Physics: Coordinates in Astronomy

Physics: Coordinates in Astronomy

1. Positions of Stars are Measured by Angles Seen from Earth

   • The positions of stars are measured by angles seen from Earth. It's like Latitude and Longitude but for the sky.
   • The Sky appears to turn as the Earth Spins, So two fixed positions have been picked by astronomers to measure from:
     • The Pole Star
       • A star which does not seem to move because it is almost directly above the north pole of Earth.
     • The Celestial Equator
       • An imaginary plane extending out from the Earth's equator.
   • The Two angles used to measure positions in the sky are:
     • Declination - Celestial Latitude, measured in degrees.
     • Right Ascension - Celestial Longitude, measured in degrees or time.

2. Planets Seem to Move in Complicated Patterns

   • All Planets in the Solar System Orbit the Sun in the same direction but diffferent speeds. The closer to the sun, the quicker the planet.
   • If you note down the position of planets everyday at the same sidereal time each night. You'll find that they seem to travel from west to east.
   • Sometimes the planet seems to change direction and go the other way fro a bit, making a loop on the way. This is called retrograde motion.
   • It happens because both the planet and Earth are moving around the Sun, so we're seeing the motion of the planet relative to the Earth.
   • Mars appears to change direction once every two or so years.