Chapter 1. The Solar System
- Upon completion of this chapter, you will be able to
state distances of objects within the solar system in terms of light-time,
describe the sun as a typical star, relate its share of the mass within
the solar system, and compare the terrestrial and jovian planets. You will
be able to distinguish between inferior and superior planets, describe
asteroids, comets, and the Oort cloud. You will be able to describe the
environment in which the solar system resides.
The solar system has
been a topic of study from the beginning of history. For nearly all that
time, people have had to rely on long-range and indirect measurements of its
objects. For all of human history and pre-history, observations were based
on visible light. Then in the 20th century people discovered how to use
additional parts of the spectrum. Radio waves, received here on Earth, have
since 1931 to investigate celestial objects. Starting with the emergence
of space flight in
1957, instruments operating above Earth's obscuring atmosphere could
take advantage not only of light and radio, but virtually the whole spectrum
(the electromagnetic spectrum is the subject of a later
At last, with interplanetary travel, instruments can be carried to many
solar system objects, to measure their physical properties and dynamics
directly and at very close range. In the 21st century, knowledge of the
solar system is advancing at an unprecedented rate.
The solar system consists of an average star we
call the sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, Neptune, and Pluto. It includes the satellites of the planets,
numerous comets, asteroids, meteoroids, and the interplanetary medium, which
permeates interplanetary space. The sun is the richest source of
electromagnetic energy in the solar system. The sun's nearest known stellar
neighbor is a red dwarf star called
Proxima Centauri, at a distance of about 4.2 light years. (A light year
is the distance light travels in a year, at about 300,000 km per second.)
Our whole solar system, together with all the local stars you can see on a
clear dark night, orbits the center of our home galaxy. This spiral disk we
call the Milky Way includes some 200 billion stars, thousands of gigantic
clouds of gas and dust, and enormous quantities of mysterious dark matter.
Interstellar space is the term given to the
space between stars in the galaxy. We are beginning to find that many stars
besides the sun harbor their own planets, called extrasolar planets.
As of January 2004 astronomers have detected about
100 planets orbiting other stars. They are all giant, Jupiter-like
planets, made mostly of gas, since current detection methods cannot reveal
smaller worlds. Their formation process is still unclear.
The image at right shows a galaxy similar to the
Milky Way, known as M100 (click the image for details). The Milky Way
has two small galaxies orbiting it nearby, which are visible from the
southern hemisphere. They are called the Large
Magellanic Cloud and the Small Magellanic Cloud. Our galaxy, one of
billions of galaxies known, is travelling through intergalactic space.
On a cosmic scale, all galaxies are generally receding from each other,
although those relatively close together may exhibit additional local motion
toward or away from each other.
The sun and planets each rotate on their axes.
formed from the same rotating disk, the planets, most of their
satellites, and the asteroids, all revolve around the sun in the same
direction as it rotates, and in nearly circular orbits. The planets orbit
the sun in or near the same plane, called the ecliptic (because it is where
eclipses occur). Pluto is a
special case in that its orbit is the most highly inclined (17 degrees)
and the most highly elliptical of all the planets. Because its orbit is so
eccentric, Pluto sometimes comes closer to the sun than does Neptune. It's
interesting to note that most planets rotate in or near the plane in which
they orbit the sun, since they formed, rotating, out of the same dust ring.
Uranus must have suffered a whopping collision, though, that set it rotating
on its side.
The most common unit of measurement for distances
within the solar system is the astronomical unit (AU). The AU is based on
the mean distance from the sun to Earth, roughly 150,000,000 km. JPL's Deep
Space Network refined the
precise value of the AU in the 1960s by obtaining radar echoes from
Venus. This measurement was important since spacecraft navigation depends on
accurate knowledge of the AU. Another way to indicate distances within the
solar system is terms of light time, which is the distance light travels in
a unit of time. Distances within the solar system, while vast compared to
our travels on Earth's surface, are comparatively small-scale in
astronomical terms. For reference, Proxima Centauri, the nearest star at
about 4 light years away, is over 265,000 AU from the sun.
||~Earth-Moon Round Trip
||~Sun to Mercury
||Sun to Earth (1 AU)
||~1.5 x Sun-Jupiter Distance
||Voyager-1 (January, 2004)
||~Next closest star
Temperatures Within the Solar System
Select the above link for the Solar System Temperature Reference showing
examples and comparing temperatures of objects and conditions from absolute
zero through planet temperatures, to those of stars. The reference also
includes temperature conversion factors and links to a conversion engine.
sun is a typical star. Its
spectral classification is "G2 V." G2 basically means it's a
yellow-white star, and the roman numeral V means it's a "main sequence"
dwarf star (by far the most common) as opposed to supergiant, or sub-dwarf,
etc. You can view
current images of the sun as seen today by the suite of instruments
aboard SOHO (Solar & Heliospheric Observatory) as it views the sun from the
Lagrange point between the Earth and the sun.
