PX158 - B1 - introduction

the "celestial sphere" is a way of mapping the position and movement of stars, sun, moon, planets, etc

a reference grid similar to latitude and longitude on earth is used

• right ascension (R.A.)
  • equivalent to longitude
  • $0$ to $360\mathrm{°}$, sometimes $0$ to $24h$
• declination (Dec.)
  • equivalent to latitude
  • $-90\mathrm{°}$ to $+90\mathrm{°}$

it can be observed that all the stars in the night sky rotate about a point called the celestial pole (approximately polaris for the northern hemisphere)

it rotates once per day

the positions of stars in the reference grid remain unchanged

the positions of the sun and the planets change throughout the year.

the sun move along the great circle called the ecliptic, and the planets move very close to it. the moon also moves very close to it

the position of the sun for the northern hemisphere:

• at the vernal equinox (${21}^{st}march$):
  $RA=0\mathrm{°}$
  $Dec=0\mathrm{°}$
• at the summer solstice (${21}^{st}june$):
  $RA=90\mathrm{°}$
  $Dec=+23.5\mathrm{°}$
• at the autumn equinox (${23}^{rd}september$):
  $RA=180\mathrm{°}$
  $Dec=0\mathrm{°}$
• at the winter solstice (${21}^{st}december$):
  $RA=270\mathrm{°}$
  $Dec=-23.5\mathrm{°}$

the sun rises and sets due east and west respectively on the equinoxes, and at the extreme north/south positions on the solstices