Zodiacal Motion of the Sun

 

  1. From week to week, the Sun and Planets appear to move against the background of fixed stars.
  2. The constellations which contain the Sun at any time during a year are zodiac constellations. For the most part, the Planets also stay within the zodiac constellations as they complete their separate journeys around the sky.
  3. The Sun follows exactly the same path through the zodiac every year, a great circle called the ecliptic. The planets, for the most, do not stray very far on either side of the ecliptic.
  4. How can the Sun's motion be observed?
    On any given day the Sun will rise about four minutes later than a star that rose simultaneously with it, in close proximity, on the day before. (Cf. Heliacal Rising.)
  5. In terms of angular measurement, this four-minute delay every 24 hours works out to the Sun's occupying a new position each morning about a degree farther eastward against the background of fixed star.
    • 1 degree is about twice the angular diameter of the Sun. For reference, it's handy to remember that the Moon and the Sun both appear about half a degree in diameter.
  6. The changing position of the Sun from one day to the next can be estimated by considering that the Sun goes once around the zodiac each year; i.e., it completes a 360 degrees circuit against the background of the fixed stars every year. Therefore, roughly:
    • 360 degrees divided by 365 days = 0.986 degrees change of position per day; or, the Sun moves about one degree per day.
    • Review: The path of its motion is the ecliptic. Any constellation that includes the ecliptic is a zodiac constellation.
  7. Zodiacal motion does not have anything to do with daily (or diurnal) motion. Keep the two types of motion distinct as you think about them. Diurnal motion appears quite differently (always going westward, for instance). Diurnal motion results from the Earth turning once around its axis every day (360 degrees per 24 hours). So for an Earth-bound observer, the Sun or Moon (or any celestial body) will appear to move 1 degree every four minutes: (24 hours per 360 degrees) x (60 minutes per hour) = 4.0 minutes per 1 degree change of position. The delay observed with the Moon and planets is less than that for the Sun.

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