Right Ascension

Right ascension is similar to longitude. The right ascension of a star is the angle between it and the celestial prime meridian. The circles on the Stellarium screenshot below show declination. The lines emanating from Polaris show right ascension.

There is one important difference, right ascension is not measured in degrees but in hours and minutes. The RA of the prime meridian is 0 h 0 min then we imagine a 24hr clock going clockwise from the meridian.

Study the diagram below which shows the northern half of the major celestial meridians. At midnight on March 21st, the Vernal Equinox, the prime meridian is as shown below. (If you lived in Greenwich).

Notice the following: Star A will culminate in 1h 45mins, at 01:45. Star B will culminate 2h and 29mins later, at 02:29.   If you look at this "clock" at midnight on every day after March 21st this clock will have moved anti-clockwise by 4 minutes. e.g. on March 27th it will be 6 x 4 = 24 minutes further on so star A will culminate at 01:21.

Your longitude, how far East or West of Greenwich you are, will also affect the time at which stars culminate.

The further East you are the sooner they will culminate, by 4 minutes for every degree East.

The further West you are the later they will culminate, by 4 minutes for every degree West.

E.g. If star A culminates at 01:21 in Greenwich the in Middlesbrough (1.13 W) it will culminate 1.13 x 4 = 4.4 minutes later.

Remember that the Sun rises in the East so if you lived more Easterly than you do now the sun would rise sooner.

The Equatorial Mount

If a telescope has an equatorial mount, such as the one below, then one can find stars with known coordinates more easily. When set up correctly rotation about one axis changes the R.A. and about another axis changes the Dec.

Many telescopes now have GOTO functions where one simply types in the R.A. and Dec. and motors turn the telescope to the right place.