Satgen335 Satellite Software Pt2 by GM4IHJ 26 Aug 95 TIME UT. It is important when using satellite software to be clear just exactly what is meant when it lists the satellite Az and El at say 1200 UT Almost all software quotes Universal Time UT, which refers to the Mean time Noon at which the Sun is observed passing due south for an observer located on the Greenwich meridian of 0 degrees Longitude near London England. - It has to be Mean or Average time, because the earths orbit round the Sun is elliptical, so the Sun actual crosses the meridian sometimes a little before Greenwich Mean time noon , or at other times of year, a little later than noon. This locking of the time statement to Greenwich England is fine for Englishmen, but almost every other location in the world uses its own local mean Sun time. Eg Eastern USA uses eastern standard time which is 5 hours behind Greenwich. Central USA is 6 hours behind, Mountain zone time USA is 7 hours behind . Pacific zone time is 8 hours, Alaska is 9 hourc and , Hawaii is 10 hours. So 0000 UT on 31 Jan is , 7pm EST, 6pm CST, 5pm MST, and 4pm PST all on the previous day the 30th Jan. For stations east of the Greenwich meridian of 0 longitude and west of the International date line, local time is correspondingly ahead of Greenwich UT by roughly 1 hour for each 15 degrees the longitude of the location is east of Greenwich. Be warned however that local time zones are not always placed at 15 degree intervals, and also be aware that many countries run some form of daylight saving tide, advancing their clocks one hour in spring and putting them back again in winter, in order to maximise the alignment of the sunlit hours to the working day. SIDEREAL TIME. Having got your station time UT correct, you can go ahead with any terrestrial DX communications, but if you are tracking satellites , they fly in STARSPACE and keep Sidereal time. This differs from the 24 hour UT clock because , in addition to its 365 approx rotation per year on its axis , the earth does one orbit round the Sun. So from the point of view of anything in starspace the earth appears to turn 366 times appron each year. Hence getting ahead of UT by roughly 4 minutes each day (24 hours in a yeer ). If it is up to date your software will calculate sidereal time for you with no action required on your part. But some older software Eg W3IWIs 1982 tracking program and copies thereof,do not calculate the sidereal time variation for you, but require hat a new amended Sidereal time variable is added to the software each year.