Pardon any slips in terms or misspellings, I am curious as to what defines
the exact time of an equinox ( transition from Summer to Fall or Winter to
Spring). I understand the Solstice is when the earth's tilt has reached
its max in one direction. So is the equinox when the tilt is halfway or
what exactly determines the exact time?
Another question, anyone have the time of the change for this year Summer
to Fall equinox?
-o- "A hallucination is only as real as the eyes that see it."
> Howdy star gazers,
> Pardon any slips in terms or misspellings, I am curious as to what defines
> the exact time of an equinox ( transition from Summer to Fall or Winter to
> Spring). I understand the Solstice is when the earth's tilt has reached
> its max in one direction. So is the equinox when the tilt is halfway or
> what exactly determines the exact time?
> Another question, anyone have the time of the change for this year Summer
> to Fall equinox?
The moment of the equinox is the moment the sun crosses the celestial
equator. Vernal is the crossing from south to north, Autumnal is from
north to south. The latter will occur at 00:22 UT of 23-Sep-93.
| (V) | "Tiger gotta hunt. Bird gotta fly.
| (^ (`> | Man gotta sit and wonder why, why, why.
| ((\\__/ ) | Tiger gotta sleep. Bird gotta land.
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| \< ) |
| ( / | Kurt Vonnegut Jr.
| | |
| ^ |
The Earth's tilt does not change, The poles always point in the same
direction; but since they are not perpendicular to the plane of Earth's
orbit, that direction appears to move closer to and farther from the Sun
as the Earth goes around the Sun. (Experimenting with a globe and a lamp
> may make this clearer.)
Actually, the earth's tilt does change, but very slowly. The axis of
the earth precesses with a period of about 25800 years and its obliquity
decreased in the past year by about 4 arc seconds.
I know this isn't the point, but there is a misconception among some
that certain things about solar system are fixed, when in fact, everything
is always in flux.
The equinox occurs when the Earth's motion about the Sun causes
the Sun to appear on the celestial equator as seen from the Earth.
When the Sun appears on the equator with apparent motion from south
to north it is the vernal equinox. The complimentary equinox six
months later, with the Sun's apparent motion from north to south, is
the autumnal equinox.
See the Astronomical Almanac for more detail.
>Without consulting any references, my recollection is that the equinox is
>defined as the point in time when the sun intersects the celestial equator.
>Speaking of misconceptions, the popular description of the equinox is that
>it is the time of year when day and night are equal. This is not true.
>Day and night are not equal on the equinox. The day when day=night is
>true is the equilux. It is always offset from the equinox by a few days.
True enough, but only because of mutation of definitions. Equinox comes
from the Latin "equal nights". In an effort to predict the vernal
equinox, which marked the beginning of many cultures' years, various
priest-astronomers associated the day of equal day and night with a
specific location of the Sun against the background stars. This is
the sort of thing that made the association of the vernal equinox with
the first point of Aries, although it now lies about midway through
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The source of confusion is not a mutation of definitions, but due to leaving
out some significant additional effects and because of the relative inaccuracy
of timing equipment available during the period when the term "equinox" came
into being. Even today, many astronomers will refer to the equinox as the
date when day=night. If you point out the shift from the equinox of the
equilux (literally, Latin for "equal light"), most astronomers will say that
day "approximately" equals night on the equinox. Of course, day "approximately"
equals night every day of the year, depending on your interpretation of
"approximately". The date when day comes closest to being equal to night
is something different.
Ok, here's another little bit of reasoning why the equilux and equinox
might not fall on the same day, even with no atmosphere and a point sun.
During the March equinox, the equation of time is changing by about 17
seconds per day. This means that noon to noon takes that much
less than 24 hours. So each daytime is about 11h59m52s long, and
each nighttime is likewise.
At the September equinox, the equation of time is changing by about 21
seconds per day. So each daytime is about 11h59m50s long, and nighttime
Now the second derivative of the equation of time is not zero, so
there will be a tiny (about 0.1 second) difference in the lengths
of day and night even on the equinox. So in general, the locus
of points on the earth's surface which actually experience a true equilux
is a very small set (left to the reader to calculate).
Of course, at this level, for the earth, the whole discussion has become
pointless, because nobody's life will be affected. Not even your local
weatherman's sunrise and sunset times will be able to show this, and it
will not determine whether or not you plant/harvest your crops today or
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What defines the equinox(es) ?
