night – one when geosat flaring ends in the evening hours because of the earth’s shadow covering up the observing point and the other time in the early morning when the earth’s shadow uncovers the observing point. This makes the middle of the night the worse time to look for flaring geosats and the time after sunset and before sunrise the best time. As the dates progress from late-September to mid-October, these two times in the table, for each latitude and date, converge towards each other on local midnight and afford the viewer a much greater (higher elevation) opportunity to see flaring geosats. This makes this process more and more exciting with each passing night. (And the full moon will not be problem for the mid-to-high latitude observers this Fall!) Kevin Fetter has helped me greatly by hosting the necessary tables on his website at: http://www.kfetter.com/jeff/geoflare.htm At the website there is a very in-depth explanation and example of how to use the tables. However, I'll give a "quick" explanation as to how they work for those more familiar with this hobby. In the webpage above, find the table that applies to your latitude and the date you want to observe. The PD Dec. column (by your latitude) and the PDRA row (by the date) define the point (PD means Purple Dot) where you want to always be looking, throughout the night. You will start observing at the PD Rise Time (PDRT) in your east (shortly after sunset) and stop when the first of two times in the body of the table is reached for your latitude and date. This is when flaring at the PD stops and the PD is eclipsed. It re-starts at the second time in the table (as the PD becomes uneclipsed) and continues until the PD sets in your west (shortly before sunrise). You will always look along your PD Dec. line from the eastern to the western horizon since this is the only place you will see geosats. The times given for PDRT and the two times in the body of the table are assuming "Local Midnight" is at 12:00AM each night. You will have to adjust the PDRT and the two times in the table for each date by determining your *actual* local midnight time. This is the time that is halfway between sunset and sunrise, based on your local timezone. If this time is after 12:00am, you will have to add it to the PDRT and the two eclipse times for that date. Here's an example of how to use the tables for me at latitude +33.06946N and on October 5. On that date for my location the halfway point between sunset and sunrise is 1:16am Central Daylight Time (CDT). I got this information from my Heavens-Above webpage under the "Sun" link, at the bottom called "Minimum Altitude". (BTW, this time changes almost every night so it must be looked up for each observing night for me.) I find the table (there are 12 of them) that applies for latitude 33 and date Oct 5 (4th table down). The point I need to watch (the PD) is at RA: 0:45:28 (the PDRA under the date) and at Dec.: -5.39deg (the PD Dec. next to the latitude 33). This point rises at 6:12pm Local Midnight Time (PDRT next to latitude 33), is eclipsed at 6:33pm Local Midnight Time, and is uneclipsed at 5:26am Local Midnight Time. I then *add* 1hr16min to those 3 times and get a corrected time of PDRT=7:28pm CDT, PD Eclipse Time=7:49pm CDT, and a PD Uneclipse Time=6:42am CDT. This means, as I watch the PD rise from 7:28pm CDT to 7:49pm CDT, the PD is uneclipsed and I stand the greatest chance of seeing a geosat flare. Then from 7:49pm CDT until 6:42am CDT the PD is eclipsed and there shouldn't be any geosats flaring *at* the PD. Then after 6:42am CDT I should be able to start seeing flaring at the PD until it sets in the west. This, at least intially, is not a lot of time to watch for geosats. But if you look right across increasing dates, you will see the eclipse (evening) and uneclipse (early morning) times begin to converge on local midnight, affording more time to see flaring geosats. On October 16 the earths shadow pulls off the geosat orbit and the RD uneclipsed *all* night! Additional information: 1.) The Date at the top of each table is based on 0 (zero) UT. As the day progresses, the sun moves lower in declination so that for those around +/-180 East/West longitude, the information may be more inacurrate. 2.) In the evening, even after you PD is eclipsed *continue* to look west of the PD along the PD Dec. line. You will see *most* of the geosats begin flaring out here. 3.) In the table, the term "NVAN" means Not Visible All Night, meaning the PD stays under the earth's shadow all night (not good for geosat flaring!). 4.) In the table, the term "VAN" means Visible All Night, meaning the PD is not eclipsed all night by the earth's shadow. This will start on October 16 and continue until next February!. 