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Coordinate Systems for Astronomy
or: How to get your telescope to observe the right object
Figure 1: Basic definitions for the Earth
Definitions - Poles, Equator, Meridians, Parallels
The rotation of the Earth on its axis lets us define a coordinate system for the surface of the Earth.
The two points where the rotation axis meets the surface of the Earth are known as the north
geographic pole and the south geographic pole (Figure 1).
The equator is the great circle perpendicular to the rotation axis and lying half-way between the
Great circles which pass through the two poles are known as meridians.
Small circles which lie parallel to the equator are known as parallels.
Question: What other poles does the Earth have, that could be confused with the geographic poles?
Figure 2: Earth Longitude and Latitude
Coordinates on the Earth – Longitude, Latitude
The longitude of a point on the Earth’s surface is measured east or west along the equator. Its value
is the angular distance between the local meridian passing through the point and the Greenwich
Meridian, which passes through the Royal Greenwich Observatory in London (Figure 2).
Because the Earth is rotating on its own axis it is possible to express longitude in time units as well
as angular units. The Earth rotates through:
360° in 24 hours (of Sidereal Time – see later), or
15° of longitude in 1 hour, or
1° of longitude in 4 minutes, or
1' of longitude in 4 seconds, or
1" of longitude in 1/15 second.
The latitude of a point is the angular distance north or south of the equator, measured along the
meridian passing through the point. It is measured in degrees.
Question: What is the approximate longitude and latitude of Johannesburg?
Figure 3: Horizon coordinates for an observer
Horizon Coordinates – Azimuth, Altitude / Elevation, Zenith Angle
The horizon coordinate system is also called the topocentric or alt-az or az-el system.
It is the simplest coordinate system as it is based on the observer's horizon. The celestial
hemisphere viewed by an observer on the Earth is shown in Figure 3.
The great circle through the zenith and the north celestial pole cuts the horizon at the north point
(N) and the south point (S). It is called the zenith meridian.
Altitude (or elevation) is the angle measured along a vertical circle from the horizon. It is
measured in degrees.
An alternative to altitude is the zenith angle, z = 90° - altitude.
Azimuth is generally defined as the angle between the vertical through the north point and the
vertical through the star, measured eastwards from the north along the horizon from 0° to 360°.
This definition applies to observers in both the northern and the southern hemispheres.
Question: How close to zenith does the Sun come in Johannesburg? When does this occur?
Figure 4: Celestial Equatorial coordinates
Celestial Equatorial Coordinates - Right Ascension, Declination
Because the altitude and azimuth of a star are different for different places on the Earth, the horizon
coordinate system is not useful for cataloguing celestial positions. A more useful coordinate
system for locating celestial objects is one based on the celestial equator and the celestial poles.
This is defined in a similar manner to latitude and longitude on the surface of the Earth. In this
system, known as the equatorial coordinate system, the analogue of latitude is the declination.
The declination (Dec. or δ) of a star is its angular distance in degrees measured from the celestial
equator along the meridian through the star. It is measured north and south of the celestial equator.
It ranges from 0° at the celestial equator to +90° at the north celestial pole and -90° at the south
celestial pole.
Right Ascension (R.A. or α) is the equivalent of longitude on the Earth. The zero point chosen on
the celestial sphere is the point at which the Sun crosses the equator, on about March 21. This is
called “The first point in Ares, γ”. It is the northern hemisphere vernal (spring) equinox, and the
southern hemisphere autumnal equinox.
The Right Ascension of an object is the angle between the meridians through the first point in Ares
and the celestial object. It is measured in units of time, from 0h to 24h along the celestial equator
eastwards (counter-clockwise) from the first point of Aries.
Figure 4 also shows the ecliptic, the path along which the Sun appears to move in the sky. Ecliptic
Coordinates are used for the positions of solar system objects, particularly the planets. Ecliptic
Longitude and Latitude are measured in degrees.
Question: Why is the ecliptic tilted, and by how much is it tilted?
