1. Basic terms and concepts

2. Measuring distances  Astronomical unit (AU) is defined as the distance from the Earth to the Sun.  1 AU is about 150 million kilometers  We measure the distances in our Solar System using the Astronomical Unit  For example: Mercury is 0.3 AU from the Sun Neptune is about 30 AU from the Sun

3. Measuring Distances  Beyond the Solar System the AU is not very useful, use light year.  One Lightyear is the distance that light travels in one Earth year.  1 ly = 300,000 km/s x 365.25 days/yr x 24 hr/day x 60 min/hr x 60 sec/min = 10 13 km We are a few light years from the nearest star

4. Celestial Sphere

5.  The Celestial Sphere is an imaginary dome on which all the stars are projected.  It appears like all the stars are at the same distance from Earth.  On Earth it is like a dome, the ground blocks the lower part of the sphere  From out in space, it would appear like a sphere with the Earth at the center.

6. Celestial Sphere

7.  The Celestial Sphere is a model that helps us to simplify the location and motion of the celestial bodies.  We can put a grid on the Celestial Sphere, much the same as lines on a globe. These can be used to locate stars and constellations

8. Coordinates of Celestial Sphere

9. Declination and Right Ascension  Lines of Declination are the lines that run East West (pink)  Use “dec” to indicate the declination Values are from +90 degrees to -90 degrees  The lines of Right Ascension “RA” are the lines that run from pole to pole Divided into 24 segments (each segment is called an hour) Coordinated given in hours, minutes, seconds.

10. Declination and Right Ascension  The point 0h, 0m, 0s is where the Sun’s path crosses the celestial equator.  The Celestial equator is the 0 degree of declination  The path of the Sun across the CS is known as the Elliptic

11. Elliptic

12.  The belt shaped region surrounding the Elliptic passes through the twelve constellations of the Zodiak  Zodiak means little animals  The position of the sun at any time of the year is related to the constellation of the Zodiak.