The Celestial Sphere

Right Ascension and Declination

Each and every astronomical object in the night sky, such as a star, a galaxy and nebulae possesses its own unique coordinate to identify its position on the celestial sphere. These coordinates are represented by ‘right ascension’ and ‘declination’.

Right ascension (RA) is a measure of hours, minutes and seconds whilst declination (DEC) is measure of degrees.

Astronomical objects are analogous to towns and cities on the Earth’s surface. Towns and cities are located on the Earth’s surface using coordinates of latitude and longitude. Longitude states how far the city is east or west along the Earth’s equator with respect to the Greenwich meridian, while latitude states how far a city is north or south of the Earth’s equator.

Similarly, right ascension (RA) locates where an astronomical object, a star for instance, is located along the celestial equator. The zero point of longitude is where a line straight down from the Greenwich Observatory in England meets the equator. The zero point for right ascension is the vernal equinox.  The vernal equinox happens twice a year and occurs when the celestial equator coincides with the equator.

To find the right ascension of a star follow an hour circle (straight down) from the star to the celestial equator. The angle from the vernal equinox eastward to the foot of that hour circle is the star’s right ascension.

EarthGrid

Right ascensions are always recorded in terms of hours, minutes, and seconds. One hour of right ascension (1h) is 15°. Since 24 x 15° = 360°, there are 24h of right ascension around the celestial equator. The celestial sphere makes one full rotation (24h of RA) in one day (24 hours of time). Thus the celestial sphere advances about 1h of RA in an hour of time.

Using the Setting Circles

First of all your altitude should be set to 53 degrees for Ireland.

The declination circle is graduated in degrees from +90 degrees to zero degrees and down to –90 degrees.

Point the telescope at Polaris and set the declination dial to 90 degrees.

Point it at a star on the celestial equator and it should read 0 degrees.

Point it at any star and it should read the declination of that star.

The right ascension circle is graduated in 24 hours with subdivisions. Unlike the declination scale which is fixed, the right ascension scale can be rotated. This is because the position of a particular right ascension is continually moving as the Earth spins on its axis. The motor on your telescope turns the right ascension axis at the same rate as the movement of the Earth. So once you have your right ascension set with a star whose right ascension you know, the star should stay in the field of view as long as the motor is running.

To use the right ascension, you have to first find a star of known right ascension. I have made a table below with the right ascensions of some stars for you. What you need to do is calculate the difference between the right ascension of the star your telescope is pointing at in the sky and the right ascension of the star you want to look at, and then dial the difference.

You will notice that there are two sets of numbers, running in opposite directions. The top one is for the northern hemisphere and the bottom for the southern hemisphere, so you need to stick with the top scale.

Setting circles is probably the thing which confuses beginners more than anything else. It even confuses the most advanced users too. Most people would prefer to find objects by star hopping or even casting around in the hope of finding the object they are looking for, rather than using the setting circles.

Star

Constellation

RA

DEC

   

h

m

°

Procyon

Canis Minor

07

39

+05

13

Sirius

Canis Major

06

45

-16

43

Betelgeuse

Orion

05

55

+07

24

Rigel

Orion

05

14

-08

12

Aldebaran

Taurus

04

35

+16

30

Capella

Auriga

05

16

+45

59

Pollux

Gemini

07

45

+28

01

Arcturus

Bootes

14

15

+19

10

Alkaid

Ursa Major

13

47

+49

18

Mizar

Ursa Major

13

23

+54

55

Alioth

Ursa Major

24

54

+55

57

Megraz

Ursa Major

12

15

+57

01

Phad

Ursa Major

11

53

+53

41

Merak

Ursa Major

11

01

+56

22

Dubhe

Ursa Major

11

03

+61

45

Deneb

Cygnus

20

41

+45

16

Mirphak

Perseus

03

24

+49

51

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