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So, what time is it in space?

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Well, that's a complicated question.

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For the astronauts on the International Space Station, and for most orbiting satellites, they run on Coordinated Universal Time, or UTC.

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It's not actually a time zone, but rather a time standard that just so happens to be the same as GMT.

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It's used to prevent time zone confusion and give a standard time to reference.

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But, for astronauts especially, it's just a number.

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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.

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Satellites are the same way.

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Time is primarily used to inform them when to execute commands and perform operations.

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Universal time is just a reference number that's easy to relate back to your local time.

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But, that's not the only time that's used for space applications.

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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.

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Earth's rotation is both irregular in the short term, and slowing down permanently in the long term.

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Earth days are getting about 1.6 milliseconds longer every single day.

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This isn't in any way noticeable in daily life, but can be very significant over time.

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As a result, we use a different time scale when dealing with orbital motion.

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Orbital motion doesn't care about the Earth's rotation at all, and must be extremely precise because of the massive distances involved.

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The most common time standards used are Julian date and terrestrial time, which are both entirely independent of Earth's rotation.

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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.

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It's a tiny difference to us, but when you're dealing with precise predictions over millions of miles, it gets very significant.

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And that's the key.

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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.

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It's important so you can know where you are, and know where things are.

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Everything in space is constantly moving, orbiting one object or another.

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Telling where it's located is intrinsically linked to what time it is.

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This gets to our next question.

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How do you tell where something is in space?

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As I said before, everything in space is constantly moving and orbiting around something else.

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All this motion is primarily controlled by gravity.

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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.

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Now you might ask, how do I tell you where it is, or even where it was?

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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.

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In space, there are an infinite number of reference frames that we can use, but we also have an added complexity.

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Since, again, everything is moving all the time, to have a frame of reference we need to incorporate time.

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There are several different ways of describing where things are, but we typically focus on two reference frames.

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One, what's easiest for us to use.

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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.

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And two, what's easiest to calculate.

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Orbital math is hard, and many times impossible, so picking a reference frame that makes your calculation as easy as possible is often necessary.

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This reference point is usually whatever has the most significant gravitational effect, and ignores all other motion.

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For satellites, we use earth.

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We want to know where they are relative to our location on the earth.

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This frame of reference is called Earth Centered Earth Fixed, or ECEF, as it is rotating with the earth and is fixed to it.

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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.

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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.

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One, translate the coordinates I observed at t0 in ECEF to ECI at t0.

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Two, translate the orbital motion of the satellite in ECI from t0 to t1, where our calculations are easier, to t1.

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And three, translate the coordinates I calculated at t1 back to ECEF from ECI.

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Steps one and three both require translating between these coordinate frames, ECEF and ECI.

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So how do we do that translation?

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Here is where we now introduce the idea of an epoch.

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An epoch is a point in time that we use as a reference to translate between these types of frames.

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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.

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That way we can calculate the translation between them by calculating how much the earth has rotated since Julian Date 2000.

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This is a simplification of the constant translation and calculation done throughout satellites, space technology, and astronomy.

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But that is why time and space are both extremely important and highly dependent on really what you're looking to do with them.