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Seasons and the Zodiac

Zodiac & Ecliptic

The ecliptic is the plane in which the earth orbits the sun. The Zodiac is the twelve constellations that contain the ecliptic. The planets also orbit very close to the ecliptic. Thus the zodiacal constellations also are where one observes the planets in the sky. The twelve constellations of the Zodiac are:

  • Sagittarius
  • Capricornus
  • Aquarius
  • Pisces
  • Aries
  • Taurus
  • Gemini
  • Cancer
  • Leo
  • Virgo
  • Libra
  • Scorpius

The Zodiac was extremely important to historical astronomy. At times many believed that the stars either directly influenced or were portents of one's fate. This view carries over today with Astrology and Horoscopes. This view, of course, carries no credence with modern astronomy. Nonetheless, the historical precedent of the Zodiac as a reference influences how both amateur and professional astronomers describe the location of objects in the sky.

Zodiac Explorer

Sun's Motion Along the Zodiac

Because the earth takes about 4 minutes longer to repoint towards the sun (the solar day is longer than the sidereal day), the location of the sun with respect to the Zodiac changes. The sky rotates from east to west. Because the sun lags behind the stars/Zodiac, the sun lags easterly every day by 4 minutes which is just under 1° per day. In 365 days, the sun will have completed one full circuit around the Zodiac.

The animation below shows the position of the sun with respect to the Zodiac at various times of year. Note the orbital axis is continually pointed in the same direction. Thus how high the sun is in the sky changes with time of year. Where the sun is located with respect to the Zodiac is its seasonal motion.

Stellar Visibility

Stars that are in the sky during the day are obscured from view because the sun is so much brighter. Thus the constellations of the Zodiac and stars with similar right ascension are viewable only during certain times of the year. For example, the constellation Sagitarius is high in the daylight sky late December early January. Thus, it isn't visible during that time of year. Rather, stars in the vicinity of Gemini are the visible stars high in the sky near midnight.