[00:14.480 --> 00:21.080]  So, what time is it in space? Well, that's a complicated question. For the astronauts
[00:21.080 --> 00:25.600]  on the International Space Station, and for most orbiting satellites, they run on Coordinated
[00:25.600 --> 00:31.980]  Universal Time, or UTC. It's not actually a time zone, but rather a time standard that
[00:31.980 --> 00:37.700]  just so happens to be the same as GMT. It's used to prevent time zone confusion and give
[00:37.940 --> 00:45.020]  a standard time to reference. But, for astronauts especially, it's just a number. On the International
[00:45.020 --> 00:50.180]  Space Station, the sun rises and sets about every 90 minutes, so there isn't any relationship
[00:50.180 --> 00:54.580]  between the sun and what time it is that's meaningful to the astronauts. Satellites are
[00:54.580 --> 00:59.620]  the same way. Time is primarily used to inform them when to execute commands and perform
[00:59.620 --> 01:06.040]  operations. Universal time is just a reference number that's easy to relate back to your
[01:06.040 --> 01:12.240]  local time. But, that's not the only time that's used for space applications. UTC is
[01:12.240 --> 01:17.560]  not an exact accounting of the passage of time, but rather strives to stay aligned with
[01:17.560 --> 01:23.900]  the rotation of the Earth, because it's primarily people on Earth that are using it. Earth's
[01:23.900 --> 01:29.160]  rotation is both irregular in the short term, and slowing down permanently in the long term.
[01:29.160 --> 01:35.140]  Earth days are getting about 1.6 milliseconds longer every single day. This isn't in any
[01:35.140 --> 01:40.600]  way noticeable in daily life, but can be very significant over time. As a result, we
[01:40.600 --> 01:45.580]  use a different time scale when dealing with orbital motion. Orbital motion doesn't care
[01:45.580 --> 01:51.080]  about the Earth's rotation at all, and must be extremely precise because of the massive
[01:51.080 --> 01:57.600]  distances involved. The most common time standards used are Julian date and terrestrial time,
[01:57.600 --> 02:02.840]  which are both entirely independent of Earth's rotation. To give an idea of just how different
[02:02.840 --> 02:10.020]  these time scales can be, over the last century, between 1900 and 2000, universal time was
[02:10.020 --> 02:17.000]  about 64 seconds slower than terrestrial time. It's a tiny difference to us, but when you're
[02:17.000 --> 02:22.400]  dealing with precise predictions over millions of miles, it gets very significant. And that's
[02:22.400 --> 02:28.720]  the key. What time it is in space isn't important so you can look at the clock on the wall and
[02:28.720 --> 02:34.240]  know if you need to go to bed. It's important so you can know where you are, and know where
[02:34.240 --> 02:40.760]  things are. Everything in space is constantly moving, orbiting one object or another. Telling
[02:40.760 --> 02:45.760]  where it's located is intrinsically linked to what time it is. This gets to our next
[02:45.760 --> 02:50.880]  question. How do you tell where something is in space? As I said before, everything
[02:50.880 --> 02:55.940]  in space is constantly moving and orbiting around something else. All this motion is
[02:55.940 --> 03:01.040]  primarily controlled by gravity. So to know where something is right now, we use where
[03:01.040 --> 03:06.560]  it was at some time in the past, and we predict where it is now, based on the laws of gravitational
[03:06.560 --> 03:11.580]  motion. Now you might ask, how do I tell you where it is, or even where it was? On earth
[03:11.580 --> 03:17.460]  I could tell you where to go based on your address, or some landmark, and these are useful
[03:17.460 --> 03:22.860]  frames of reference that we can use to fully communicate where something is. In space,
[03:22.860 --> 03:27.680]  there are an infinite number of reference frames that we can use, but we also have an
[03:27.680 --> 03:32.880]  added complexity. Since, again, everything is moving all the time, to have a frame of
[03:32.880 --> 03:37.500]  reference we need to incorporate time. There are several different ways of describing where
[03:37.500 --> 03:43.260]  things are, but we typically focus on two reference frames. One, what's easiest for
[03:43.260 --> 03:47.040]  us to use. For example, if I want to take a photo of the moon, I would want to know
[03:47.040 --> 03:53.180]  where the moon is in relation to where I'm going to be when I have my camera. And two,
[03:53.180 --> 03:59.100]  what's easiest to calculate. Orbital math is hard, and many times impossible, so picking
[03:59.280 --> 04:05.020]  a reference frame that makes your calculation as easy as possible is often necessary. This
[04:05.020 --> 04:10.000]  reference point is usually whatever has the most significant gravitational effect, and
[04:10.000 --> 04:14.840]  ignores all other motion. For satellites, we use earth. We want to know where they are
[04:14.840 --> 04:18.880]  relative to our location on the earth. This frame of reference is called Earth Centered
[04:18.880 --> 04:25.960]  Earth Fixed, or ECEF, as it is rotating with the earth and is fixed to it. It's the same
[04:25.960 --> 04:31.340]  as GPS coordinates, but it's much, much easier to calculate the position of the satellite
[04:31.340 --> 04:35.500]  when we ignore the rotation of the earth, which is a different frame of reference called
[04:35.500 --> 04:41.300]  Earth Centered Inertial, or ECI. So if I saw where a satellite was in relation to me, in
[04:41.300 --> 04:47.580]  the ECEF frame at time t0, and I need to know where the satellite will be in relation
[04:47.580 --> 04:56.040]  to me in the ECEF frame at time t1, we actually need to calculate three different steps. One,
[04:56.040 --> 05:04.240]  translate the coordinates I observed at t0 in ECEF to ECI at t0. Two, translate the orbital
[05:04.240 --> 05:11.280]  motion of the satellite in ECI from t0 to t1, where our calculations are easier, to
[05:11.300 --> 05:20.080]  t1. And three, translate the coordinates I calculated at t1 back to ECEF from ECI. Steps
[05:20.080 --> 05:26.700]  one and three both require translating between these coordinate frames, ECEF and ECI. So
[05:26.700 --> 05:31.280]  how do we do that translation? Here is where we now introduce the idea of an epoch. An
[05:31.280 --> 05:35.820]  epoch is a point in time that we use as a reference to translate between these types
[05:35.820 --> 05:43.100]  of frames. Specifically, we use the J2000 or Julian Date 2000 epoch to translate between
[05:43.100 --> 05:51.780]  ECEI and ECEF as we set the ECEI reference frame to be the same as the ECEF at that time.
[05:51.780 --> 05:56.400]  That way we can calculate the translation between them by calculating how much the earth
[05:56.400 --> 06:02.700]  has rotated since Julian Date 2000. This is a simplification of the constant translation
[06:02.700 --> 06:08.080]  and calculation done throughout satellites, space technology, and astronomy. But that
[06:08.080 --> 06:13.200]  is why time and space are both extremely important and highly dependent on really what you're
[06:13.200 --> 06:14.480]  looking to do with them.