Without consulting any references, my recollection is that the equinox is
defined as the point in time when the sun intersects the celestial equator.
Speaking of misconceptions, the popular description of the equinox is that
it is the time of year when day and night are equal. This is not true.
Day and night are not equal on the equinox. The day when day=night is
true is the equilux. It is always offset from the equinox by a few days.
In addition, the day of equilux occurs at somewhat different dates on
different places on Earth. Defining equinox = equilux would mean we had
to abandon one universal moment for the equinox, equally valid all over
By the way, when does equilux occur at the North and South poles? :-)
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Nybrogatan 75 A, 6 tr, S-114 40 Stockholm, Sweden
BL->Could someone perhaps elucidate the reason for the time difference between
BL->equinox and equilux? Simple geometry would make them the same.
The "simple geometry" does not account for refraction, nor for the
non-zero angular size of the Sun. So the official (Astronomical Almanac)
definition of Sun rise and set are the instants when the refracted upper
limb of the Sun is tangent with a flat (i.e., sea level) horizon. This is
roughly 2 or 3 minutes from the simple geometry case.
X SLMR 2.1a X Oh, Oh, where's my rain filter?
--- Maximus/2 2.01wb
At low latitudes, that is. At high latitudes the difference may be
considerably greater. Here at 60 deg north, the difference is at least
4 minutes (at the equinoxes), growing to 6-7 minutes at the solstices.
Further north the difference is even greater.
Could someone perhaps elucidate the reason for the time difference between
equinox and equilux? Simple geometry would make them the same.
-- Bill Lawson
According to my software, they would fall on the same day were the sun a
point source and the earth had no atmosphere. These geometric sunrises
and sunsets are for lat 27 20N, lon 82 32W. Geometric sunrise or sunset is
the time the sun would have been split in half by an ideal horizon if there
were no refractive effects of the atmosphere.
Vernal Equinox 1993 Autumnal Equinox 1993
20-Mar-93 9:41 local time 22-Sep-93 20:12 local time
19-Mar rise 6:38:50 set 18:37:16 21-Sep rise 7:21:59 set 19:23:53
20-Mar rise 6:37:44 set 18:37:47 22-Sep rise 7:22:26 set 19:22:43
21-Mar rise 6:36:37 set 18:38:18 23-Sep rise 7:22:53 set 19:21:34
Two factors extend the actual photoperiod to greater than 12 hours on
the day of the equinox. One is that when the sun is on the horizon you
can still see half of it. The other is that the atmosphere refracts the
sun's rays and allows you to see it still, even after it is geometrically
below the horizon. Hence, equilux is after the autumnal equinox and before
the vernal equinox. How many days before or after depends upon your latitude.
: At low latitudes, that is. At high latitudes the difference may be
: considerably greater. Here at 60 deg north, the difference is at least
: 4 minutes (at the equinoxes), growing to 6-7 minutes at the solstices.
: Further north the difference is even greater.
In fact, neither the poles nor the equator do ever experience equilux.
Horizontal refraction is 35' =(approx.) 0.6 degrees,
i.e. when the Sun seems to cross the horizon it is really 0.6 degrees
Due to the ellipticity of the orbit of the earth, the Sun has a
northen declinations for 186 days per year, and a southern declinations
for 179 days. In the vicinity of the equinoxes, the declination of the
Sun changes about 0.4 degrees per day. Thus the Sun's crossing of the
horizon will be 1.5 days early in spring and 1.5 days late in the
autumn at both poles. So if you define the Sun as being above the horizon
when its midpoint is above it you get:
North pole, Sun above horizon 189 days, below 176 days
South pole, - " - 182 days, below 183 days
At the equator, where the Sun theoretically would be above the horizon
for 12 hours per day, the sunlight time is prolonged by 2.4 minutes
int the morning and evening, ( 1 degree = 4 minutes of time ).
Thus the Sun will be above the horizon for 12h 5m and below for 11h 55m.
The change of the equation of time is much smaller than 5m per day.
Since all these times vary continuously with the latitude, there is
an area around the North pole, a smaller one around the South pole
and one around the equator where equilux never occurs.
: Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
: Nybrogatan 75 A, 6 tr, S-114 40 Stockholm, Sweden
: InterNet: pau...@saaf.se
Dept. of Scientific Computing
Uppsala University, SWEDEN