Talk:Rotation period

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Rotation period is meaningless without a reference direction. Presumably these are all siderial periods? Gordon Vigurs 10:53, 28 June 2006 (UTC)

There isn't much more to say about rotation periods. Shouldn't this be merged with a general article on planetary motion? Gordon Vigurs 07:52, 11 July 2006 (UTC)

Well, the chart of periods is nice for the various moons and planets. The table is of sidrial periods. But maybe this article could be folded in with Orbital period? Careful, if you change this, though. Look out for the likes of the Lunar month entry, which references this article (rotation period). Pigkeeper 05:32, 10 October 2006 (UTC)

Also, Rotation period is not just for astronomy, but for any body which rotates. Says so here: Pigkeeper 05:44, 10 October 2006 (UTC)

Table precision[edit]

The period of Uranus is 0.718333333 days? I don't believe it. The table needs direct references. Melchoir (talk) 05:22, 25 March 2008 (UTC)

In fact, I happen to have one... Melchoir (talk) 05:24, 25 March 2008 (UTC)

Difference between Rotation Period value, as per referenced Solar System Dynamics (23.93419 hours = 86163.084 seconds) and Sidereal Day (23 h 56 m 4.1 s = 23.93447 hours = 86164.092 seconds) is 1.008 seconds, and NOT 8.4 milliseconds. Is this an errata or am i missing something? —Preceding unsigned comment added by Jamrb (talkcontribs) 22:47, 15 July 2008 (UTC)

Nice catch, Jamrb. It's an error. Your calculations check out - given the information on the page. But it appears that the tabulated figure for earth's rotational period is defective. It was either mis-transcribed or miscalculated - probably the latter, as it differs from the generally published values by just about one second. Until edited in March 2008, the table had a very different figure. I lack access to the Murray & Dermott book and thus am unable to check the cited source. (Were this value correct, it would raise havoc with established parameters, as divergence with the sidereal day would be > 100 times too fast, and in the wrong direction). Per the figures (unfortunately not sourced) in the Earth's rotation article, an 8 millisecond difference is readily apparent. An 8.4 ms difference is referenced here. Pending correction of the presently-sourced value, I will put in a separately-referenced figure. Congratulations - you found a fairly serious error. In terms of the accuracy to which the earth's motions are known, one second per year is far from minor. Hertz1888 (talk) 05:50, 16 July 2008 (UTC)

Tropical year versus sidereal year[edit]

In this article it is stated:

"Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day"

But in the article Year it is said:

The sidereal year is the time taken for the Earth to complete one revolution of its orbit, as measured against a fixed frame of reference (such as the fixed stars, Latin sidera, singular sidus). Its duration in SI days of 86,400 SI seconds each is, on average:
365.256 363 004 days (365 d 6 h 9 min 9.7676 s) (at the epoch J2000.0 = 2000 January 1 12:00:00 TT).
The tropical year is "the period of time for the ecliptic longitude of the Sun to increase by 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons. The mean tropical year is approximated by 365 days, 5 hours, 48 minutes, 45 seconds."

Can it possibly be that what here is called "sidereal day" in reality is called "tropical day" and that "sidereal day" in reality is with respect to the stars of the celestial sphere as the name says, i.e that nothing is "misnamed"

Is there any authoritative text from for example IAU where these terms are defined?

Stamcose (talk) 14:56, 11 June 2011 (UTC)