The sun dominates the gravitational field of the
solar system; it contains about 99.85% of the solar system's mass. The
planets, which condensed out of the same disk of material that formed the
sun, contain only about 0.135% of the mass of the solar system. Satellites
of the planets, comets, asteroids, meteoroids, and the interplanetary medium
constitute the remaining 0.015%. Even though the planets make up only a
small portion of the solar system's mass, they do retain the vast majority
of the solar system's angular momentum. This storehouse of momentum can be
utilized by interplanetary spacecraft on so-called "gravity-assist"
Mass Distribution Within the Solar System
gravity creates extreme pressures and temperatures within itself, sustaining
a thermonuclear reaction
fusing hydrogen nuclei and producing helium nuclei. This reaction yields
tremendous amounts of energy, causing the material of the sun to be plasma
and gas. These thermonuclear reactions began about 5 x 109 years
ago in the sun, and will probably continue for another 5 x 109
years. The apparent surface of the sun has no clean physical boundary, as
solid planets do, although it appears as a sharp boundary when seen from the
distance of Earth. Click the SOHO solar image at right for more details
about the image.
The sun rotates on its axis with a period of
approximately 25.4 days. This is the adopted value at 16° latitude. Because
the sun is a gaseous body, not all its material rotates together. (This
solar fact sheet describes how rotation varies with latitude). Solar
matter at very high latitudes takes over 30 days to complete a rotation. Our
star's output varies slightly over an
11-year cycle, during which the number of sunspots changes.
The sun's axis is tilted about 7.25 degrees to the
axis of the Earth's orbit, so we see a little more of the sun's northern
polar region each September and more of its southern region in March. As
viewed from the Earth's surface, the Sun subtends roughly half a degree of
arc upon the sky (as does the Moon, at this period in cosmic time.)
The sun has strong magnetic fields that are
associated with sunspots and coronal mass ejections, CMEs (also called solar
flares). A sunspot is a relatively cool area that appears dark against the
hotter face of the sun. Sunspots are formed when magnetic field lines just
below the sun's visible surface are twisted, and reach though the
photosphere. CMEs are huge magnetic bubbles of plasma that
erupt from the sun's corona and travel through space at high speed. View
this spectacular 5-Mbyte
SOHO movie of CMEs in August, 1999.
The solar magnetic field is not uniform, and it is
very dynamic. Solar magnetic field variations and dynamics are targets of
major interest in the exploration of the solar system.
These and many other aspects of the sun are the
subjects of ongoing research.
Our Bubble of Interplanetary Space
The "vacuum" of interplanetary
space includes copious amounts of energy radiated from the sun, some
interplanetary and interstellar dust (microscopic solid particles) and gas,
and the solar wind. The solar wind, discovered by
Eugene Parker in 1958, is a flow of lightweight ions and electrons
(which together comprise plasma) thrown from the sun. The solar wind
inflates a bubble, called the heliosphere, in the surrounding
interstellar medium (ISM).
The solar wind has a visible effect on comet tails.
It flows outward from our star at about 400 km per second, measured in the
vicinity of Earth's orbit, and the
found that it approximately doubles its speed at high solar latitudes.
Diagram courtesy Dr. Gary Zank, University of Delaware
The boundary at which the solar wind meets the ISM,
containing the collective "solar" wind from other local stars in our galaxy,
is called the heliopause. This is where the solar wind and the sun's
magnetic field stop. The boundary is theorized to be roughly
teardrop-shaped, because it gets "blown back" to form a heliotail, as the
sun moves through the ISM (toward the right in the diagram above). The sun's
relative motion may also create an advance bow shock, analogous to that of a
moving boat. This is a matter of debate and depends partly on the strength
of the interstellar magnetic field.
But before it gets out to the heliopause, the solar
wind is thought to slow to subsonic speeds, creating a termination shock.
This appears at the perimeter of the green circle in the diagram. Its actual
shape, whether roughly spherical or teardrop, depends on magnetic field
strengths, as yet unknown.
In the diagram above, temperatures are theorized;
none have been actually measured beyond the termination shock. Note that
even with the high particle temperatures, their density is so low that
massive objects like spacecraft remain very cold (as long as they are
shaded, or distant, from the sun).
