WEBVTT 00:14.480 --> 00:17.300 So, what time is it in space? 00:17.300 --> 00:20.060 Well, that's a complicated question. 00:20.440 --> 00:28.160 For the astronauts on the International Space Station, and for most orbiting satellites, they run on Coordinated Universal Time, or UTC. 00:28.180 --> 00:34.920 It's not actually a time zone, but rather a time standard that just so happens to be the same as GMT. 00:34.920 --> 00:40.060 It's used to prevent time zone confusion and give a standard time to reference. 00:40.160 --> 00:43.980 But, for astronauts especially, it's just a number. 00:44.300 --> 00:54.040 On the International Space Station, the sun rises and sets about every 90 minutes, so there isn't any relationship between the sun and what time it is that's meaningful to the astronauts. 00:54.040 --> 00:55.620 Satellites are the same way. 00:55.620 --> 01:00.820 Time is primarily used to inform them when to execute commands and perform operations. 01:01.240 --> 01:07.100 Universal time is just a reference number that's easy to relate back to your local time. 01:07.100 --> 01:11.120 But, that's not the only time that's used for space applications. 01:11.640 --> 01:23.320 UTC is not an exact accounting of the passage of time, but rather strives to stay aligned with the rotation of the Earth, because it's primarily people on Earth that are using it. 01:23.640 --> 01:29.160 Earth's rotation is both irregular in the short term, and slowing down permanently in the long term. 01:29.160 --> 01:33.980 Earth days are getting about 1.6 milliseconds longer every single day. 01:33.980 --> 01:39.640 This isn't in any way noticeable in daily life, but can be very significant over time. 01:39.640 --> 01:44.220 As a result, we use a different time scale when dealing with orbital motion. 01:44.380 --> 01:52.700 Orbital motion doesn't care about the Earth's rotation at all, and must be extremely precise because of the massive distances involved. 01:52.700 --> 02:01.280 The most common time standards used are Julian date and terrestrial time, which are both entirely independent of Earth's rotation. 02:01.300 --> 02:14.060 To give an idea of just how different these time scales can be, over the last century, between 1900 and 2000, universal time was about 64 seconds slower than terrestrial time. 02:14.080 --> 02:22.000 It's a tiny difference to us, but when you're dealing with precise predictions over millions of miles, it gets very significant. 02:22.000 --> 02:23.120 And that's the key. 02:23.120 --> 02:30.600 What time it is in space isn't important so you can look at the clock on the wall and know if you need to go to bed. 02:30.600 --> 02:35.600 It's important so you can know where you are, and know where things are. 02:35.660 --> 02:40.380 Everything in space is constantly moving, orbiting one object or another. 02:40.480 --> 02:44.660 Telling where it's located is intrinsically linked to what time it is. 02:44.660 --> 02:46.700 This gets to our next question. 02:46.800 --> 02:49.720 How do you tell where something is in space? 02:49.720 --> 02:54.820 As I said before, everything in space is constantly moving and orbiting around something else. 02:54.820 --> 02:57.840 All this motion is primarily controlled by gravity. 02:57.840 --> 03:07.300 So to know where something is right now, we use where it was at some time in the past, and we predict where it is now, based on the laws of gravitational motion. 03:07.300 --> 03:11.160 Now you might ask, how do I tell you where it is, or even where it was? 03:11.160 --> 03:22.120 On earth I could tell you where to go based on your address, or some landmark, and these are useful frames of reference that we can use to fully communicate where something is. 03:22.120 --> 03:28.720 In space, there are an infinite number of reference frames that we can use, but we also have an added complexity. 03:28.720 --> 03:35.380 Since, again, everything is moving all the time, to have a frame of reference we need to incorporate time. 03:35.620 --> 03:41.360 There are several different ways of describing where things are, but we typically focus on two reference frames. 03:41.620 --> 03:44.360 One, what's easiest for us to use. 03:44.360 --> 03:51.740 For example, if I want to take a photo of the moon, I would want to know where the moon is in relation to where I'm going to be when I have my camera. 03:51.740 --> 03:54.980 And two, what's easiest to calculate. 03:54.980 --> 04:04.840 Orbital math is hard, and many times impossible, so picking a reference frame that makes your calculation as easy as possible is often necessary. 04:04.880 --> 04:11.860 This reference point is usually whatever has the most significant gravitational effect, and ignores all other motion. 04:11.860 --> 04:13.640 For satellites, we use earth. 04:13.640 --> 04:16.780 We want to know where they are relative to our location on the earth. 04:16.780 --> 04:25.200 This frame of reference is called Earth Centered Earth Fixed, or ECEF, as it is rotating with the earth and is fixed to it. 04:25.200 --> 04:38.240 It's the same as GPS coordinates, but it's much, much easier to calculate the position of the satellite when we ignore the rotation of the earth, which is a different frame of reference called Earth Centered Inertial, or ECI. 04:38.240 --> 04:55.040 So if I saw where a satellite was in relation to me, in the ECEF frame at time t0, and I need to know where the satellite will be in relation to me in the ECEF frame at time t1, we actually need to calculate three different steps. 04:55.040 --> 05:02.640 One, translate the coordinates I observed at t0 in ECEF to ECI at t0. 05:02.640 --> 05:11.800 Two, translate the orbital motion of the satellite in ECI from t0 to t1, where our calculations are easier, to t1. 05:11.800 --> 05:19.660 And three, translate the coordinates I calculated at t1 back to ECEF from ECI. 05:19.660 --> 05:26.240 Steps one and three both require translating between these coordinate frames, ECEF and ECI. 05:26.240 --> 05:28.340 So how do we do that translation? 05:28.340 --> 05:31.140 Here is where we now introduce the idea of an epoch. 05:31.140 --> 05:36.620 An epoch is a point in time that we use as a reference to translate between these types of frames. 05:36.620 --> 05:51.780 Specifically, we use the J2000 or Julian Date 2000 epoch to translate between ECEI and ECEF as we set the ECEI reference frame to be the same as the ECEF at that time. 05:51.780 --> 06:00.100 That way we can calculate the translation between them by calculating how much the earth has rotated since Julian Date 2000. 06:00.100 --> 06:07.760 This is a simplification of the constant translation and calculation done throughout satellites, space technology, and astronomy. 06:07.760 --> 06:14.480 But that is why time and space are both extremely important and highly dependent on really what you're looking to do with them.