Download 3.2a Right Ascension and Declination

3.2a
demonstrate an understanding of the terms ‘right ascension’ and declination
When we look at a map, we locate the position of a place using latitude and
longitude. If we look out at the night sky and imagine the stars are painted on a
sphere around the Earth, we locate a particular celestial object in a similar way using
declination (dec) and right ascension (RA).
Comparing the Earth with the night sky:declination (dec) is equivalent to latitude
right ascension (RA) is equivalent to longitude
Declination (dec)
Declination is the angular distance north or south of the celestial equator
Declination
North of the celestial equator
is positive
+90˚ = North Celestial Pole
NORTH CELESTIAL POLE
CELESTIAL
EQUATOR
Declination
South of the celestial
equator is negative
-90˚ = South Celestial Pole
SOUTH CELESTIAL POLE
Picture credit : (Earth) NASA’s Earth Observatory
We need reference points for latitude or declination from which to measure:Reference point
EARTH
NIGHT SKY
VERTICAL
EQUATOR
Latitude
CELESTIAL EQUATOR
Declination
In the diagram below, looking out from the Earth, if we imagine the line of our
equator extended into space, we get the celestial equator. Instead of latitude,
astronomers use the word declination. The equator is given as 0˚. Whereas latitude
may be 30˚N, anything north of the celestial equator is plus = dec +30˚. Anything
south of the celestial equator is minus = dec -30˚ (equivalent to latitude 30˚S).
DECLINATION
(degrees)
+30˚
0˚
CELESTIAL
EQUATOR
-30˚
Above the North Pole is dec +90˚ and above the South Pole is dec -90˚
Picture credit : (Earth) NASA/JSC-Apollo17
For the purposes of this explanation, the celestial sphere has been reduced. A
person has the sky curving round behind them – the view above shows the full band
of stars between declination +30˚ and -30˚.
Right Ascension (RA)
If a person was able to see the night sky shown above for a full day, the full band of
stars would pass in front of them, moving steadily towards the right.
The longitude reference point was more difficult. Many countries have laid claim to
the Prime Meridian – the Chinese once used a gate from the Forbidden City to
Tiananmen Square to mark the line. The GREENWICH PRIME MERIDIAN was chosen
as the 0˚ reference point in 1884.
The scale of Right Ascension is like a clock. The Earth spins every 24 hours (actually
every 23 hours 56 minutes) and the clock was used to divide the night sky. Every
hour, another 15˚ of the night sky passes an observer. Just as with a clock, parts of
an hour were divided into minutes.
Reference
point
EARTH
NIGHT SKY
HORIZONTAL
GREENWICH MERIDIAN
Longitude
VERNAL/SPRING EQUINOX
Right Ascension
GREENWICH PRIME MERIDIAN
360˚ 0˚
Yours truly standing at the Royal Observatory in Greenwich with my right foot in the
east and my left foot in the west! The PRIME MERIDIAN is the line between my feet –
the reference line for LONGITUDE for the whole world.
As there are 360˚ in a circle, the circle around the world could be referenced – a
location could be given in degrees eg on the opposite side of the world to
Greenwich, the longitude is 180˚. On the celestial sphere, half way round the
sphere is 12 hours 0 minutes, which is the value of Right Ascension for a star at that
point.
The night sky moves across the sky as shown by the arrows in the diagram
(ANGLED AT 23.5˚ because of the tilt of the Earth’s axis)
Autumn Equinox
Right Ascension
= 12 hr 0 min
(half way round the circle)
NORTH CELESTIAL POLE
Summer Solstice
Right Ascension
= 6 hr 0 min
(a quarter of the way
round the circle)
CELESTIAL
EQUATOR
23.5˚
Winter Solstice
Right Ascension
= 18 hr 0 min
(three quarters of the way
round the circle)
RA up to
24hr 0min
SOUTH CELESTIAL POLE
Spring Equinox
Right Ascension
= 0 hr 0 min
(the starting point of
measurements)
Picture credit : (Earth) NASA’s Earth Observatory
The key part for measuring Right Ascension is when to start the clock. The decision
was taken to choose the Spring Equinox – when the path of the Sun cuts the celestial
equator moving northwards. Having decided on this reference point, an astronomer
situated at the Prime Meridian in Greenwich just needed to observe the stars at that
moment. Once the stars on the line at 0 hours 0 minutes had been recorded, the
rest of the night sky could be catalogued by careful observation. One hour later,
stars on the Prime Meridian line were given a Right Ascension value of 1 hour 0
minutes. Values for Right Ascension for a number of stars are shown in the
following chart:Star name
Constellation
Right Ascension (RA)
Polaris
Ursa Minor
2 hours 32 minutes
Sirius
Canis Major
6 hours 45 minutes
Regulus
Leo
10 hours 08 minutes
Deneb
Cygnus
20 hours 42 minutes
During 24 hours, the Earth makes a full rotation through 360˚ (15˚ per hour). The
stars drift across the night sky from East to West. In the diagram below, vertical
lines are shown for every 2 hours on the portion of the night sky between about dec
+30˚ and -30˚.
+30˚
0˚
-30˚
24
22
20
18
16
14
12
10
8
6
4
2
0
22
20
18
16
RIGHT ASCENSION (HOURS)
DRIFT OF STARS ACROSS THE NIGHT SKY
+30˚
0˚
-30˚
16
14
12
10
8
6
4
2
24
/0
RIGHT ASCENSION (HOURS)
16 HOURS LATER, THE STARS HAVE DRIFTED ACROSS THE NIGHT SKY
The reference point for the night sky is the point at which the Sun crosses the
celestial equator moving North (the Vernal/Spring equinox). Two hours later, the
stars will have drifted 30˚ across the night sky. The term Right Ascension (RA) in
effect represents the hours (angular distance eastwards) from the reference point
that a particular celestial object will be.
As an example, the celestial coordinates of the star BETELGEUSE in the constellation
Orion are:Right Ascension = 5 hours 52 minutes : Declination = +7 degrees 24 minutes.
There are many different ways that star maps can be presented. A map may show
the stars for a month or over a year. The star map shown below shows the band of
stars seen between about dec +30˚ and -30˚.
BETELGEUSE
Right Ascension = 5 hours 52 minutes : Declination = +7 degrees 24 minutes
+30˚
0˚
-30˚
24
22
20
18
16
14
12
10
8
RIGHT ASCENSION (HOURS)
More detail on star maps
shows the constellations
like Orion and usually
gives information about
the locations of other
celestial objects found in
the region
eg star clusters like (M45)
and nebulae (M42)
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