5.) If you want to continue looking for geosats after the season officially ends (on October 16), just keep looking at your PD Dec. and a PDRA that is exactly opposite the sun's RA. Please let me know if you have any questions and please tell me about any observations or lack there of you've made. I would like to thank Kevin Fetter for hosting my files, Greg Roberts for throwing out my earlier equations with incredible observations last April, ushering this new data, and Rod Austin for confirming that Greg was correct! M The first thing you must know is where to look. For a given latitude, you always look along the same *declination* (Dec.) each night from the eastern horizon to the western horizon. The right ascension (RA) you will look at will be at the RA that is exactly opposite the sun's RA (add or subtract 12 hours from the sun's RA). So each night for a given observer, the declination doesn't change but the RA does. I call the this point the "Purple Dot" or PD and in the tables below you will see the terms "PD Dec." for Purple Dot Declination and "PDRA" for Purple Dot Right Ascension. When a geosat is at this point in your sky *and* this area is *not* eclipsed by the earth's shadow, there is the maximum chance of the geosat flaring. The second thing you must know is when, each night, to look at the Purple Dot (PD). These times are driven by when the earth's shadow eclipses the PD in the evening ("Eclipse Starts") and when the earth's shadow uneclipses the PD in the early morning ("Eclipse Ends"). Even though the PD rises, crosses the local meridian, and then sets at the same time each night (relative to "Local Midnight" - more about this later) the eclipse times change greatly each night. How do these times change? On September 23rd almost everyone (except those on the equator) had no chance to see their PD uneclipsed in their night sky. In other words, the PD was "Not Visible All Night" - NVAN - a term I use in the tables. Then at some date later than September 23 (for the Northern Hemisphere) the PD will rise in the east in the evening uneclipsed and move some distance west before being eclipsed by the earth's shadow, which catches it from the east. It will stay eclipsed throughout the middle of the night until some time in the early morning, before it sets, when it will become uneclipsed by the shadow racing off faster to the west. During very middle of the night you should not see any flaring geosats *at* the PD. Throughout the following nights, the eclipse and un-eclipse times converge on Local Midnight - giving the observer greater and greater opportunities to see flaring geosats at their PD. Then on October 16, the PD will be "Visible All Night" - VAN - for everyone. If you go to this webpage, you will see 12 tables, each 72 columns in width. There are 6 tables for Latitudes 0 - +39N and 6 tables for Latitude +40N - +80N. Choose your 6 tables based on your latitude. Each latitude has 6 tables for 6 different sets of dates - 4 days in length each. Across the top of each table is the date *at 0.0UT*, the sun's declination for each date at 0 UT, and the Purple Dot's Right Ascension (PDRA) for each date. Down the left column is the observer's latitude, the Purple Dot Rise Time (PDRT), and the Purple Dot Declination (PD Dec.). The body of the table is made up of, for each latitude and date, the eclipse start and end times *based on Standard Local Midnight* which is always 12:00am. Your actual Local Midnight time is the point halfway through the night between sunset and sunrise and is based on your timezone and longitudinal offset. You can get this from your Heavens-Above site by clicking on the "Sun" link in your main page and reading the "Minimum Altitude" time at the bottom of the page. THE TIMES IN THE TABLES ARE MEANINGLESS UNLESS YOU ADJUST THEM TO YOUR ACTUAL LOCAL MIDNIGHT TIME. For me on Oct 5 my actual local midnight time will be 1:16am Central Daylight Time. I will then *add* 1 hour 16 minutes to PDRT for my latitude and the two times in the table for my latitude on October 5. On Oct 7, the my actual local midnight time changes to 1:15am CDT, so I will add 1 hour and 15 minutes to my PDRT and the two times under 7-Oct. So here is an example on how to use the tables for myself on October 5. First I would go to the webpage above and find the table has my latitude down the left side and the date October 5 across the top. In this case it's the 4th table down from the top of the page. I would next find my latitude (I live at +33.06946N) at 33. The next number over is the PDRT or the time at which my PD rises. It's 6:12pm Standard Local Midnight time. It will *always* be 6:12pm *every day of the year*. Then I will apply my offset, describe above, by adding 1hr 16min, to get 7:28pm Central Daylight Time. This is the actual time, on my watch, that PD will rise uneclipsed. The next number over is the PD Dec., so I will always track this declination (-5.39 deg.) all the way from the eastern to the western horizon. The PD will move right down this declination each night for me. Then I track over to the 5-Oct column and the two numbers there, 6:33PM and 5:26AM are the times on Oct 5 that I see my PD first get eclipsed and then later uneclipsed. I convert these two times to my actual time by adding 1hr 16min to each of them to get 7:49pm CDT and 6:42am CDT. At the top of the column by the Oct 5 data is the PDRA which is 0HR 45MIN 28SEC. So what this all means is that on Oct 5, on a star chart I will mark the location (with a purple marker!) RA: 0:45:28 and Dec.: -5.39deg. I will then face my eastern horizon and at 7:28pm CDT this point in the starfield will rise *un-eclipsed* with opportunity as each geosat drifts left in front of that point to have that geosat flare. At 7:49pm CDT, the PD will be eclipsed from the east, so I will no longer see flaring at the PD until 6:42am CDT the next morning when geosats, to my west, begin flaring at the PD. As the days after Oct 5 go by, the 7:49pm time moves toward local midnight and so does the 6:42am time - giving more and more dark night time to see optimally flaring geosats. ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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