FINDING OUR PLACE IN THE UNIVERSE
By Ronald D. Ferdie, Tim Hunter, and James McGaha
Where are we in the Universe?
Observational results from the Hubble Space Telescope (HST) and
advances in other satellite and ground-based astronomy have
greatly enhanced our ability for determining distances in the
Universe compared to prior times when the largest functional
telescope in the world was the Mount Palomar 200-inch. One
hundred years ago, the known Universe was the Milky Way. Thirty
years age, the known Universe stretched around us some five
billion light years with cautious suspicion it stretched perhaps
thirteen to fifteen billion light years in space as well as
time. Twenty years ago, quasars were considered enigmatic super
energy structures. Today, we feel they formed early in the
history of the Universe and are energetic galaxy nuclei.
This article summarizes how we determine our place in the
Universe by building upon different overlapping yardsticks to
measure distances. However, these yardsticks are still built
upon a "house of cards" wherein parallax methods used to
directly and precisely determine “close” distances to earth are
then in turn used to support other increasingly less precise
yardsticks for determining distances to far away Milky Way stars
and nearby galaxies (Cepheid variable stars, supernova
explosions, and planetary nebulae brightness) which in turn are
used to support other yardsticks (spiral galaxy surface
brightness fluctuations, elliptical galaxy fundamental plane and
red shift determinations) for determining distances to remote
galaxy clusters and quasars (Sky & Telescope December 1983,
pages 516; Sky &Telescope February 2002, pages 18-19).
This house of cards technique for overlapping distance scales
allows us to literally take a ruler to the Universe. However, it
is fraught with uncertainty and must constantly be re-evaluated.
If you change or modify a parameter anywhere in one method of
determining distances, then the downstream distance scales are
changed, and our view of the Universe can radically change,
particularly at its distant fringes.
Also, each method is limited to a certain scale;
for example, if the stars of a particular galaxy cannot be
individually resolved, the technique for measuring distance by
using the spectral classification or absolute magnitude of
selected stars in the galaxy cannot be used for this galaxy, and
its distance must be inferred by the next, more indirect and
less precise method in the chain of distance scales.
The methods for finding our place in the Universe are summarized
in an overlapping set of scales:
Figure one. Diagram showing the overlapping scales used to
measure the Universe.
The cited modern distances to celestial objects are based on a
number of sources, such as the Hipparcos and Tyco Catalogs.
From approximately 1600 to 1850, the distances to the Moon, the
Sun, the planets, and the nearby stars were determined with
reasonable accuracy. During this time, astronomers began to
realize the vast size of the Universe far exceeded the distance
scales used by mankind on the Earth. The mile and kilometer
became too limiting to describe the vast distances to nearby
stars and beyond. For the planets, miles and kilometers are
still used for their distances. For a larger view of the Solar
System, the Astronomical Unit (AU), the mean distance of the
Earth from the Sun (1.496 x 10E13 cm or 92.92 x 10E6 miles), is used
. It describes distances to the outer planets and the Kuiper
Belt of asteroids beyond Pluto and the Oort Cloud of comets
beyond at the outer fringes of our solar system.
Even the AU is, however, too small to easily describe distances
to even the closest of the stars. Here, the AU measurement unit
is supplanted by the light year (5.865 billion miles), the
parsec (3.26 light years), the kiloparsec (3260 light years) and
Mpc (3.26 million light years). However, it is the light year
that has emerged as the most popular measurement to use since it
accents how far back in time we are seeing the images of remote
DIRECT OBSERVATION FROM THE EARTH'S SURFACE
The distances to the Moon, Sun, and planets in our solar system
can be measured using simple trigonometry. Simultaneous
observations of these objects made by observers at two points
sufficiently spread apart on the Earth’s surface allow for
trigonometric calculation of the distance to the object. Since
the 1960’s, direct observation from the Earth also includes the
use of laser and radar measurements. Highly accurate distances
to the Moon to the fraction of an inch have been obtained by
using laser beams going from ground-borne telescopes to optical
reflectors left on the Moon’s surface by U.S. astronauts.
