Question about STS-116 launch times

From: Ralph McConahy (rwmcconahy@earthlink.net)
Date: Fri Dec 08 2006 - 12:43:12 EST

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    A question for the orbital mechanics gurus:
    
    I plugged the STS-116 nominal (ISS In Plane) launch times from Spaceflightnow.com into a spreadsheet, then created a column that displayed how much time earlier each day the launch will occur compared to the previous day (see below). Comparing the day by day "Earlier by" times, it is interesting how it is equal every other day, with a +/- ~3m10s alternating from one day to the next. Could someone explain why the delta-time jumps like this instead of a constant change from day to day?
    
      Ralph McConahy
      38.3306N,  75.6970W, -25m (WGS84)
     
     
    STS-116 Nominal Launch Times	
    			
     	 PM (EST)	 Earlier	
     9-Dec	 8:47:34	  by: 	
    10-Dec	 8:25:02	 22:32	
    11-Dec	 7:59:20	 25:42	
    12-Dec	 7:36:48	 22:32	
    13-Dec	 7:11:06	 25:42	
    14-Dec	 6:48:34	 22:32	
    15-Dec	 6:22:52	 25:42	
    16-Dec	 6:00:20	 22:32	
    17-Dec	 5:34:37	 25:43	
    18-Dec	 5:12:06	 22:31	
    19-Dec	 4:46:23	 25:43	
    20-Dec	 4:23:52	 22:31	
    21-Dec	 3:58:09	 25:43	
    22-Dec	 3:35:38	 22:31	
    23-Dec	 3:09:55	 25:43	
    24-Dec	 2:47:24	 22:31	
    25-Dec	 2:21:41	 25:43	
    26-Dec	 1:59:10	 22:31	
    
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