start	end	text
14480	21080	So, what time is it in space? Well, that's a complicated question. For the astronauts
21080	25600	on the International Space Station, and for most orbiting satellites, they run on Coordinated
25600	31980	Universal Time, or UTC. It's not actually a time zone, but rather a time standard that
31980	37700	just so happens to be the same as GMT. It's used to prevent time zone confusion and give
37940	45020	a standard time to reference. But, for astronauts especially, it's just a number. On the International
45020	50180	Space Station, the sun rises and sets about every 90 minutes, so there isn't any relationship
50180	54580	between the sun and what time it is that's meaningful to the astronauts. Satellites are
54580	59620	the same way. Time is primarily used to inform them when to execute commands and perform
59620	66040	operations. Universal time is just a reference number that's easy to relate back to your
66040	72240	local time. But, that's not the only time that's used for space applications. UTC is
72240	77560	not an exact accounting of the passage of time, but rather strives to stay aligned with
77560	83900	the rotation of the Earth, because it's primarily people on Earth that are using it. Earth's
83900	89160	rotation is both irregular in the short term, and slowing down permanently in the long term.
89160	95140	Earth days are getting about 1.6 milliseconds longer every single day. This isn't in any
95140	100600	way noticeable in daily life, but can be very significant over time. As a result, we
100600	105580	use a different time scale when dealing with orbital motion. Orbital motion doesn't care
105580	111080	about the Earth's rotation at all, and must be extremely precise because of the massive
111080	117600	distances involved. The most common time standards used are Julian date and terrestrial time,
117600	122840	which are both entirely independent of Earth's rotation. To give an idea of just how different
122840	130020	these time scales can be, over the last century, between 1900 and 2000, universal time was
130020	137000	about 64 seconds slower than terrestrial time. It's a tiny difference to us, but when you're
137000	142400	dealing with precise predictions over millions of miles, it gets very significant. And that's
142400	148720	the key. What time it is in space isn't important so you can look at the clock on the wall and
148720	154240	know if you need to go to bed. It's important so you can know where you are, and know where
154240	160760	things are. Everything in space is constantly moving, orbiting one object or another. Telling
160760	165760	where it's located is intrinsically linked to what time it is. This gets to our next
165760	170880	question. How do you tell where something is in space? As I said before, everything
170880	175940	in space is constantly moving and orbiting around something else. All this motion is
175940	181040	primarily controlled by gravity. So to know where something is right now, we use where
181040	186560	it was at some time in the past, and we predict where it is now, based on the laws of gravitational
186560	191580	motion. Now you might ask, how do I tell you where it is, or even where it was? On earth
191580	197460	I could tell you where to go based on your address, or some landmark, and these are useful
197460	202860	frames of reference that we can use to fully communicate where something is. In space,
202860	207680	there are an infinite number of reference frames that we can use, but we also have an
207680	212880	added complexity. Since, again, everything is moving all the time, to have a frame of
212880	217500	reference we need to incorporate time. There are several different ways of describing where
217500	223260	things are, but we typically focus on two reference frames. One, what's easiest for
223260	227040	us to use. For example, if I want to take a photo of the moon, I would want to know
227040	233180	where the moon is in relation to where I'm going to be when I have my camera. And two,
233180	239100	what's easiest to calculate. Orbital math is hard, and many times impossible, so picking
239280	245020	a reference frame that makes your calculation as easy as possible is often necessary. This
245020	250000	reference point is usually whatever has the most significant gravitational effect, and
250000	254840	ignores all other motion. For satellites, we use earth. We want to know where they are
254840	258880	relative to our location on the earth. This frame of reference is called Earth Centered
258880	265960	Earth Fixed, or ECEF, as it is rotating with the earth and is fixed to it. It's the same
265960	271340	as GPS coordinates, but it's much, much easier to calculate the position of the satellite
271340	275500	when we ignore the rotation of the earth, which is a different frame of reference called
275500	281300	Earth Centered Inertial, or ECI. So if I saw where a satellite was in relation to me, in
281300	287580	the ECEF frame at time t0, and I need to know where the satellite will be in relation
287580	296040	to me in the ECEF frame at time t1, we actually need to calculate three different steps. One,
296040	304240	translate the coordinates I observed at t0 in ECEF to ECI at t0. Two, translate the orbital
304240	311280	motion of the satellite in ECI from t0 to t1, where our calculations are easier, to
311300	320080	t1. And three, translate the coordinates I calculated at t1 back to ECEF from ECI. Steps
320080	326700	one and three both require translating between these coordinate frames, ECEF and ECI. So
326700	331280	how do we do that translation? Here is where we now introduce the idea of an epoch. An
331280	335820	epoch is a point in time that we use as a reference to translate between these types
335820	343100	of frames. Specifically, we use the J2000 or Julian Date 2000 epoch to translate between
343100	351780	ECEI and ECEF as we set the ECEI reference frame to be the same as the ECEF at that time.
351780	356400	That way we can calculate the translation between them by calculating how much the earth
356400	362700	has rotated since Julian Date 2000. This is a simplification of the constant translation
362700	368080	and calculation done throughout satellites, space technology, and astronomy. But that
368080	373200	is why time and space are both extremely important and highly dependent on really what you're
373200	374480	looking to do with them.