Many references to an “equinox” occur in metal, undoubtedly referencing the importance of equinoxes to ancient, agrarian-based cultures. A notable reference to the word in metal occurs as a song title in the somewhat infamously twice recorded album Obsessed By Cruelty by Teutonic thrash metal giants Sodom.
My personal preference is for the second recording, done for a proper, domestic German release on Steamhammer. Compare the two recordings of the song for yourself. A glance at the broken English lyrics of the song reveals nothing of note, astronomy related, anyway.
The vernal equinox, also known as the First Point in Aries, is a location on the Celestial Sphere where the Ecliptic crosses the Celestial Equator (the opposite point of crossing is the autumnal equinox). The Celestial Sphere is an imaginary construct that reflects the geometry of the sky as seen from Earth. Our perspective on Earth depicts the entire sky as a sphere of essentially infinite radius, centered on the Earth, with all celestial objects residing on the inside surface of the sphere. Many cultures of the past (and some of the present, unfortunately), notably the Aristotelian Greeks, viewed the Celestial Sphere as a literal picture of a geocentric universe with all celestial motion tracing a perfect circle about the Earth.
The model, though entirely incorrect as a literal interpretation of the structure of the Universe, does a good job of predicting and explaining diurnal motion, lunar phases, and the seasons. Even subtle, complicated motions such as precession were initially explained in terms of a geocentric universe. As I’ve previously described, the geocentric model falls apart completely when trying to explain planetary motion, ultimately leading to the Copernican revolution, referenced quite nicely on Heliocentric and Anthropocentric by The Ocean.
The following graphic (click on it) depicts the basic idea of the Celestial Sphere.
The Celestial Poles are the projections of the Earth’s geographic poles on to the Sphere. As the Earth rotates in a Solar day, we see the daily, or diurnal, motion of objects as a series of concentric circles centered on the celestial poles.
This motion is easily photographed from the Earth’s surface as a star trail photograph, which beautifully depicts this effect.
A Solar day is a rotation of the Earth with respect to the Sun, equal to 24 hours. However, the Earth is orbiting the Sun; therefore, a Sidereal day, a rotation of the Earth 360 degrees with respect to the background stars, is a bit shorter (see graphic below). This rotation requires 23h 56min, four minutes less than a Solar day. The Earth requires another four minutes to rotate such that the Sun is in the same position as seen from the Earth. The effect of this difference is that we see the stars rise and set four minutes earlier every day; therefore, the visible constellations in the night sky slowly change through the year.
The word sidereal (well, the Latin root sideris, anyway) is referenced once in awhile in metal. The Latin origin of the word means “from the stars,” giving rise to all sorts of references to cosmic infinity and so on. The recent album from Italian black metal band Nox Illunis, In Sideris Penumbra, is an example (the words “sidereal” and “penumbra” have nothing to do with each other, by the way, although both are astronomical terms).
The Celestial Equator is the Earth’s geographic equator projected on to the sky; hence, the Celestial Equator is 90 degrees from the Celestial Poles and splits the Celestial Sphere into two hemispheres. The Ecliptic is a projection of the Earth’s orbit around the Sun (click on the following graphic).
As the Earth orbits the Sun, we see the Sun move slightly eastward along the Ecliptic, from one Solar day to the next. The Ecliptic is inclined 23.5 degrees with respect to the Celestial Equator because the Earth’s equatorial plane is inclined by the same amount with respect to the Earth’s orbital plane about the Sun, giving the Earth seasons.
The vernal equinox is sometimes referred to as the First Point in Aries due to its designation as the origin for measuring right ascension, a celestial coordinate analogous to longitude. The following two maps depict the position of the vernal equinox. This is a closeup of the area from the Pocket Sky Atlas.
Of course, you cannot observe the vernal equinox in the night sky on the day of the vernal equinox (March 20-21st), simply because the Sun is at this point on that date! In other words, wait six months, say, September, to see it high in the night sky (boxed area).
The entire Celestial Sphere, however, is precessing slowly due to a subtle gravitational effect exerted upon the Earth by the Sun, causing the Earth to precess like a top with a period of about ~26,000 years. This results in the Ecliptic moving along the Celestial Equator at a rate of about 50 arc seconds per year, also causing the dates that the Sun spends in each constellation of the zodiac (the constellations through which the Ecliptic passes) to subtly change over time. Never mind that astrology was dreamt up 5,000 or so years ago, and that the Earth has precessed by about 1/5th of a period since that time, changing the zodiac “signs” associated with birth dates.
For that matter, because of precession, the vernal equinox is no longer in Aries, but Pisces, instead.
Incidentally, the recent “uproar” by the scientifically illiterate, pseudoscience/ religious/ creationism-obsessed, moronic American public about the “inclusion” of the constellation Ophiuchus in the zodiac is due to the official designation of constellation boundaries set by the International Astronomical Union (IAU) in 1930. The Sun, for example, spends about three weeks within the official boundaries of Ophiuchus, compared to only one week in Scorpius….
3 comments:
Thanks, man - this was a great read, as always.
As someone who works on ancient Greco-Roman astronomy, I was amazed by the recent uproar about the effects of precession on the zodiac. Astronomers have known about precession since Hipparchus (2nd century BC), and his successors distinguished between constellations and signs of the zodiac, with the latter shifting by 50 arc-seconds a year to match the precession. So why all the fuss two milliennia later?
Thanks. The fuss is always the illiterate public. The situation is noticeably getting worse here in the U.S.
Cool, I really enjoyed that one.
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