Additionally, radar has been used from the Earth to obtain
return signals from the Moon, Venus, near Earth passing
asteroids, and other nearby solar system bodies. The radio
telescope at the Arecibo Observatory in Puerto Rico can transmit
and receive radio signals as far out as 9.6 AU (one hour and
twenty minute travel time for the speed of light). This is the
distance to Saturn, and the telescope has been used to observe
and measure distances to Saturn and Titan.
Trigonometric parallax is a direct measurement technique to
determine close star distances using the Earth’s orbit around
the sun as a baseline. A nearby star appears displaced relative
to more distant reference stars when photographed or imaged six
months apart (Parallax
- Wikipedia, the free encyclopedia). The displacement
relative to the background star images can be equated to a
parallax angle, and the mean distance to the star solved by
trigonometry. Alan Hirshfeld in his wonderful book Parallax presents a detailed history of “the race to measure the cosmos.”
A shortened version of this history to measure the parallax of
the nearest stars is also found in his article in the November
2001 issue of Sky & Telescope, pages 38 –45. The brief
historical overview of parallax measurements below is derived
from these and other sources.
Soon after the Polish astronomer Nicholas Copernicus suggested
in the latter part of the 1500’s that the Earth and planets
circled the Sun rather than circling the Earth, it became
recognized that distances to the stars might be measured by
using the Earth’s orbit as a baseline. While this concept became
apparent in the 1600’s, there were many obstacles to overcome
before it was first successfully employed in the mid 1800’s.
First, it took quite some while for the concept that stars were
other suns to be recognized. Second, no one appreciated how vast
the distances to the stars were and the resultant difficulty
that would entail to obtain accurate parallax measurements.
Moreover, it was unknown whether stars were spread out at
different distances from the Earth and whether they had
differing brightness and mass.
Early attempts at parallax measurements were unsuccessful
because the available equipment was not precise enough. In the
late 1500’s the Danish astronomer nobleman Tycho Brahe was the
first to try to determine the parallax of a star, but, at best,
his pre-telescopic measuring equipment could only determine star
positions to one arc minute. In 1669 and again in the 1720’s,
Englishmen Robert Hooke and James Bradley, respectively,
attempted to determine the parallax displacement of Gamma
Draconis, a second magnitude star. They compared its position to
a nearby dimmer star they assumed to be much farther away. Both
of them made measurements over many months. Gamma Draconis was
chosen, since it is bright star, and Hooke and Bradley assumed
it is close by the Earth relative to other stars. They liked it
because it had a dim “background” star in the same telescopic
field of view convenient for their measurements. Also, Gamma
Draconis passed nearly overhead at the latitude of London where
they made their measurements.
Bradley’s results showed that the star displayed an unmistakable
shift, but the shift was opposite to what was expected!
Eventually Bradley realized that this shift was a natural
phenomenon, the aberration of light caused by the Earth’s motion through space. This was the first
direct proof the Earth moves through space rather than being a
fixed body about which all other solar system objects revolve.
Thus, while Copernicus first proposed that the solar system
objects, including the Earth, revolve around the Sun in the late
1500’s, and Galileo was punished in part for supporting the
Copernican hypothesis, proof of the Earth’s motion through space
was not obtained until the 1720’s! The aberration of light is
common to all stars, and it must be reduced out of the
calculation for determining the trigonometric parallax of a
In the late 1700’s and early 1800’s, William Herschel also
attempted to find parallax motion of a star using his “double
star” method wherein the brighter star of a double star pair was
assumed to be much closer than the dimmer one. He assumed all
stars were equally intrinsically bright.
Herschel observed and measured the very precise motion of
several hundred close pairs of bright and dim stars over more
than twenty years, but he found that most of these “double
stars” were not random alignments that could be used to
determine the parallax of the brighter star. Instead, Herschel
discovered that many were actually real double stars, which were
bound together by mutual gravitation orbiting a common center of
mass. For truly randomly aligned optical double stars,
Herschel’s observations still lacked the accuracy to
successfully determine the parallax of the brighter star.
However, his failure clearly indicated that indeed most stars
must be very far away.