Talk:Sidereal day

I just made the year a sidereal year. That changes the sidereal day slightly, according to the following calculation:

86400000 3652564 3662564 */ . 86164099 ok
86400000 3652422 3662422 */ . 86164090 ok

phma

pronounciation

side-real or sigh-der-ial? 86.128.51.192 20:30, 30 April 2006 (UTC)

I believe it is pronounced as "sigh-dear-ee-uhl"

tropical day and sidereal day

Hello all of you,

The idea of Sidereal day is related to the rotation of the earth with relation to the stars (so when the mean star is in median) 23.934472 Hours, correct? The Solar day is the day related to the mean sun (very close to 24.0000 Hours). Beside this two days another 'Days' exists: a Day of 24 Hours (Hour is defined by using atomic time) and I think this "Day" is the length one refers to when say e.g. that the sun takes 365.24218962 DAYS. Correct?

We also have the pure mean rotational duration of the earth, so the mean time it takes to rotate 360 degrees looking at its own axis. I call this the Tropical day and it is 23.9344697 Hours long. The relation between Sidereal day and Tropical day is the luni-solar precession (25,770 Years) [ precession#Precession_of_the_equinoxes ]

Are the above deduction correct? And one question is related to the mean Sidereal day. It refers to "the hour angle of the vernal equinox" in Explanatory Supplement to the Astronomical Almanac (p. 48). THAT would say that the Sidereal day has nothing to do with the stars... I don't understand this... I am investigating this for my web-site: http://www.iol.ie/~geniet/eng/moonfluct.htm (where all durations are seen as 'mean')

Can someone help me here?

All the best,

Victor

(William M. Connolley 16:50, 2004 Apr 21 (UTC)) Not sure... I think tropical day is probably equinox-to-equinox.

vernal equinox is not a physical object ?

Previously the article said "An apparent sidereal day is the time it takes for the Earth to turn 360 degrees in its rotation; more precisely, is the time it takes the vernal equinox to make two successive upper meridian transits. ... Midnight, in sidereal time, is when the vernal equinox crosses the upper meridian."

That doesn't make any sense to me. The vernal equinox is a point in time (near March 21), not a physical object that can cross the meridian.

I changed it to say

"An apparent sidereal day is the time it takes for the Earth to turn 360 degrees in its rotation; more precisely, is the time it takes a typical star to make two successive upper meridian transits."

Is "a typical star" OK, or is there some subtlety I'm missing ?

-- DavidCary 06:01, 15 May 2004 (UTC)

To answer your question, yes the vernal equinox is a point in time, more specifically the day when the sun crosses the celestial equator going north. That event happens in the same location on the celestial sphere every time it happens. That point is where Right Ascension is by definition 0 hours. Probably because it wasn't given any other name 0h RA is commonly called the vernal equinox. The use of "typical star" is perfectly alright, but the transit of the vernal equinox is when the sidereal day begins.

Starfoxy 14:28, 3 Sep 2004 (UTC)

"There are 366.2422 sidereal days in a tropical year, but 365.2422 solar days"

Am I missing something or is that a typo? I might be missing something but I'd take that to mean they're 1.0000 days apart

Solar day vs sidereal day:

I am still not clear on this: how does one actually measure a sidereal day ? Is it by using a star other than the sun as a reference point ? Is the point of this that there is a discrepancy between the results of that versus using the sun as a reference point ?

Obviously, the solar day will vary between latitudes and times of the year, but if you add up all the solar days in a year, don't they average out to the same as a sidereal year ?

I am trying to grasp this, I really am! Thanks, Omega Man 01:25, 7 December 2006 (UTC)

Just a note - did a small edit since somebody had vandalized the photo description. TR

a sidereal day is simply how long it takes for a planet to spin on it's axis one full rotation. I don't like the fact that they call it an apparent sidereal day. A solar day is how long it takes the sun to reach the same position in the sky. This is variable because not only does it take into account the fact that the earth is rotating, but it's also revolving, therefor the solar day isn't the same ... every day. Imasleepviking ( talk ) 20:11, 6 August 2007 (UTC)