Zodiacal Motion of the Sun
- From week to week, the Sun and Planets appear to move against
the background of fixed stars.
- 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.
- 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
- 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.)
- 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
- 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.
- 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
- Review: The path of its motion is the ecliptic. Any
constellation that includes the ecliptic is a zodiac
- 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.