Figure 5: Precession of the Earth's axis
Precession – Epoch, Precession
Having settled on Right Ascension and Declination as the natural coordinate system for most
celestial objects, we find that the RA and Dec of celestial objects very inconveniently change with
time. The reason for this is that the direction in which the Earth’s axis points is changing, owing to
the tidal pull on the not quite spherical Earth. This effect is called Precession (Figure 5).
As a result, an Epoch must be stated in respect of all RA, Dec coordinates. Standard epochs for
catalogues of objects were B1900, B1950 and J2000. Here B standards for Besselian and J for
Julian, and describe subtle changes of definition.
To get the current position of an object in order to observe it, its coordinates must be precessed
from the catalogue epoch to the date and time of observation.
Generic Equatorial Coordinates plotted on a rectangular grid are shown in Figure 6.
Figure 6: RA, Dec coordinate system
Figure 7: Galactic Coordinates
Galactic Coordinates – Longitude, Latitude
For objects in the Milky Way, the Galactic coordinate system is also used. The direction of zero
longitude and latitude is towards the centre of the Milky Way. This coordinate system does not vary
with time, unlike RA, Dec. As an early attempt at defining the origin of the Galactic Coordinates
was found later to be inaccurate, a second zero point was determined, and in this system Galactic
longitude, latitude are called Lii, Bii (Figure 7).
The centre of the Milky Way, ~7000 parsecs away towards Lii = 0o, Bii = 0o, lies in Sagittarius, at
J2000 RA 17h 45m 37s, Dec -28o 56' 10''. It passes over South Africa once per day.
The Galactic anti-centre direction, at Lii = 180o, Bii = 0o, lies north of the well known constellation
of Orion, at J2000 RA 5h 45m 37s, Dec 28o 56' 10'' Figure 8).
Figure 8: Galactic coordinates mapped onto B1950 Equatorial coordinates
Figure 9: Radio sky at 408MHz in J2000 Equatorial coordinates (Haslam et al.
1982). Shown using rectangular projection, with RA in degrees.
Radio emission from the sky is shown above in equatorial coordinates (Figure 9) and below in
Galactic coordinates (Figure 10).
Figure 10: Radio sky at 2300MHz in Galactic coordinates (Jonas et al. 1998).
Left one-third from Stockert telescope, Germany. Right two-thirds from
HartRAO 26m telescope. Shown using the Mollweide equal area projection.
Figure 11: Solar day and sidereal day
Time – Universal Time, Sidereal Time, Julian Date
Universal Time comes in more than one form. Universal Time Coordinated (UTC) is also known
as Greenwich Mean Time (GMT). It is the time measured by a evenly running clock positioned
on the Greenwich Meridian.
The world is divided into Time Zones; 15 degrees change in longitude is equivalent to an hour's
change in time. South Africa is roughly 30 degrees east of Greenwich, and so South African
Standard Time (SAST) is set to be 2 hours ahead of UTC.
Our “year” is technically the Earth's “tropical period”. This is the time that elapses between two
alignments of its axis of rotation with the Sun, or 365.242 days. The Earth's orbital (sidereal)
period around the Sun is 365.256 days. The 0.014 day (= 20 minutes) difference is caused by
precession, which has a period of 25770 years (see above), as 365.25/25770 = 0.014 day/yr.
The Earth turns once on its axis in 23 hours 56 minutes and 04 seconds. This is called a sidereal
day. Stars rise at the same time each sidereal day.
However we set our watches to a length of day matching the mean solar day. The Earth has to turn
for an extra ~1o to get the Sun back to the same place in the sky, as it moves almost 1o per day in its
orbit around the Sun. The length of the mean solar day is the well known 24 hours.
To point our telescopes we have to make allowance for the actual, irregular rotation period of the
Earth. UT1 is the time scale used to measure this. Leap seconds are inserted in UTC at intervals to
keep UTC and UT1 within 0.9 of a second.
Measuring the changing length of day (UT1) is done by a global network of radio telescopes using
quasars far out in the universe as fixed radio beacons. The HartRAO 26m radio telescope is part of
this network.
A time scale divided into years with months of different lengths and numbers of days per month and
year is very awkward when dealing with time series of data. To get around this the Julian Date
(JD) was devised. This is the number of decimal days since 12h00 UT of January 1, 4713 BCE.