The white lines in the diagram represent charged
particles, mostly hydrogen ions, in the interstellar wind. They are
deflected around the
heliosphere's edge (the heliopause). The pink arrow shows how neutral
particles penetrate the heliopause. These are primarily hydrogen and helium
atoms, which are mostly not affected by magnetic fields, and there are also
heavier dust grains. These interstellar neutral particles make up a
substantial fraction of the material found within the heliosphere. The
little black + in the green area represents the location of Voyager 1 at 80
AU in January of 2001. Voyager 1 is humanity's most distant object, and in
2004 the spacecraft is returning
evidence that it is probably beginning to encounter the termination
shock now, at a distance of just over 90 AU from the Sun.
The solar wind changes with the 11-year solar
cycle, and the interstellar medium is not homogeneous, so the shape and size
of the heliosphere probably fluctuate.
The solar magnetic field is the dominating magnetic
field within the heliosphere, except in the immediate environment of planets
which have their own magnetic fields. It can be measured by spacecraft
throughout the solar system, but not here on earth, where we are shielded by
our planet's own magnetic field.
The actual properties of the interstellar medium
(outside the heliosphere), including the strength and orientation of its
magnetic field, are important in determining the size and shape of the
heliopause. Measurements that the two Voyager spacecraft will make in the
region beyond the termination shock, and possibly beyond the heliopause,
will provide important inputs to models of the termination shock and
heliopause. Even though the Voyagers will sample these regions in discrete
locations, this information will result in more robust overall models.
For further information on this vast subject and
its many related topics, search the web for "heliosphere," "Alfven waves,"
"pickup ions," and "local interstellar cloud."
Quiz-1: The Sun and Interplanetary Space
Chapter 1. The Solar System CONTINUED
The Terrestrial Planets
The planets Mercury, Venus, Earth, and Mars, are called terrestrial
because they have a compact, rocky surface like Earth's terra firma. The
terrestrial planets are the four innermost planets in the solar system. None
of the terrestrial planets have rings, although Earth does have belts of
trapped radiation, as discussed below. Only Earth has a substantial
planetary magnetic field. Mars and the Earth's Moon have localized regional
magnetic fields at different places across their surfaces, but no global
Of the terrestrial planets, Venus, Earth, and Mars have significant
atmospheres. The gases present in a planetary atmosphere are related to a
planet's size, mass, temperature, how the planet was formed, and whether
life is present. The temperature
of gases may cause their molecules or atoms to achieve velocities that
escape the planet's gravitational field. This contributes to Mercury's lack
of a permanent atmosphere, as does its proximity to the source of the
relentless solar wind.
The presence of life on Earth causes oxygen to be abundant in the
atmosphere, and in this Earth is unique in our solar system. Without life,
most of the oxygen would soon become part of the compounds on the planet's
surface. Thus, the discovery of oxygen's signature in the atmosphere of an
extrasolar planet would be significant.
lacks an atmosphere to speak of. Even though most of its surface is very
hot, there is strong evidence that water ice exists in locations near its
north and south poles which are kept permanently-shaded by crater walls.
This evidence comes from Earth-based radar observations of the innermost
planet. The discovery of permanently shaded ice at the poles of Earth's Moon
strengthens arguments that the indications of ice on Mercury may be real.
atmosphere of carbon dioxide is dense, hot, and permanently cloudy, making
the planet's surface invisible. Its best surface studies have come from
landers and imaging radar from orbiting spacecraft.
as of November 2001, is still the only place known to harbor life. And life
has flourished here since the planet was young. Our home planet is also
unique in having large oceans of surface water, an oxygen-rich atmosphere,
and shifting crustal sections floating on a hot mantle below, described by
the theory of plate tectonics.
Moon orbits the planet once every 27.3 days at an average distance
of about 384,400 km. The Moon's orbital distance is steadily increasing at
the very slow rate of 38 meters per millenium. Its distance at this point in
its history makes the Moon appear in the sky to be about the same size as
the Sun, subtending about half a degree.
atmosphere, also carbon dioxide, is much thinner than Earth's, but it
sustains wispy clouds of water vapor. Mars has polar caps of carbon dioxide
ice and water ice. The planet's surface shows strong evidence for extensive
water coverage in its distant past, as well as possible evidence for water
flow in small springs during recent times.
JPL's first spacecraft, Explorer 1, carried a single
scientific instrument devised and operated by James Van Allen and his team
from the University of Iowa. Early in 1958 the experiment discovered bands
of rapidly moving charged particles trapped by Earth's magnetic field in
toroidal, or doughnut-shaped regions surrounding the equator. The
illustration below shows these belts only in two dimensions, as if they were
sliced into thin cross-sections.
