Thursday word: sidereal

[prettygoodword.livejournal.com]

sidereal (sai-DEER-ee-uhl) - adj., of, relating to, or determined by the stars; (Astron.) (of time) measured by the apparent motion of the fixed stars.


The sidereal day is slightly shorter than the solar ("synodic") day because, as the earth rotates, it also moves around the sun -- so to return to having the sun in the same place, it has to rotate further than to return a fixed star to the same place. Thus the progression of stars through the seasons. Sidereal time is irrelevant to most of us who aren't astronomers, but vitally important to them -- more important, probably, than knowing when the sun goes down and comes back up. The word was first used in this spelling in 1634, alteration of earlier sideral, borrowed in 1594 from French sidereal, from Latin sīdereus, starry, astral, from sīdus, star, constellation, probably from the PIE root *sweid-, to shine.

On the wall was a clock with three dials, showing the local, mean solar, and sidereal times.

---L.

Comments

[trellia-chan.livejournal.com]

When pronounced correctly, that's a pretty word! :-)

[theidolhands.livejournal.com]

Ah, thanks for pointing that out, I did go "side real" in my head. I've also often used the term "side real" without realizing that it had a scientific basis as well.

[prettygoodword.livejournal.com]

I often mispronounce it as si-DER-ee-uhl, if that's any consolation.

[prettygoodword.livejournal.com]

It is!

[sallymn.livejournal.com]

I saw that somewhere the other week and though "lovely word..." and didn't write it down, so thank you!

[prettygoodword.livejournal.com]

De nada!

[ersatz-read.livejournal.com]

I want that clock.

[prettygoodword.livejournal.com]

Me too! I found one that's mean solar, mean sidereal, and right ascension of sun + moon, but that's not as useful -- I mean, you can get local from that, plus knowing relative position of moon is a bonus, but it's not the same.