Modified Julian Days (MJD) are also used. They start at 0h00 UT, and are defined as
MJD = JD – 240000.5
The Julian Date on 2009 August 5 at 10h00 UT was 2455048.875, and the MJD was 55048.375.
Using Sidereal Time – Mean, Apparent, and Hour Angle
As with all longitude-like coordinates, a zero point has to be set for sidereal time. This is the
sidereal time at the Greenwich meridian, or Greenwich Mean Sidereal Time (GMST). It is the
elapsed time since the zenith meridian transit of the vernal equinox at the Greenwich meridian.
Local Mean Sidereal Time (LMST) is then the current Greenwich Mean Sidereal Time plus the
observer's longitude (East longitude is positive), converted from degrees to hours.
LMST[h] = GMST[h] + Longitude[o] / 15
A small correction (< 1.15 seconds of time) to this for nutation (18.6 year period wobble of the
Earth's pole) gives the Local Apparent Sidereal Time (LAST). This equals the Right Ascension
of all bodies currently crossing the observer's zenith meridian.
Observatories often display clocks showing the Local Sidereal Time to aid observers. For example,
see .
The Hour Angle of an object is the angle of an object east or west of the observers zenith meridian.
Hour Angle = Local Sidereal Time – Right Ascension of Date
or simply:
A negative Hour Angle translates as the time until an object crosses the zenith meridian, a positive
Hour Angle as the time since it has crossed the zenith meridian.
Question: The LST in Johannesburg is 22h 30m. Where in the sky is 3C48, one of the radio
galaxies used for flux calibration, whose coordinates are approximately RA 01h 34m +32o 54'?
Figure 12: Atmospheric refraction
Ground-based telescopes have to observe through the Earth's atmosphere. Refraction by the
atmosphere increases the apparent elevation angle of an object (Figure 12). The effect is largest for
objects close to the horizon. Refraction by the atmosphere is greater by about 8% for radio waves
compared to visible light. Refraction also depends on the height of the observatory above sea level
and on the water vapour content of the atmosphere.
A simple approximation to the radio refraction is:
Δz = (n - 1) x tan(z)
where Δz (radians) is the apparent increase in elevation angle at zenith angle z (radians)
n is the refractive index
n – 1 ~ 3 x 10-4 for radio waves at sea level
n – 1 ~ 2.5 x 10-4 for radio waves at Hartebeesthoek, at altitude 1400m.
Question: The horizon for the radio telescopes at Hartebeesthoek varies from 3o to 12o elevation,
as they are located in a valley. What is the approximate radio refraction in degrees at those two
elevation angles?
Figure 13: Definition of the Astronomical Unit and Parsec
Distances – Astronomical Unit, Parsec
For relatively small astronomical distances the Astronomical Unit (AU) can be used. This is
defined as the mean Sun-Earth distance, which is 149.6 x 106 km (Figure 13).
For large distances the Parsec (pc) is used. This is defined as being equal to the distance from the
Sun at which the mean Sun-Earth distance (1 AU) subtends an angle of 1 arc second (Figure 13).
One parsec equals 3.086 x 1013 km, or 3.26 light years.
Figure 14: Parallax measurement
A consequence of the Earth's motion around the Sun is that nearby objects in space appear to move
forwards and backwards against the background of very distant objects, through parallax (Figure
14). The amount of parallactic movement is proportional to their distance, so the observed parallax
provides an absolute measure of their distance. Optically, this has long been used to measure the
distance to nearby stars.
Radio telescopes separated by thousands of kilometres can measure parallaxes of milliarcseconds
(mas). These networks of radio telescopes are now being used to measure the distances to starforming regions in the Milky Way, using the annual parallax of methanol and water vapour masers
embedded in the star-forming clouds. The aim is to accurately map the spiral arms of the Milky
Way. The HartRAO 26m telescope has been used to search for suitable masers for this project.
We have:
where d = distance to source in parsecs
p = parallax angle of source in arcseconds.
Question: The methanol masers in the star-forming region G9.62+0.20 were found by Sharmila
Goedhart in her monitoring with the HartRAO 26m telescope to show regular flaring, the first ever
found in masers in a star-forming region. The parallax of the masers has been measured to be 0.194
mas. What is their distance?
