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Right Ascension & Declination

Compare the positions of stars on an image to their sky coordinates.

Right Ascension (RA) is comparable to longitude on Earth.  It is measured in units of time:  hours, minutes, and seconds. Each hour compares to 15 degrees.  Right Ascension increases towards the East with 0.0 hours of RA being at the place in the sky where the Sun crosses the celestial equator on the first day of spring (Vernal Equinox).

Declination (Dec) is comparable to latitude on Earth.  The Celestial Equator is a projection of the Earth's Equator onto the sky.  North of the Celestial Equator is considered + Dec.   South of the Celestial Equator is considered - Dec.  Declination is measured in degrees, arcminutes, and arcseconds. There are 60 arcminutes in a degree and 60 arcseconds in an arcminute.

Castor is a bright star in Gemini.  It is a multiple star system.
If you compare the RA and Dec for the two bright stars of the system, you will notice that the A component is just East and South of the B component.

Compare the RA and Dec positions for Castor A and Castor B.

What is the difference in RA?  ______________________
h = hours   m = minutes   s = seconds

What is the difference in Dec?  ______________________
d = degrees   ' = minutes   " = seconds

Castor C is a third component of the system.

Pollux is the brightest star in Gemini.   Read the RA and Dec.  Compare these coordinates to Castor. 

Is Pollux East or West of Castor?  ____ 

How do you know?______________________________

Is Pollux North or South of Castor?  ____

Explain_______________________________________

HOU educators download .fts images of Castor and Pollux.

01/24/2000

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