Introduction An to Radio
Astronomy
Lecture 1 some basic astronomy
Where on earth.?
Far out in the uncharted
backwaters of the unfashionable
end of the western spiral arm of
the Galaxy lies a small
unregarded yellow sun.
Orbiting this at a distance of
roughly ninety-two million miles
is an utterly insignificant little
blue green planet whose apedescended life forms are so
amazingly primitive that they still
think digital watches are a pretty
neat idea.
D Adams, The Hitch Hikers Guide to
the Galaxy, Random House 1979.
The solar system sizes to scale,
relative distance to sun not.
(NASA, Wikipedia).
Early history of astronomy
Astronomy is a natural
science studying
celestial objects (stars,
planets, comets,
galaxies etc) and extraterrestrial phenomena
(CMBR).
One of the oldest
sciences prehistoric
cultures left artefacts
(Stonehenge).
Early civilizations
(Babylonians, Greeks,
Chinese, Indians, and
Maya) made methodical
observations of night
sky.
Invention of telescope
(1608) required before
the modern science
could develop.
Our galaxy
Spiral galaxy, approximately 100 000 light
years diameter.
Solar system located approx 30 000 light
years from core.
Geocentric vs. Heliocentric models
Until early 17th century
(Kepler, Galilei)
geocentricity ruled
(although sun-centred
models had been
proposed in 3rd century
BC).
Even today, many
astronomical concepts
(celestial sphere) are
essentially geocentric.
1568 illustration of
Ptolemaic geocentric
system
Modern science
From 17th century, rapid
developments.
Newton Principia of 1687
laid foundations of
celestial mechanics; also
developed reflecting
telescope (1668).
Lacaille produced
extensive star catalogues.
Herschel discovered
Uranus in 1781.
Euler et al worked on
three body problem;
improved by Lagrange
and Laplace; improved
predictions of orbital
motion.
19th century advances in
spectroscopy and
photography impacted on
astronomy.
20th century
Existence of our own
galaxy, Milky Way, only
proven in 20th century!
Other galaxies then found
- and expansion of the
Universe.
Exotic objects discovered:
quasars, pulsars.
Exotic predictions made:
black holes, neutron
stars.
Cosmology made great
strides (Big Bank, CMBR).
Observational astronomy
added new parts of EM
spectrum:
Radio
IR
Optical (traditional)
UV
X-ray
Gamma-ray
Some basics of the Earth
Located 149.7 million km
from sun - 1 AU (by defn).
Mean diameter 12 742
km.
Orbit around sun in
365.24 sidereal days (leap
years every 4 years,
unless divisible by 100
and not divisible by 400!)
Axis tilted approx 23.5
degrees relative to plane
of ecliptic.
The Earth, see from Apollo 17
The orbit of the earth
Note that the direction of earths rotation (west to east) is in the same
direction as its orbit around the sun (a prograde orbit).
Solar vs sidereal days
Because the Earth
rotates both about its
own axis, and about the
sun, the solar day
(between the sun at its
highest) and sidereal
day (stars ditto) differ
(by approx 1 degree - 4
min - per day, or one
day in the year).
Earth (solar) time
The day as we use it is
actually the mean solar day,
averaged over the year.
Because firstly, the earth
moves in a slightly elliptical
orbit, and secondly, the
earths axis is tilted relative
to its plane of orbit, the
length of the true solar day
actually varies (noticeably)
during the year.
Near the Dec solstice, the
day is lengthened by about
30 seconds; near the March
(and Sept) equinoxes,
shortened by about 18 (21)
secs.
Result is that local noon is
not constant (by our clocks)
during the year; for
instance, latest sunset in
the W Cape occurs well into
January (not midsummers
day).
Sunrise, sunset and length of day in
Cape Town
The equation of time
This equation gives the
difference between
apparent solar time (i.e.
measured with a sundial) and mean solar
time.
Negative means the sun
is slow, i.e. noon is
delayed and the sun
sets later.
Eqn of time (contd)
As noted, it is due to
two superimposed
effects with different
periods:
The ellipticity of the
earths orbit (dark
dash-dot; annual
period)
The inclined axis of
rotation of the earth
(mauve broken line,
biannual period).
Rgd former, earths moves
fastest at perihelion (closest to
earth) and slowest at aphelion
around 4 mins of arc per day
difference.
Eqn of time (contd 2)
Even if the earth moved at
constant speed, the
apparent motion of the sun
would still vary.
See below and opp, central
(1) is vernal equinox, ditto
(2) summer solstice (N.H.)
Location on earth
Lines of latitude and
longitude provide
location in a variant of
essentially spherical
coordinates here,
(,).
Prime meridian runs
through Royal
Observatory,
Greenwhich. (Selected
1884).
