It's worth remembering that all these things were well understood long ago.
Ptolemy wrote a whole chapter about the changing length of the solar day in the
Almagest nearly 2000 years ago….
On the inequality in the solar days
……it seems appropriate to add a brief discussion of the subject of the
inequality of the solar day. A grasp of this topic is a necessary prerequisite,
since the mean motions which we tabulate for each body are all arranged on the
simple system of equal increments, as if all solar days were of equal length.
However, it can be seen that this is not so. The revolution of the universe
takes place uniformly about the poles of the equator. The more prominent ways
of marking that revolution are by its return to the horizon, or to the
meridian. Thus one revolution of the universe is, clearly, the return of a
given point on the equator from some place on either the horizon or the
meridian to the same place; and a solar day, simply defined, is the return of
the sun from some point either on the horizon or on the meridian to the same
point. On this definition, a mean solar day is the period comprising the
passage of the 360 time-degrees of one revolution of the equator plus
approximately 0;59 time-degrees, which is the amount of the mean motion of the
sun during that period; and an anomalistic solar day is the period comprising
the passage of the 360 time-degrees of one revolution of the equator plus that
stretch of the equator which rises with, or crosses the meridian with, the
anomalistic motion of the sun in that period. This additional stretch of the
equator, beyond the 360 time-degrees, which crosses the horizon or meridian
cannot be a constant, for two reasons: firstly, because of the sun’s apparent
anomaly; and secondly, because equal sections of the ecliptic do not cross
either the horizon or the meridian in equal times. Neither of these effects
causes a perceptible difference between the mean and the anomalistic return for
a single solar day, but the accumulated difference over a number of solar days
is quite noticeable……” Almagest III 9, translated by G.J. Toomer
What is given above, describes - if you read it carefully - the two components
of the equation of time. (0:59 time degree above = 59/60 of a degree)
Kevin
On 14 Sep 2011, at 22:44, Tom Laidlaw wrote:
This article has an extra 71 sec, but it does not say exactly which day. On
the other side it has a deffciency of 75 sec.
Apparent solar time, sometimes called true solar time, is determined by the
daily apparent motion of the observed Sun. It is based on the interval
between two successive returns of the Sun to the local meridian. The length
of a solar day varies throughout the year, and the accumulated effect of
these variations (equation of time) produces seasonal deviations of up to 16
minutes. Why, you may ask: Earth’s orbit is elliptical and the Earth’s axial
tilt. Consequently, apparent solar days are shorter in March (26–27) and
September (12–13) than they are in June (18–19) or December (20–21). In 2010,
the greatest Universal Time(UT1) interval between apparent midnights (at
Greenwich) is 86,471 seconds and the shortest interval is 86,325 seconds.
Ref. http://www.universetoday.com/14700/how-long-is-a-day-on-earth/
Tom
From: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] On
Behalf Of Tom Laidlaw
Sent: Wednesday, September 14, 2011 2:25 PM
To: 'Marcelo'; 'Frank Evans'
Cc: 'Sundial'
Subject: RE: longest day
According to a wi9kipedia article the longest Apparent Solar Day is Dec. 22
at 24 hrs. 29.9 sec.
http://en.wikipedia.org/wiki/Solar_time#Apparent_solar_time
Does it have a name? How about Longest Apparant Solar Day
Tom Laidlaw
From: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] On
Behalf Of Marcelo
Sent: Wednesday, September 14, 2011 12:49 PM
To: Frank Evans
Cc: Sundial
Subject: Re: longest day
I used to think that the longest day was the summer solstice. Methinks that
you refer to some effect from the Equation of Time. And I don't know if there
is a name for it.
2011/9/14 Frank Evans frankev...@zooplankton.co.uk
Greetings, fellow dialists,
I tried to astonish my grandson by telling him that this was the longest day
of the year. (I think this is correct; today is 24 hours and 22 seconds
long.) He replied by asking if there was a name for this day. He said he
couldn't find anything on the interwebs.
I am being outplayed by a grandson! Any help with a name, please?
Frank 55N 1W
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