## celestia-developers

 [Celestia-developers] Apollo 11 Orbit From: Clint Weisbrod - 2003-09-28 02:11:43 Attachments: Message as HTML ```Hi All, With 3 kids running around now, it's hard to contribute much to Celestia = but I did notice something I thought I'd investigate. I suspect the Apollo 11 elliptical orbit parameters are significantly = inaccurate. I was following Apollo 11 above the moon and noticed that = the altitude was too high. Also noticed the orbital speed was very slow, = almost imperceptibly so. A single orbit took nearly 5 hours or 0.2 days = as is currently set in apollo.ssc. I visited a few sites, gathered some information and made some = calculations to adjust the orbit parameters in this file. http://www-pao.ksc.nasa.gov/history/apollo/apollo-11/apollo-11.htm = quotes the Apollo 11 lunar orbit with apogee of 186 km and perigee of = 183 km. Using the volumetric mean radius of the moon (1737.1 km) as = quoted from = http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html, one can = compute the eccentricity of the orbit as about 0.0558. The semimajor = axis of the orbit is 1923.1 km. Assuming the orbit to be roughly = circular, the period of the orbit is about 2.099 hours or 0.0875 days. Updating apollo.ssc with these values yielded some unexpected results, = however. I expected the Apollo 11 lunar orbiter to stay between 183 and = 186 km above the moon. Instead, it varied between 293 km and 78 km. = Interestingly, the sums of these two sets of altitudes (183 + 186, 293 + = 78) are roughly equal. This suggests to me that somehow the center of = the orbiters' orbit is not at the center of the moon. If the center of = the moon were located about 107 km away from Apollo 11 orbits' center = and on the semimajor axis, you would get the results these values = produce. Don't know how that's possible, though. I'm going to investigate the code a little bit, but if anyone has any = insight into this problem, please don't hesitate to share it. Best regards, Clint Weisbrod.```