Determining latitude and longitude
(pre-GPS)
Latitude can be determined
from altitude of celestial
pole (=latitude).
At sea, this is done by
observing the sun's angle at
noon (i.e., it when reached
its highest point in the sky,
or culmination).
Longitude is more difficult.
On land, can be determined
from transit (crossing the
meridian) of star with
accurately measured coordinates.
At sea, needs accurate
knowledge of UTC (GMT);
combined with measure of
local noon, gives longitude.
Until mid 18th century,
clocks not sufficiently
accurate (invention of
marine chronometers).
The celestial sphere and spherical
(positional) astronomy (location in the
heavens)
The celestial sphere is an
imaginary sphere of arbitrarily
large radius, concentric with the
Earth and rotating upon the same
axis.
All objects in the sky can be
thought of as projected upon the
celestial sphere.
Projected upward from Earth's
equator and poles are the
celestial equator and the celestial
poles.
In geocentric models, celestial
sphere was viewed as a physical
reality rather than a geometrical
projection.
Celestial coordinate systems
Horizon system of coordinates
Equatorial coordinates
Ecliptic coordinates
Galactic coordinates
Horizon system of coordinates
Plane through observing point
parallel to local horizon is
plane of reference.
Zenith is point overhead.
Nadir is point underfoot.
Coordinates given by azimuth
(ref: N, clockwise) and altitude
(ref: horizon).
Meridian circle is great circle
passing through celestial poles
and zenith.
Useful for altazimuth
mounting (telescope steerable
around vertical and horizontal
axes).
Equatorial coordinates
Right ascension (abbrev.
RA; symbol ) and
declination, (abbrev.
dec; symbol ) are the
astronomical terms for
the two coordinates of
a point on the celestial
sphere when using the
equatorial coordinate
system.
Declination
Declination in astronomy is comparable to
geographic latitude, but projected onto the
celestial sphere.
Declination is measured in degrees north and
south of the celestial equator.
Points N of the celestial equator have positive
declinations, while those S have negative
declinations.
Right ascension
Right ascension is the celestial equivalent of terrestrial longitude
.
Both and measure an angle that increases toward the east as
measured from a zero point on an equator.
For former, the zero point is the Prime Meridian on the geographic
equator.
For latter, the zero point is known as the first point of Aries (for
historical reasons Aries has shifted over the years), which is the
place in the sky where the Sun crosses the celestial equator at the
March equinox.
customarily measured in hours, minutes, and seconds, with 24
hours being equivalent to a full circle (360 degrees).
1 hour of right ascension = 124 of 360, or 15 degrees of arc; 1 of
= 15 of arc, etc.
Hour angle
Another astronomical term is hour angle.
The hour angle (HA) of an object is equal to the
difference between the current local sidereal
time (LST) and the right ascension () of that
object:
HA = LST
Thus, the object's hour angle indicates how much
sidereal time has passed (or is still to pass, if
negative) since the object was (or will be) on the
local meridian.
Movement of heavenly bodies
For an observer on earth,
stars etc appear to move
in circular paths.
Depending on the
latitude of the observer
and the declination of the
object, it may: rise and
set; remain visible
continuously
(circumpolar); or remain
entirely below the
horizon.
Rising and setting of stars
Assume that stars etc
are fixed to inner
surface of CS.
As earth rotates about
p1-p2 from W to E, stars
appear to move in
opposite direction, and
in a circular motion.
Assuming observers
view unobstructed, his
horizon is plane X-Y.
Rising and setting contd.
which when drawn to
scale (as CS is at infinity) is
great circle NESW (note that
this depends on latitude).
Star on celestial equator rises
in east at E, sets at W, with
equal time above and below
horizon.
Star A1 is visible for shorter
time; A2 for more; stars above
small circle NT are always
visible (to this observer).
Celestial horizon depends on
observers latitude (sketch is
for one in N. Europe).
Astronomical distances
Astronomical distances are enormous.
Common measurements:
Astronomical unit (distance earth-sun: 149.7
million km, 500 light seconds)
Light years (closest star Alpha Centauri, 4.3 light
years)
Parsecs (closest star just over 1pc)
Parsecs
The parsec (parallax of one
arcsecond; symbol: pc) is a unit of
length, equal to just under 31 trillion
(311012) kilometres (about 19
trillion miles), 206265 AU, or about
3.26 light-years.
Defined as the length of the adjacent
side of an imaginary right triangle in
space. The two dimensions that
specify this triangle are the parallax
angle (defined as 1 arcsecond) and
the opposite side (defined as 1
astronomical unit (AU), the distance
from the Earth to the Sun).
d=1/p, where d is the distance in pc,
and p is the parallex angle, in sec of
arc.
Hyperdrive, worm holes etc