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Cycles

• The Earth spins on its axis roughly once every 24 hours, in a counterclockwise sense when seen from above the North Pole.
  • A solar day is about 4 minutes longer than a sidereal day, as shown by this animation.
• The Moon orbits the Earth roughly once a month.
• The Earth orbits the Sun roughly once a year.
  • The Earth's orbit about the Sun defines the ecliptic plane, in which all the other planets orbit as well.
  • 12 constellations lie on the ecliptic plane and define the Zodiac.
  • From Earth's perspective, the Sun passes one Zodiac constellation per month and cycles through all 12 once a year.
• The Earth's North-South axis is not fixed in space but rather slowly precesses with a period of 26,000 years.
• During the precession, the Earth's axis traces out an imaginary conical surface in space and a circle on the celestial sphere.
  • The Celestial North Pole or CNP (i.e., the projection of the Earth's axis onto the northern sky) moves about 1° along this circle every 72 years (360x72 = 26,000).
  • Stars that happen to lie at or near this circle would represent the North Star at the time. Polaris is the North Star today.
    • Vega was the North Star 13,000 years (i.e., half a precession cycle) ago and will be again 13,000 years from now.
    • Thuban was the North Star about 5,000 years ago and will be again in about 21,000 years from now.
  • Earth's climate may be affected by this precession, a phenomenon known as Milankovitch cycles.
• Stars follow well defined orbits about the center of the galaxy.
• Our Sun orbits the center of the Milky Way with a period of 230 million years.

Movement in the sky

• To the naked eye, planets look like stars.
• However, they do not twinkle like the stars and they do not keep the same annual rhythms.
• The planets are known as wanderers because they change their positions with respect to the background stars. (The term "planet'' is derived from the Greek word for "wanderer.")
• From Earth, planetary motion appears uneven and somewhat complex.
• Planets generally appear to drift eastward against the background of fixed stars.
• Occasionally, planets appear to stop, reverse direction  (i.e., move westward), and then continue eastward again.
• This temporary backward movement is called "retrograde motion".
• Retrograde motion is an illusion related to to the movement of the Earth-based observer. This applet demonstrates how Mars and the Sun move across the Zodiac and the retrograde motion of Mars. Click here to read more about retrograde motion.
• In the course of a day (due to the rotation of the Earth on its axis):
  • Stars rise in the east and set in the west.
  • Stars follow circular paths centered on the Celestial Poles. Time-lapse photography reveals these beautiful star trails.
  • Stars sufficiently close to the North or South Celestial Pole always appear above the horizon; these are known as circumpolar stars (relative to the location of the observer).
• In the course of a year, stars and constellations appear to drift westward.
• Over a period of a human lifetime, the stars do not noticeably move relative to one another.

Ecliptic plane

• The Earth's orbit about the Sun defines the ecliptic plane.
• All the planets orbit in nearly the same plane--the ecliptic plane.
• The path of the Sun in the sky defines the edge-on view of the same ecliptic plane.
• As seen from the Earth, the other planets follow paths which are similar to that followed by the Sun.
• The 12 constellations which lie on the ecliptic are known as the Zodiac.
• All planets in the sky wander across the Zodiac (and never outside it).

Seasons

• The seasons result primarily from the tilt of the Earth's axis.
  • The Earth's slightly (around 1%) varying distance to the Sun during the course of its orbit is not a major factor. In fact, in the northern hemisphere, the Earth is nearest the Sun (perihelion) in Winter and farthest (aphelion) in Summer.
  • The tilt relative to the Sun determines how direct (i.e., perpendicular) sunlight is. The more direct the sunlight, the more concentrate the light energy, the warmer the surface.
  • When the North Pole is tilted toward the Sun, the northern hemisphere experiences summer and southern hemisphere experiences winter.
  • When the North Pole is tilted away from the Sun, the northern hemisphere experiences winter and the southern hemisphere experiences summer.
  • For comparison, the orbit of Mars is significantly more elliptical than that of Earth. Consequently, Mars's seasons are affected both by its tilt (which is very similar to that of the Earth) as well as by its orbital position. Because the Martian winter in the northern hemisphere occurs at the perihelion, the northern winter is mild and brief, while the southern winter is cold and long.
• With regard to the seasons, there are 4 special points in the Earth's orbit about the Sun.
  • Summer solstice: Around June 21, the sun is highest in the sky and the North Pole experiences 24 hours of sunlight.
  • Autumnal equinox: Around September 21, the day is 12 hours long (and getting shorter) and the night is 12 hours (and getting longer).
  • Winter solstice: Around December 21, the sun is lowest in the sky and the North Pole experiences 24 hours of darkness.
  • Vernal equinox: Around March 21, the day is 12 hours (and getting longer) and the night is 12 hours (and getting shorter).
• The southern hemisphere experiences the opposite seasons.
• The Tropic of Cancer, or Northern tropic, marks the most northerly latitude at which the sun can appear directly overhead at noon at the June solstice. The equivalent latitude in the southern hemisphere is called Tropic of Capricorn, or Southern tropic.
• The Arctic Circle marks the southern extremity of the polar day (24 hour sunlit day, often referred to as the "midnight sun") and polar night (24 hour sunless night). On the Arctic Circle these events occur, in principle, exactly once per year, at the June and December solstices, respectively. The equivalent latitude in the southern hemisphere is called the Antarctic Circle.

