In order to write and run the script in this answer I typed "Greenwich sidereal time right now?" into google (or at least tried to) and got two numbers.

Greenwich sidereal time was 09:51:07 but my local sidereal time was ten minutes earlier at 09:40:50.

I'm in UTC +8 so the ten minute difference is not because I'm ~8 hours away.

What causes this small difference in sidereal times?

mi tiempo sideral no es tu tiempo sideral

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    $\begingroup$ I repeated your search and got the exact same numbers displayed in Google, so the numbers in that screenshot are clearly not accurate - and clicking through that link gives me a different pair of numbers. (I am very much not at the same longitude as you, by the way.) $\endgroup$
    – HDE 226868
    Commented May 16, 2021 at 0:48
  • $\begingroup$ @HDE226868 ya I figured that out too, and had to redo my linked answer :-) That must be a caching thing. Still there's the offset when I go to Heavens Above live: i.sstatic.net/6SKKA.png via heavens-above.com/… $\endgroup$
    – uhoh
    Commented May 16, 2021 at 0:53
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    $\begingroup$ That sounds like a bug. Your local sidereal time is 0h0m0s when the First Point of Aries (i.e., RA 0h0m0s) crosses your meridian. So there's no way that there should only be a 10 minute difference if you're 8 hours east of Greenwich. This site looks ok: localsiderealtime.com/sidereal.html $\endgroup$
    – PM 2Ring
    Commented May 16, 2021 at 2:56

1 Answer 1


I don't think the Google Search is passing any coordinates to the Heavens Above website to get the results the Search is displaying.

If you click through on the provided link in the search, Heavens Above gives the following default location:

Default Heavens Above Location Screenshot.

Location: Bristol (51.4770°N, 2.5690°W)

Since Bristol is 2.57° West of the Greenwich Meridian, its Local Sidereal Time is about 10 minutes earlier than Greenwich.

  • $\begingroup$ I was having strange results with heavensabove using https://www.heavens-above.com/whattime.aspx?lat=25&lng=121.5&alt=68&tz=GMT&cul=en before (see comment under question), but now it is giving reasonable results i.sstatic.net/rqEGh.png I'm in UTC +8 and it's 8 hours and 6 minutes later here than there. 121.5 East is should be 121.5/15 = 8.1 hours ahead and that's exactly what it's saying. Okay I'm feeling confident that I understand sidereal time again, thanks! $\endgroup$
    – uhoh
    Commented May 16, 2021 at 10:43
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    $\begingroup$ @uhoh Interesting. I'm still getting the Bristol location from the direct link that spawns when I google "Greenwich sidereal time right now." heavens-above.com/… Heavens-Above GmbH seems to be located in Munich, so maybe they've got servers in Bristol? $\endgroup$
    – notovny
    Commented May 16, 2021 at 21:27
  • $\begingroup$ Yes, that does indeed sound plausible. With google getting so good it's important to remember that it's still just a search algorithm and the results are "suggestions" rather than actual answers. We should google things AYOR (at your own risk) :-) i.sstatic.net/0bWfh.png $\endgroup$
    – uhoh
    Commented May 16, 2021 at 22:50

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