I am trying to understand the relationship (quantitatively) between JulianDate, UTC, and the epoch of Two Line Elements. It's a deep subject, especially if you include relativity, and a long subject - there is a lot of history. But here what I need is just a bare-bones understanding of how these three things are related quantitatively, on earth, plus/minus a few years from now, to milli-second accuracy.
I roughly understand that UTC increase by 1 second every second, with the usual rules for hours, minutes, days, years, leap years and leap seconds.
I don't understand JulianDate. That links to a long article which states many facts, but after reading, I still don't understand what it is quantitatively. If I line up UTC agains Julian Day are they parallel (except for UTC's leap years/seconds)? Is there a mapping that I can understand or do I have to use a computer program or web site or some other black box to convert between the two?
Here are two examples of situations where I feel lost. I print additional information to make sure I understand some things - the questions are in bold:
Example 1 - time on my computer and in Skyfield:
from skyfield.api import now, JulianDate, load import time def test(): a, b = now(), time.gmtime() # check the times print a.tt_tuple() print time.asctime(b) test() (2016.0, 1.0, 31.0, 11, 45, 28.927114605903625) Sun Jan 31 11:44:20 2016
Both of those are derived from my computer's system time (I think!) which is up-to-date. There is a difference of about 69 seconds. What is that?
Example 2 TLE epoch:
from skyfield.api import now, JulianDate, load import math data = load('de421.bsp') earth = data['earth'] TLE = """ 1 25544U 98067A 16031.25992506 .00006019 00000-0 97324-4 0 9994 2 25544 51.6430 25.8646 0006733 62.5910 66.7566 15.54344726983528 """ ISS = earth.satellite(TLE) print ISS._sgp4_satellite.satnum, " NORAD satellite number of ISS in the TLE" print ISS._sgp4_satellite.epoch, " The epoch of the TLE" frac = 0.25992506 # fraction of day in the epoch of the TLE" print round((((frac*24.-6.)*60.-14.)*60.-17.)*1E+06), " the microseconds of the epoch" print ISS._sgp4_satellite.mo*(180./math.pi), " TLE Mean Anomaly - of ISS *at* epoch?" print JulianDate(utc=(2016, 1, 31, 6, 14, 17.525184)).tt, " What is this number??"
25544 NORAD satellite number of ISS in the TLE 2016-01-31 06:14:17.525184 The epoch of the TLE 525184.0 the microseconds of the epoch 66.7566 TLE Mean Anomaly - of ISS *at* epoch? 2457418.76071 What is this number??
What is the exact meaning of the last number, and how can I convert the value of this epoch to UTC (if I wanted to)?