Daily path of the Sun

• Because of the Earth's West-to-East rotation, the Sun appears to rise in the East and set in the West.
• Strictly speaking, the Sun rises exactly in the East and sets exactly in the West on only two days of the year--the two equinoxes.
• During summer in the Northern Hemisphere, the Sun rises north of East and sets north of West. In winter, the Sun rises south of East and sets south of West. Only on the Spring and Fall Equinoxes does the Sun rise exactly in the East and set exactly in the West.
• When the Sun rises north of East, it passes higher through our sky on its way to the western horizon. Summer days are long because the Sun must travel through more sky before setting. The farther south the Sun rises, the lower it stays in our sky and the shorter the days.
• On the longest day of the year, the Summer Solstice, the Sun reaches its highest altitude in our sky at midday. At midday the Sun is due South and it crosses the meridian, an imaginary line that divides your sky into eastern and western halves. Each day after the Summer Solstice, the Sun's altitude at midday will be slightly lower until it reaches its lowest midday altitude on the Winter Solstice, the shortest day of the year.

Summer and winter solstices

Summer in the northern hemisphere Winter in the southern hemisphere

Winter in the northern hemisphere Summer in the southern hemisphere

NOTE: This is not to scale; in reality, the sun is a lot bigger to scale, and Earth is a lot further away.

Astrological ages

• In the same manner in which individuals born at different times of the year are thought to be dominated by different astrological signs, astrologers also tend to view different historical periods as being dominated by the influence of particular signs.
• Because of the precession of the Earth's axis, the vernal equinox moves through all the constellations of the Zodiac over the 26,000 year precession period. This phenomenon is known as the precession of equinoxes. The period during which the vernal equinox remains in a particular Zodiac sign is known as an astrological age and each age lasts about 2150 years (one-twelfth of 26,000 years).
• Presently, the vernal equinox is in the constellation Pisces.
  • This age is commonly described as the age of faith and religion.
  • It began roughly with the birth of Christ, about 2000 years ago.
  • The New Testament is replete with fish symbolism.
  • It should not be too surprising, therefore, that the symbol of Christianity has been fish.
• The preceding age was the Age of Aries, often described by astrologers as an age of aggression and enterprise.
  • The Arian Age ushered in efforts to replace polytheism with monotheism.
  • This is the age of Moses and the Old Testament.
  • The symbol of Aries can be seen as representing the power of multiple gods streaming down into a single god-head.
  • The Jews still blow the shofar (ram’s horn) in commemoration.
• The age of Taurus (the bull) preceded the Age of Aries.
  • The 'golden calf' represents represents idol worshiping in general and a bull deity in particular, prominent in this age.
  • Moses condemning his own people upon finding them worshiping a 'golden calf' after coming down from Mount Sinai. This condemnation ushers in the next age, the age of the Old Testament, the Age of Aries.
• The vernal equinox is approaching Aquarius.
  • According to astrological mysticism, the age of Aquarius is characterized by spiritual enlightenment and unusual harmony and understanding in the world. Unfortunately, there is no evidence that the position of the vernal equinox with respect to the constellations of the Zodiac will bring such harmony.
  • "The Dawning of the Age of Aquarius" was a song in the musical Hair, which celebrated the Age of Aquarius.
• This interesting video ("Zeitgeist") on the topic of astrotheology discusses how the various religions and mythologies were born from observations of cycles in the sky.

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