Satgen250 Satellites and Calendars Pt1 by GM4IHJ 8th Jan 94 BID of this msg is SGEN250 Please use this BID if you retransmit the msg There may at first reading, seem to be no connection between a satellite in space and, the Calendar we use on the Earth. If however you and your tracking software do not keep Earth ( Greenwich Mean Time UT) and Space Sidereal Time in step, you will soon lose your satellites. So as most tracking software calendar problems occur around 1 January New Year, this may be a good time to talk about a few of the problems. For practical purposes the Earth is said to go around the Sun. It does so in roughly 365.25 days. So to stay in line the Romans of Julius Caesars day added an extra leap year day every 4th year. This sensible Julian Calendar worked reasonably well up to the 15th century. But because Earth's orbital rate is once around the Sun in more exactly 365.2425 days, it began to be obvious that something must be done or Summer would begin to occur in the Northern Hemisphere in December. At this point the Papal authorities in Rome sensibly agreed to add an additional statement to the leap year rule by which a leap year was any year whose number was exactly divisible by 4, unless it was also divisible exactly by 100. The only exception to this being a year which was divisible by 4, 100 and 400 which also became a leap year. So 1900 is a not a leap year but 2000 will be a leap year ( divisible by 400). The change was instituted in Catholic countries by Pope Gregory . Hence this modern calendar is called the Gregorian calendar. Needless to say England and other Protestant countries refused to have anything to do with this Papist nonsense. However even they capitulated a hundred years or so later. So by the end of the 18th century Christian countries were using a calendar which kept winter in its proper place in December. Though please note that Muslims , Chinese and other groups use quite different calendars. Unfortunately the story cannot be decently buried at that point, because people began to take more notice of the stars, which lead to the realisation that the stars in the sky were passing across a fixed meridian Eg Greenwich 4 minutes earlier in time each successive day , so the Sidereal star time was roughly 4/1440 x 365= roughly one day out each year. The reason for this time slip of almost exactly one rotation is really quite simple . Not only does our Earth rotate 365.425 times in a year as it goes round the Sun. Its actual trip around the Sun adds one more rotation as seen from the relatively fixed stars in the sky. So when we talk about things in space, be they planets , stars or satellites , we need to remember the 366.425 revolutions which the Earth makes with respect to the sats and the stars. All this is probably interesting mumbo jumbo to most satellite users. All the average user wants is software that takes care of all this for him. Such software exists although it did not come into amateur radio use until 1983 by which time the Spectrum and Trash80 were replacing the dear old ZX81. So do be careful. I still encounter amateurs who want to know the sidereal time correction for each new year , because their software ( which is mostly copied from W3IWIs original ) does not automatical deal with sidereal time. How is the matter resolved in modern software. Read all about it and more important, how to check it, next week in Part 2. 73 de GM4IHJ @ GB7SAN