Celestial Sphere (Hirst)

Christine Hirst, West Ranch High School Astronomy and Earth Science teacher

Principles

• Celestial coordinate systems in the celestial sphere
• Celestial motion
• stellar observing
• Using the celestial sphere as a means of navigation
• right ascension and declination
• latitude and longitude
• seasons

Standards

• Students will know that the sun is a star at the center of our solar system
• Students will know that the sun moves throughout our sky as a result of Earth's movement around it

Procedure

1. The small yellow sun can be used to rotate through the signs of the zodiac
2. Earth can be rotated and positioned in relation to the sun to simulate time of day and year
3. The date and time of day can be set along the vertical axis to illustrate the visible stars and constellations from a given point on Earth at any given point of year

Explanation

Seasons and the changing view of the night sky are observed as a result of the inclination of Earth's axis, our rotation around the sun and our changing perspective of our night sky.  As we revolve around the sun and rotate on our axis it is easy to assume, as the ancients did, that we are the center of it all.   We see the stars appear to move from hour to hour and one night to the next, and "wandering stars" which move independently of background stars, later named planets.  What we are really observing is the sky changing as we rotate on our axis throughout the sky at a speed of 465 meters/second.   Because we are also revolving around the sun, our sky is different at different times of the year, which the ancients used to determine planting seasons.  Celestial motion and distance are also determined by using the baseline of Earth's orbit to measure changes in a stars location.

1.Are the stars all equidistant from Earth, as portrayed in the model?

Answer: although the model portrays this, this is a simplified view of our perspective from Earth.  The difference sizes are a result of apparent magnitude, which is a combination of intrinsic luminosity and distance from the observer.

2. Are the constellations all equal distance from each other, so that the sun and planets move through them at equal times?
Answer: the constellations vary in size, and the sun takes longer to move through some than others.  This is one reason why the current astrological signs are not in conjunction with the sign of year. For example, Cancer, a summer sign, is out near Winter.

3. How can you tell where a star is located in the sky?

Answer:    Coordinate systems similar to latitude and longitude are use to track stars on the celestial sphere.  They are called declination (similar to latitude) and right ascension (similar to longitude. These are projected onto the night sky and used to track the location of a star.  Unlike latitude and longitude, which are fixed locations on Earth, stars appear to move throughout the night, and position changes.

4. Do stars really move, or is it simply a result of our rotation?

Answer: although the stellar or nightly motion we observe is a result of daily rotation, stars do actually move.  Stars have observed radial and transverse velocity, which is towards us and horizontal to us.  The star with the largest rate of proper motion is Barnard's Star, which is a low mass red dwarf star about 6 light years from Earth.

5. Why do the planets appear to move faster than the stars, or faster than other planets?

Answer: the planets are located closer to us, so appear to travel faster across the sky.  Planets with shorter periods of revolution (closer to the sun) appear on the horizon sometimes multiple times a year, such as Venus.  Planets with larger orbits, such as Jupiter, travel slower across the sky, however Saturn appears to trail behind the revolution of Jupiter as it has a larger orbit.  Apparent retrograde motion is observed as a result of the orbit of Earth "overtaking" the orbit of inner planets, namely Mars, as Mercury does not appear high on our horizon due its' short orbital period.

Everyday examples of the principles illustrated

• changing nightly view of celestial objects
• seasons
• changing sun angle and angle of insolation