Bronx Prep Charter School

Bronx Prep Charter School

Science Dept. – Physics

Lab Experiment

The Celestial Sphere

Objective:

-  To learn to use a star map by identifying the right ascension and declination of a given set of stars.

-  To learn to use a star map by identifying stars at a given right ascension and declination.

Materials:

Star Maps

Background:

We can construct a coordinate system for the sky in order to locate the positions of heavenly objects. What we can do is project certain points of the Earth’s coordinate system onto the sky.

Procedure:

1.  Find the projection on the Earth’s poles on the celestial sphere. We call these the North Celestial Pole and the South Celestial Pole.

2.  Project the Earth’s equator onto the celestial sphere. We call this the Celestial Equator.

3.  Find the path of the Sun on the celestial sphere as we see it from the Earth. We call this path the ecliptic.

4.  Note where the Sun crosses the Celestial equator going North. We call this point the vernal equinox. The point where the Sun crosses going South is called the autumnal equinox .

5.  The points the Sun reaches most north and most south from the Celestial Equator are called the summer solstice and the winter solstice.

6.  Angles above and below the celestial equator are called angles of declination (Dec.). The declination of a star is its angular distance measured in degrees between the Celestial Equator and the star. The object’s declination is positive (+) if it is north of the Celestial Equator and negative (-) if it is south. Figure 1. Points on the Celestial Equator have a declination of 0 degrees. The North Celestial Pole is Dec. = +90 degrees. The South Celestial Pole is Dec. = -90 degrees.

7.  Beginning at the vernal equinox and proceeding easterly, we can measure angles called Right Ascension (R.A.). The right ascension of an object is the angular distance measured along the celestial equator between the vernal equinox and the point on the equator that intersects an hour circle passing through the object. An hour circle is a great circle that passes through both celestial poles as well as through the object itself. The right ascension coordinate is measured in hours, with subdivisions of minutes and seconds of time, instead of 360 angular degrees all around the celestial equator. Right ascension has a total of 24 hours.

Exercise 1:

Name the celestial objects on the data page with the designated locations.

Exercise 2:

Find the R.A. and Dec. of the stars listed in Exercise 2 on the data page.

Questions:

1.  What is the R.A. and Dec. of the Sun on your birthday?

2.  What constellation forms the background for the Sun on this date?

3.  How many degrees of arc are equivalent to one hour of R.A.?

4.  Why are the days March 21 and September 23 given the name equinox?