The processing for JWST's alignment is done on Earth. How long does it take for a signal from Earth to reach the JWST?

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    $\begingroup$ What did your own research leave unclear, please? $\endgroup$ Jul 20, 2022 at 23:13
  • $\begingroup$ @PM2Ring If that's true, why can you not here and now re-state them? $\endgroup$ Jul 28, 2022 at 22:29
  • $\begingroup$ @PM2Ring What do you mean? I saw a link to JWST's alignment, leading nowhere useful. What did I miss? How "How long does it take for a signal from Earth to reach the JWST?" belongs to Astronomy, rather than, EG, aerospace or astronautics is up to you. What difference do you see between "How many light seconds…" and "How far…"? Google tells us it's about 1.5 million km; NASA's and other specialist WWW will give better answers. Isn't that about 5.003 light seconds, ignoring the huge room for error in "5.1 +/- 1" in PM 2Ring's Answer? Is anything there humorous? $\endgroup$ Jul 28, 2022 at 23:14
  • $\begingroup$ @PM2Ring So sorry. I skipped over that as someone echoing my own thoughts; ie, how could it not be the next worst thing to self-evident? $\endgroup$ Jul 29, 2022 at 21:28
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    $\begingroup$ @Robbie Ok. :) I guess we should delete this stuff. $\endgroup$
    – PM 2Ring
    Jul 29, 2022 at 22:35

2 Answers 2


Almost 5.1 seconds, plus or minus around 1 second.

Here's a daily plot (at midnight) of the light travel time from the JWST to the centre of the Earth, courtesy of JPL Horizons, using a script derived from the one in this answer. There are more graphics & scripts related to the JWST here. Times are in TDB.

Light travel time from JWST to Earth, 2022-2024

The light travel time to a location on the Earth's surface has a small extra variation due to the Earth's rotation. And of course, you can't get a direct line of sight signal from the JWST when you're on the wrong side of the Earth. ;)

Here's an hourly plot for today of the distance from the JWST to its control centre, the Space Telescope Science Institute, which is located on the Johns Hopkins University campus. I used longitude -76.622987, latitude 39.332887, altitude 0.073 km as the coordinates.

Light travel time, JWST to StSci, 2022-Jul-19

Here's my plotting script, running on the SageMathCell server.


According to NASA, the JWST is 1.5 million kilometers away, which is about 5 light seconds.

For comparison, the distance to the Moon is about 1.3 light seconds, Mars ranges from about 3 to 22 minutes, and to the Sun is 8 minutes.


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