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EARLY COSMOLOGY TUTORIAL TOPICS Dr. Brian Monson has created this group of pages as a supplement for the unit on early astronomy that is taught in most college astronomy courses and has graciously granted ScienceMaster permission to republish them. It is by no means a complete discussion of this topic. To explore more work by Dr. Monson please visit his Planetary Conjunctions page.

CONTENTS Motions of the Celestial Bodies Geocentric Models First Iteration Second Iteration Heliocentric Models First Iteration Second Iteration Third Iteration Fourth Iteration

Motions of the Celestial Bodies

To locate objects in the sky, we imagine that the earth is surrounded by a large sphere and that all the celestial objects are attached to the inside surface. We call this the celestial sphere. The motions of these objects are due to a combination of the daily rotation of this sphere about the earth and their individual motions along the sphere's surface. To find objects on the celestial sphere, we place some markings on its surface. The celestial poles are projections of the earth's rotation axis out onto the sphere. Likewise, the celestial equator is just the earth's equator transferred out to the sphere. The horizon is the circle where a plane tangent to your location on the earth intersects the celestial sphere. Objects on the sphere that are above the horizon are visible to you. The zenith is the point on the celestial sphere that is directly overhead at your location.

We can specify the location of an object on the celestial sphere in two different ways. We can draw circles parallel and perpendicular to the celestial equator (called declination and right ascension respectively). The other way would be to use circles parallel and perpendicular to your horizon (called altitude and azimuth respectively). In both cases, the coordinates of the object are the numbers of the circles which pass through the object's location. Coordinates measured relative to the horizon change with time and location on the earth since the celestial sphere rotates around the earth and every location has its own unique horizon.

The sun has two components to its motion. The first is its daily motion. It rises in the east, climbs to its highest point when it's due south and then sets in the west. It takes 24 hours, on average, for a complete cycle. The path the sun follows on any given day is called a diurnal circle.

The sun also has a slow west to east motion compared to the stars. It moves along a track called the ecliptic making a complete circle in about 365.24 days. This means it moves roughly 1° per day. The traditional zodiac constellations lie near the ecliptic and the sun moves through them at a rate of about 1 per month.

The ecliptic is inclined to the celestial equator by about 23°. When the sun is on the part of the ecliptic that lies north of the equator, the sun's diurnal circle will cross the horizon in the northeast and northwest and it will come closer to the zenith. This means that the sun will be above the horizon longer and the sun's rays will strike the ground more directly. Both effects lead to increased temperatures. When the sun is south of the equator the opposite happens.

The stars have the simplest motion of all. They are fixed on the celestial sphere so they rotate around the earth once every 23 hours and 56 minutes. This amount of time is called a sidereal day.

The moon also rises in the east and sets in the west every day like the sun. It also moves around the celestial sphere from west to east in 27.3 days. As it does this the moon goes through a phase cycle. The moon goes from being barely visible to full and back to barely visible in 29.5 days. Ancient astronomers realized that the existence of phases meant that moon shines only by reflected light because if the moon made its own light, it would always be full. The 2 day difference between its orbital period and its phase cycle is due to the eastward motion of the sun along the ecliptic. Since the sun is moving, it takes those extra days for the sun, moon, and earth to come back to the original alignment.

Ancient astronomers only knew about the first 5 planets; the others are too dim to see without a telescope. The planets rise and set daily just like the other objects. They also move from west to east like the sun and moon. This motion takes from 88 days for Mercury up to 30 years for Saturn. All the planets occasionally reverse and for a time they move east to west relative to the stars. This is called retrograde motion. The image to the left shows the position of Mars plotted on successive nights. Note the large loop when Mars undergoes retrograde motion.