Figure 15: Stellar Aberration, showing telescope
pointing direction without aberration
Astronomical (Stellar) Aberration
Telescopes are not fixed in space, they are on a moving Earth. As the speed of light is finite, this
causes the apparent position of objects to shift. We have to point the telescope at the apparent
position of the object to allow for this stellar aberration (Figure 15).
q = angle to true position of star (D-B-C)
φ = angle (D-A-C) at which telescope must actually point to see the star owing to
v = horizontal motion to right of telescope owing to Earth velocity in orbit or rotation on own axis
h = length of telescope tube
c = speed of light
h / c = time taken for light to travel down telescope tube
v h / c = horizontal distance travelled by telescope while light passes down the tube
To solve for the aberration angle q – φ = B-D-E
in triangle BDE, sin (q - φ) = opposite / hypoteneuse = b / h
in triangle EAB, sin φ = b / (v h / c)
rearranging, b = (v h / c) sin φ
substituting for b: sin (q - φ) = v h / c . sin φ / h
so sin (q - φ) = v / c sin φ
hence as (q - φ) is small, the aberration angle q – φ ≈ sin (q – φ) = v / c sin φ
The maximum value possible for the aberration is v / c, the ratio of the two velocities. This is called
the constant of aberration.
Annual aberration was used by Bradley in 1727 to measure the speed of light.
Question: What is the constant of aberration in units of arc seconds for (a) annual aberration,
caused by the Earth orbiting the Sun, and (b) for daily aberration, for the Earth turning on its own
Figure 16: International Celestial Reference Frame comprises
The International Celestial Reference Frame
To measure radio source positions and parallaxes accurately, reference sources with very accurately
measured positions must be available. To create this reference frame, radio telescope networks
have measured, and continue to measure, the positions of hundreds of quasars across the sky. This
is called the International Celestial Reference Frame (ICRF), shown in Figure 16.
The HartRAO 26m telescope has played a key role in this project, being one of the few southern
radio telescopes equipped for this. All the telescopes observe the same object at the same time, in a
technique known as Very Long Baseline Interferometry (VLBI). Observations are made at two
widely spaced frequencies (2.3 and 8.4 GHz) in order to measure and correct for the (varying)
effects of the Earth's atmosphere and ionosphere.
The networked radio telescopes are able to measure source positions to better than one milliarcsecond, whereas ground-based optical telescopes can achieve about 100 milli-arcseconds. As a
result this extra precision, the radio source-based reference frame replaced the optical FK5
reference frame in 1998.
The ICRF is crucial for comparing the positions of objects observed at different wavelengths across
the electromagnetic spectrum, i.e. through multi-wavelength astronomy. In order to understand
the relationship between the emission seen at different wavelengths, it is necessary to accurately
align and overlay the images taken at each wavelength.
The Gaia spacecraft, to be launched in December 2011, is designed to set up a new optical
reference frame aiming at 24 micro-arcsecond accuracy, which will be aligned with the ICRF. The
radio reference frame itself will be improved by moving to higher frequencies (up to 32GHz) which
will provide higher precision.
Figure 17: Changing distance between SA and USA
The International Terrestrial Reference Frame – GNSS, SLR, DORIS
At the same time as radio source positions are being measured precisely in the ICRF, so the
positions of the participating radio telescopes are also determined with high precision. After taking
out short-term effects such as tides in the solid Earth, we are left with long term motions of the
radio telescopes (Figure 17), which measure the motion of the tectonic plates on which they are
located (Figure 18). The accurate absolute positions of the these telescopes means that systems that
measure relative positions can be co-located with them, and the absolute positions transferred to
these systems. They include the Global Navigation Satellite Systems (GNSS) such as the US
Global Positioning System (GPS), Russian GLONASS and European Galileo, Satellite Laser
Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite
(DORIS). These four systems are located at Hartebeesthoek, making it a key station in the
International Terrestrial Reference Frame (ITRF). There are many scientific and practical
spinoffs from the precise position measuring that these systems carry out.
gure 18: Radio telescope motion vectors
Question: What uses of high precision position measurement with GPS can you think of ?