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In a graphic describing Cassini's final approach to Saturn NASA lists the timestamps as 4:55 am PDT (ERT). What does this mean? I am assuming that PDT mean pacific time, but I can't find out what ERT means.

Also when talking about time, is it the time when something actually happens, or, is it when the light actually gets to us? enter image description here

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ERT is Earth Received Time. I.e., when we find out about the event.

source

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    $\begingroup$ Ha! Guess that answers both parts of the question well enough! $\endgroup$ – Shane Sep 14 '17 at 21:13
  • $\begingroup$ It's the only time that makes sense, since the distances are large enough that syncing a clock on Cassini to one on the Earth runs into special relativity issues. Saturn is a bit over a light-hour away (it varies, of course, but it's always more than 66 light-minutes according to a quick google search and rudimentary conversions), which means that if an observer who is stationary relative to the sun thinks the clocks are in sync, then an observer moving with the Earth would think they are up to a third of a second off. Juno, at its top speed, would observe them a whole second off. $\endgroup$ – Arthur Sep 15 '17 at 9:07
  • $\begingroup$ @Arthur OTOH, I can see where using local time instead of received time could be easier. It is kind of weird when event B happens 20 min after event A and there is 25 min between events. $\endgroup$ – Shane Sep 15 '17 at 20:54
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As answered before, ERT is when something is retrieved/received at Earth, which may be a while after its occurrence when talking about interplanetary/beyond communication (low Earth orbital data transfers are pretty fast). PDT is Pacific Daylight Time, which is UTC-7. PST is UTC-8, which is used at different times of the year because of daylight savings (despite my opinion of it being redundant for most people in today's world).

For the second question, time is confusing. But for the most part, we use when light (or other energy) is available to our senses, or often more preferably, our data collectors. Not only is it annoying to subtract the small amount of time (relative to Earthly sources) from when light reaches the eyes, but when talking about spacebound objects, such as stars, we don't have any means of knowing what is occurring at the spot, as a star can die and we can still see its light from the past traveling to us, due to our location being many lightyears away.

I hope this helps with your curiosities! (Also correct me if I somehow got something wrong here, which I don't believe I did)

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PDT is Pacific Daylight Time. PST is Pacific Standard Time. A lot of the communication with space probes is done at JPL (Jet Propulsion Labratory), which is in California, so that's probably why that time zone was chosen. Presumably the probe itself doesn't have much use for Daylight Savings Time.

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  • $\begingroup$ @Acccumulation The question has mostly figured out what PDT means and is seeking clarification on what ERT means. Your answer could stand to be improved in this regard. $\endgroup$ – called2voyage Sep 15 '17 at 16:49
  • $\begingroup$ The ERT issue seems to have been answered. The OP seems to have been unaware of the daylight savings/standard distinction being conveyed in the nomenclature, which seems to me to be something worth clarifying. $\endgroup$ – Acccumulation Sep 15 '17 at 18:11
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    $\begingroup$ Comments are for clarifications. Answers should attempt to directly address the central question. $\endgroup$ – called2voyage Sep 15 '17 at 18:13
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    $\begingroup$ The title of the post asks what PDT means, and "I am assuming that PDT mean pacific time" is implicitly a query as to whether that assumption is correct. $\endgroup$ – Acccumulation Sep 15 '17 at 18:18
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    $\begingroup$ No, the title asks what 'PDT (ERT)' means. The OP put quotes around the whole expression to clarify that the full expansion was needed. A few sentences to clarify part of that is not sufficient for an answer. Look, I'm just giving you advice to try to help you to improve. You are free to disregard it at your own risk. $\endgroup$ – called2voyage Sep 15 '17 at 18:20

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