can anyone help me make sense of Right Asenscion?

[waynea]

try as I might I still cannot get this right in my head conceptually

The best explanation I've found comes from Patrick Moore who wrote  "a star culminates when it reaches the highest point above the observers horizon and is in his meridian. The first point of Aries therefore culminates once a day (often of course when the sun is above the horizon). RA is the difference between the culmination of the first point of Aries and the culmination of the star concerned. Rigel culminates 5 hours 12 minutes after the first point, therefore the RA of Rigel is 5 hours 12 minutes"

That's clear. But how do I work out when the First Point of Aries itself culminates? I get that RA is fixed with respect to location. Im in London UK, and with a current sidereal time of 09.28

Why is culmination sometimes when the sun is above the horizon and sometimes below given that RA doesn't vary with the observers location? changing position of the earth as it goes round the sun?

 

Hope someone can explain this to me!!

 

[vlaiv]

Hi and welcome to SGL.

43 minutes ago, waynea said:

changing position of the earth as it goes round the sun?

Indeed.

You can think of celestial sphere being fixed and Sun/Earth system embedded in center. In Spring earth is on one side of the sun with respect to celestial sphere and in autumn it is on the other side. That is why there are "seasonal constellations" - like Orion that is observed in winter and Cygnus that is observed in Summer.