[image: Parnassus,_Andrea_Appiani_(1811).jpg]

*Apollo and the Muses*
This lesson's attendant mythological figure, Apollo, has the dubious
distinction of being the most complex of all the Olympians.    He has
served as the god of music, poetry, prophecy and even the Sun.    Here we
see him performing on the lyre in the company of the muses, the goddesses
that inspired humans in their artistic and intellectual endeavors.   The
muses were often mistakenly believed to have been Apollo's daughters.
Instead, their father Zeus assigned Apollo the daunting task of instructing
them in art, music, history and, alas, astronomy.
Depending on the poet one consults, Apollo can appear either sympathetic or
malevolent.   Despite his divine beauty and potency, he is perhaps best
known for his ignominious failures to pursue women.     He once famously
chased the reluctant Daphne who preferred to be transformed into a tree
rather than allow Apollo to capture her.    In another tale, Apollo
promised the beautiful woman Cassandra the gift of prophecy in exchange for
carnal favors.  She agreed, but after having received the gift, Cassandra
refused to submit.   Although the enraged Apollo could not withdraw the
gift, he did smite her with a nasty curse.  While she retained her
prophetic powers, Apollo made sure that nobody would believe her
prophecies.
Even today the name "Cassandra" is applied to those whose correct
predictions about the future have gone unheeded.


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THE DAILY ASTRONOMER
Wednesday, April 1, 2020
Remote Planetarium 3:  Gliding Along the Zodiac

Yesterday we imposed our all too human constructs onto the otherwise
unblemished sky.  Our aim is to develop a grid work enabling us to
systematically study the Universe.    That aim is, after all, astronomy's
principal mission.

We devised three main constructs:

*-The Celestial Equator*:  the projection of Earth's equator onto the sky.
  Were it a real line, it would extend between due east and due west and
would be visible from every location on the planet.    For an equatorial
observer it would pass directly overhead.  For an observer at the north
pole or south pole, it would be aligned with the horizon.     For a
northern hemisphere observer, the celestial equator's maximum angle would
be above the due south point and would equal 90 - the observer's latitude.
Portland, Maine's latitude is approximately 43 degrees north and so the
celestial equator passes 90 - 43 = 47 degrees above due south.

-*The Meridian: *  the line connecting due north and due south.  This
meridian is specific to the observer's location.   Your meridian is not the
same as mine unless we were both standing precisely on the same longitude
line.

*-The Ecliptic: *the undulating path the Sun follows throughout the year.
 For us northern hemisphere observers, it attains its maximum altitude of
23.5 degrees north of the celestial equator on the June solstice and a
minimum altitude of 23.5 degrees south of the celestial equator on the
December solstice.
[One would say that the Sun's summer solstice declination is 23.5 degrees
and its winter solstice declination is -23.5 degrees.]

Today we explore the Zodiac, the retinue of thirteen constellation regions
through which the Sun travels each year.    These constellations are
aligned along the ecliptic.     We should mention that some astronomers
become slightly sniffy whenever we employ the term "zodiac," as it smacks
of astrology, the ancient mystical practice of using celestial objects to
predict the future and determine a person's predominant personality
traits.       What can we say in our defense except that,well, every single
symbol astronomers use for the planets and the constellations, themselves,
all derive from astrology.    The sniffy astronomers tend to forget, or at
least refuse to acknowledge, that astronomy arose from astrology much as
chemistry was borne out of alchemy.  Therefore, we'll employ the term
"zodiac" despite the consequent damnation of our immortal souls.

The Zodiac is immensely important to night sky observations because it is
within the band around it that one will find the moon and planets.   An
Earthbound observer will never find Venus next to Orion's knee or the
crescent moon resting in the Big Dipper's bowl.  (Too bad!  That sight
would be fantastic!)

We provide a list of the Zodiac constellations.  Next to each constellation
is the time period in which the Sun appears to be moving through it.

