inclined neither away from nor towards the Sun, the centre of the Sun being
in the same plane as the Earth's equator. The term equinox can also be used
in a broader sense, meaning the date when such a passage happens. The name
"equinox" is derived from the Latin equinox (equal) and nox (night), because
around the equinox, the night and day are approximately equally long. It may
be better understood to mean that latitudes +L and -L north and south of the
Equator experience nights of equal length.
At an equinox, the Sun is at one of two opposite points on the celestial
sphere where the celestial equator (i.e. declination 0) and ecliptic
intersect. These points of intersection are called equinoctial points:
classically, the vernal point and the autumnal point. By extension, the term
equinox may denote an equinoctial point.
An equinox happens each year at two specific moments in time (rather than
two whole days), when there is a location (the subsolar point) on the
Earth's equator, where the centre of the Sun can be observed to be
vertically overhead, occurring around March 20/21 and September 22/23 each
year.
Although the word equinox is often understood to mean "equal [day and]
night," this is not strictly true. For most locations on earth, there are
two distinct identifiable days per year when the length of day and night are
closest to being equal; those days are referred to as the "equiluxes" to
distinguish them from the equinoxes. Equinoxes are points in time, but
equiluxes are days. By convention, equiluxes are the days where sunrise and
sunset are closest to being exactly 12 hours apart.
Equinoctial Propagation
-----------------------
Here in Melbourne, daytime winter propagation conditions have ended, with
the emergence of the summer daytime propagation pattern beginning.
The continuing very low level of solar activity has meant that long-distance
propagation on frequencies above about 12 MHz over darkness, or
semi-darkness paths, is a non-event, but distance propagation above 12 MHz
on sunlit paths is excellent, and will continue to be so.
I look forward to researching the traditional summer propagation events,
represented by long-distance noon reception from Asia and Africa over
mostly daylight transmission paths, via Specialized Modes. This is
dominated by propagation via reflection off the top of Sporadic-E layers,
with no intermediate ground reflection.
Watch out for this, if you are in South Eastern Australia!
New Transmission Season
-----------------------
The B10 season kicks off on October 31, 2010, and there will be many
surprises in frequency usage and spectrum occupancy, as broadcasters move
many services down to the lower frequencies, and make greater use of relay
facilities.
Strangely, there are clusters of diehards out there who still wish to
be entertained or informed by listening to SW radio, even though the
technology is now obsolescent, with all sorts of exciting alternative
platforms now available.
Spectrum Occupancy on 13MHz
---------------------------
The 13 MHz band continues to be source of technical inspiration and
wonderment, with longpath mega-signals from Europe, Africa and the Middle
East, and shortpath signals from Asia dominating this spectrum in the
Melbourne afternoons.
Here is a summary of principal occupancies on Monday September 27, 2010,
between 0530 and 0600, from my favourite field monitoring site in the Olinda
Forest, 35 km each of central Melbourne, using the Eton E5 and a 3 m
antenna.
13610 CHINA CNR1
13630 AUSTRALIA RA-Shepparton English
13650 IRAN VOIRI Spanish
13660 CYPRUS BBC Arabic
13670 CHINA PBS-Xinjiang Uyghur
13690 AUSTRALIA RA-Shepparton English
13700 CHINA CNR8 Kazakh
13710 MADAGASCAR VOA-Talata French
13720 ABU DHABI Sudan Broadcasting Service, Dhabbaya, English
13740 IRAN VOIRI Dari
13750 IRAN VOIRI Bosnian
13760 MARIANAS RFA-Tinian Mandarin
13765 VATICAN VR SMG French
13775 RUSSIA VOR-Vladivostok English
13790 IRAN VOIRI Arabic
13800 IRAN VOIRI Arabic
13810 GERMANY R. Farda, Lampertheim, Farsi
13820 CYPRUS BBC Russian
13840 MADAGASCAR NHK-Talata French
13860 SRI LANKA R. Farda, Iranawela, Farsi
Regards from Melbourne!
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