The following video from the Explaining Science YouTube channel explains the equatorial celestial coordinate system widely used by astronomers
Video Description
Describes the celestial sphere, an imaginary sphere at a great distance centred on the Earth and the equatorial coordinate system: right ascension (RA) and declination (dec) used in astronomical catalogues to specify an object’s position for an observer on the Earth’s surface on the celestial sphere.
Discusses the various corrections we need to make for: precession, stellar aberration, proper motion of the object etc.. to get from the standard (J2000) equatorial coordinates given in an astronomical catalogue to the local RA and dec coordinates for an observer. Explains how we transform from the local RA and dec of an object to the azimuth and elevation in the sky, at a given time on a given date, and discusses how the light from objects near the horizon is bent (refracted) upwards by the Earth’s Atmosphere. This makes them appear slightly higher in the sky than they would be if the Earth had no atmosphere.
Interesting fact – the link with the Geocentric model
Interestingly, because the equatorial coordinate system is based upon the celestial sphere, it can be considered to originate from the obsolete Geocentric model in which the Earth is the centre of Universe.
Explaining Science 