Celestial Observations

The celestial sphere It is difficult for an observer on Earth to explain the motions of the Earth, moon, and celestial objects. All of these objects appear to be located on a surface of a sphere surrounding the earth (the celestial sphere) the line around the edge of the Earth where the celestial sphere meets Earth is the horizon. The sun rises and sets at the horizon.

The Zenith The point on the celestial sphere that is directly above an observer's position on Earth's surface is called the zenith of the observer. An observer at the North Pole has Polaris (the north star) at their zenith

Positions of celestial objects can be described using the altitude-azimuth system a) altitude – distance, in degrees, above the horizon. b) azimuth – distance, in degrees, measured clockwise from due north (0 degrees azimuth)

Do Stars Move? all celestial objects appear to move from east to west across the sky the apparent daily motion is an arc, a portion of a circular path, just like the sun. the rate of apparent motion is 360/24 hrs, or 15 degrees/hr Stars that are between the northern horizon and Polaris never set, and move in counterclockwise circles (circumpolar stars)

Stars that rise in the east and set in the west make circular arcs across the sky, just like the sun. The apparent daily motion of celestial objects is due to the earth's rotation. The earth is rotating under objects. Observing stars from day to day, they shift slightly to the west each day, appearing in their original position one year later.

Revolution is the movement of one celestial object around another. The path along which an object travels during its revolution is its orbit. Less than 3000 stars are actually visible to the naked eye at night. Star patterns have come to be known as constellations.

Common Constellations The Big Dipper Orion

Draco

Cassiopia

A constellation of facts Many constellations get their names from ancient myths, legends. Of the 88 common constellations, 70 are visible in New York state. The circumpolar constellations are ones that are visible every night of the year. During the earth's revolution, the night side faces different regions of space. Each constellation rises 4 minutes earlier than the night before.

Planetary Motions Planets look like stars in the sky, but their motions are different.   planets appear to move from east to west, like stars, on a nightly basis. Over time (multiple nights), the planets seem to change positions, moving eastward relative to the stars behind them.

Models to Explain Celestial Motion Geocentric Model– earth is at the center of the Universe. This was held to be true by the Greeks, and other civilizations that were influenced by the Greeks. The geocentric model was proposed by Ptolemy 1900 years ago.

Geocentric Model Assumptions The earth is stationary – not rotating; all celestial objects move around it at fixed distances. This model explains all daily motions of sun, planets, and stars as they move across the sky. Retrograde (backward) motion of planets were explained as smaller circular motions (epicycles) around the earth. Ptolemaic model was accepted for over 1000 years. Very complicated mathematically

Heliocentric Model Nicolas Copernicus proposed the heliocentric model (sun-centered) just before his death in 1543. Astronomy was a hobby, not his main vocation.

Heliocentric Theory: Main Points earth and planets revolve around the sun retrograde motion of planets is explained by the slower orbital velocity as the distance from the sun increases

Apparent Diameter of Planets Another aspect of observing planets are changes in the apparent diameter of the planets – they seem to get bigger or smaller over time, just like the moon does. When planets are closer to the earth, they naturally appear to be bigger. Features of planets, such as the Great Red Spot on Jupiter, can be observed to change positions over time, indicating that the planets are rotating.

Time and Earth Motions Using star positions on two consecutive nights, the earth makes one complete rotation in 23 hrs 56 min, and 4 s. This is called the sidereal day.   The earth also revolves around the sun. When revolution and rotation. The position of the sun (solar noon) is more conveniently used than star positions. The time between solar noons is used more commonly than the sidereal day, and is slightly longer.