The planhthV (Planetes)
The stars all cross the sky in absolutely parallel circular paths, exactly the same way, day after day. Because of this, we often refer to them as the fixed stars. But there are a number of celestial objects which, although they rise and set in almost the same way as the stars, gradually change their position relative to neighboring stars. These objects were referred to in Hellenic (ancient Greek) times as the planhthV (planetes), or "wanderers". This is where we get the word, "planets", which we use to describe a large body moving around the Sun. But in ancient times, the term meant any of the seven objects known to move among the stars -- the Sun, the Moon, and the five naked-eye modern-day planets -- Mercury, Venus, Mars, Jupiter and Saturn (of course, the ancient names of these objects were different from these English names). We no longer think of the Sun and Moon as planets, but in terms of their sky motions, they are as much "wanderers" as the other planets.
The Motions of the Sun and Moon
We will cover the motions of the "planetes" in more detail in the next few pages, but for now, let's consider their motions in the simplest of terms.
For the Sun and Moon, the motions are quite simple. Each day they rise in the east, cross the sky, and set in the west, just as the stars do. But in addition, they gradually move to the east, among the stars. For the Moon, the motion is relatively easy to see, because it is possible to see stars when the Moon is up, and its motion is quite fast -- going all the way around the sky in only 27.3 days. For the Sun, the motion is harder to see, because when the Sun is up, the sky is too bright to see any stars, and the motion of the Sun among the stars is much slower, requiring a full year. But even for the Sun, it is possible to tell, by careful measurement of its position, and a comparison of that position with stellar positions, that it is moving.
The paths that the Sun and Moon follow are very nearly the same, being exactly circular paths, which are tilted, on the average, about 23 1/2 degrees relative to the Celestial Equator. As a result, they are sometimes in the northern half of the sky, sometimes in the southern half, and sometimes in-between. Since, as already discussed, for people in the Northern hemisphere, stars which are in the northern half of the sky are up more than half the time, and those in the southern half of the sky are up less than half the time, the two objects are up longer at those times when they are in the north, and up less when they are in the south. In addition, when in the north, they rise north of east, and set north of west, whereas when they are in the south, they rise south of east, and set south of west.
These changes in position were well known even in prehistoric times, and ancient monuments such as Stonehenge are believed to have been, in addition to probable religious purposes, astronomical observatories of a sort, being used to keep track of the north-south motion of the Sun and probably, in some cases, the Moon.
The Motions of the Other "Planets"
For the remaining "wanderers", the motions are far more complicated. On the average, they also follow the tilted circular paths that the Sun and Moon follow, albeit over different periods of time -- about a year for Mercury and Venus, almost two years for Mars, nearly twelve years for Jupiter, and nearly thirty years for Saturn. As a result, the changing positions of Mercury, Venus and Mars are noticeable within just a few nights, whereas weeks or months might be required to see a substantial change in the position of Jupiter and Saturn.
The motion of Mars and Saturn over a three-day period, in June of 2006. Each planet is moving to the east among the stars (in this case, near the Beehive Cluster, or M44), but since Mars takes less than two years to go around the Sun, and Saturn takes nearly thirty years, the motion of Mars is much faster. (June 14 image Tunc Tezel, apod060617; June 17 image Chris Schur, apod060622); animation Courtney Seligman)
In addition, however, these five objects exhibit something which is very complex, and more than a bit strange, in some ways -- retrograde motion.
Retrograde motion means motion which is backwards, relative to the usual, direct, motion. Since the usual motion of the planets (including the Sun and Moon) is a slow eastward motion among the stars, retrograde motion refers to a temporary turning back to the west, either in the form of an "ess" or a "loop", as shown below.
(modified version of NASA SP-4212)
The motion of Mars, during its retrograde motion of 1965. The normal direction of motion is to the east (on the left), but as noted in the diagram, when Mars is near opposition (opposite the Sun in the sky), its motion turns to the west (on the right), becoming retrograde, and performing a retrograde loop or ess.