*Pisces the Fish*  [March 12 - April 18]
*Aries the Ram*  [April 19 - May 13]
*Taurus the Bull *[May 14 - June 21]
*Gemini the Twins* [June 22 - July 20]
*Cancer the Crab *[July 21 - August 10]
*Leo the Lion* [August 11 - September 15]
*Virgo the Maiden* [September 16 - October 30]
*Libra the Scales* [October 31 - November 22]
*Scorpius the Scorpion* [November 23 - November 29]
*Ophiuchus the Serpent Charmer *[November 30 - December 17]
*Sagittarius the Archer *[December 18 - January 18]
*Capricornus the Seagoat* [January 19 - February 15]
*Aquarius the Water Bearer* [February 16 - March 11]

[image: file-20200113-103974-7f1x7p.jpg]


[image: map_ophiuchus_sm.jpg]

We see the thirteen constellations pictured above.  Notice that we had to
steal a different image for Ophiuchus as it is the oft-neglected zodiac
constellation, one that astrologers tend not to recognize.

We would like to call your attention to a few matters related to the
zodiac.

First, the night sky is divided into eighty-eight rectilinear regions named
for the each of the eighty eight constellations officially designated by
the International Astronomical Union.    The dates listed above pertain to
the dates when the Sun appears to be within the region defined by the
associated constellation.

Secondly, the astrological zodiac is much neater and more fair to the
twelve constellations contained within it.  The astrological zodiac places
the Sun in each constellation for approximately 30 days.  The astronomical
zodiac is not as proportionate.   For instance, the Sun requires 45 days to
"travel" through Virgo, but only nine days to traverse the Scorpius
region.

Thirdly, the dates we listed might be confusing if you read newspaper
horoscopes.   The astrological dates are different from the astronomical
ones because the astrological zodiac refers to a 2000 year old calendar.
Yes, that does matter because these dates slowly change over time due to
precession, the wobbling of Earth's axis.  This wobbling causes the sun's
position along the zodiac to shift by about one degree every 73 years.  The
Sun used to be in Aries on the vernal equinox.  Now, it is located in
Pisces, but is slowly moving.  In the late 26th century, the Sun's vernal
equinox position will shift into Aquarius.

Finally, the Zodiac constellation club is very exclusive.    It doesn't
permit new members.   Once a zodiac constellation, always a zodiac
constellation.    If a constellation is outside the zodiac, it will never
work its way into it.*

To understand why the Sun appears to "move through" constellations we must
hoist ourselves up into outer space and observe how Earth moves relative to
the Sun and stars.    Earth revolves around the Sun and completes one orbit
every year.   As Earth moves, the Sun appears to move through different
constellations because of Earth's changing position relative to it.  The
graphic below shows the Sun between Earth and the stars comprising Taurus
the Bull.   At this time, the Sun blocks our view of Taurus and therefore
is appearing to move "through it."

[image: download.png]


We next note that Earth is tilted on its axis by 23. 5 degrees from the
vertical.    This orientation explains the ecliptic's undulations.  On the
June solstice, the northern hemisphere is aligned as closely toward the Sun
as possible.  At this time the Sun is 23.5 degrees north of the celestial
equator.  (Yes, that angle is equal to Earth's tilt.)   On the December
solstice, the northern hemisphere is pointed as far away from the Sun as
possible.  At that time, the Sun is 23.5 degrees south of the celestial
equator.


[image: 420px-Axial_tilt_vs_tropical_and_polar_circles.svg.png]



The Zodiac is so important to observational astronomy because the moon and
the planets, save Pluto, appear to travel quite close to it.     They do so
because the solar system is disc-shaped and the planes of the moon's orbit
and that of the other planets are closely aligned with Earth's orbital
plane, which we naturally call the ecliptic.

As we proceed through our study of solar system bodies we will find
ourselves returning quite often to this retinue of thirteen constellations
through which the Sun travels.

Tomorrow, we visit the closest of all celestial bodies, the moon.




*We should mention that constellations do change as a consequence of proper
stellar motions.   Over the courses of hundreds of thousands  of years, the
constellation patterns will dissipate.    However, over the "short term,"
always a funny phrase in astronomy, the zodiac constellations will remain
the same.





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