The belts that carry Van Allen's name have two areas of
maximum density. The inner region, consisting largely of protons with an
energy greater than 30 million EV, is centered about 3,000 km above Earth's
surface. The outer belt is centered about 15,000 to 20,000 km up, and
contains electrons with energies in the hundreds of millions of EV. It also
has a high flux of protons, although of lower energies than those in the
Flight within these belts can be dangerous to electronics
and to humans because of the destructive effects the particles have as they
penetrate microelectronic circuits or living cells. Most Earth-orbiting
spacecraft are operated high enough, or low enough, to avoid the belts. The
inner belt, however, has an annoying portion called the South Atlantic
Anomaly (SAA) which extends down into low-earth-orbital altitudes. The SAA
can be expected to cause problems with spacecraft that pass through it.
This table compares features of the terrestrial planets in
terms of the values for Earth. Light minutes are often used to express
distances within the region of the terrestrial planets, useful because they
indicate the time required for radio communication with spacecraft at their
distances. If you click on the planet's name at the top of the table, you'll
see a complete set of technical data for the planet, with a comparison to
Here is a more extensive table of planetary data.
|Mean distance from sun (AU)
|Light minutes from sun
|Mass (x Earth)
|Equatorial radius (x Earth)
|Orbit period (Earth years)
|Mean orbital velocity (km/s)
|Surface atmospheric pressure (bars)
|Global Magnetic field
Mean Distances of the Terrestrial Planets from Sun
Orbits are drawn approximately to scale.
Asteroids, or minor planets, orbit the Sun mostly between Mars and
Jupiter. They are covered in the
section following the Planets.
The Jovian Planets
Jupiter, Saturn, Uranus, and Neptune are known as the Jovian
(Jupiter-like) planets, because they are all gigantic compared with Earth,
and they have a gaseous nature like Jupiter's -- mostly hydrogen, with some
helium and trace gases and ices. The Jovian planets are thus referred to as
the "gas giants" because gas is what they are mostly made of, although some
or all of them probably have small solid cores. All have significant
planetary magnetic fields, rings, and lots of satellites.
Jupiter is more massive than all the other planets combined. It
emits electromagnetic energy from charged atomic particles spiraling through
its strong magnetic field. If this sizzling magnetosphere were visible to
our eyes, Jupiter would appear larger then the full Moon in Earth's sky. The
trapped radiation belts near Jupiter present a hazard to spacecraft as do
Earth's Van Allen belts, although the Jovian particle flux and distribution
differ from Earth's. Bringing a spacecraft close to Jupiter presents a
hazard mostly from ionized particles. Spacecraft intended to fly close to
Jupiter must be designed with radiation-hardened components and shielding.
Spacecraft using Jupiter for gravity assist may also be exposed to a harsh
radiation dose. Instruments not intended to operate at Jupiter must be
protected by being powered off and by having detectors covered.
the farthest planet easily visible to the unaided eye, is known for its
extensive, complex system of rings, which are very impressive even in a
small telescope. Using a small telescope one can also discern the planet's
oblateness, or flattening at the poles. Continued study of Saturn's ring
system can yield new understandings of orbital dynamics, applicable to any
system of orbiting bodies, from newly forming solar systems to galaxies.
which rotates on its side, and
are of similar size and color, although Neptune seems to have a more active
atmosphere despite its much greater distance from the sun. Both planets are
composed primarily of rock and various ices. Their extensive atmosphere,
which makes up about 15% the mass of each planet, is hydrogen with a little
helium. Neither of these planets were known to the ancients; Uranus was
discovered in 1781, Neptune in 1846.
is neither a rocky terrestrial planet nor a Jovian gas giant. It is a Kuiper
Belt Object, composed of material left over after the formation of the other
Comets in the next section). Kuiper Belt Objects were never exposed to
the higher temperatures and solar radiation levels of the inner solar
system. Pluto has large quantities of nitrogen ice, and simple molecules of
carbon, hydrogen and oxygen. They remain on Pluto as a sample of the
primordial material that set the stage for the evolution of the solar system
as it exists today, including life. Its nitrogen atmosphere will precipitate
out onto the surface as snow when its orbit takes it much farther from the
sun than it is today. Since it has not yet been visited by a spacecraft,
comparatively little is
known about this small, distant body. Objects that orbit the Sun beyond
Neptune's orbit are known as trans-Neptunian objects.
The gas giants have numerous satellites, many of which are
large, and seem as interesting as any planet. Small "new" satellites of the
Jovian planets are being discovered every few years.
Jupiter's Galilean satellites, so named because
Galileo Galilei discovered them in 1610, exhibit great diversity from each
other. All four can be easily seen in a small telescope or binoculars.
(pictured here) is the closest of these to Jupiter. Io is the most
volcanically active body in
the solar system, due to heat resulting from tidal forces (discussed further
in Chapter 3) which flex its crust. Powerful Earth-based telescopes can
observe volcanoes resurfacing Io continuously. Europa is covered with an
extremely smooth shell of water ice. There is probably an ocean of liquid
water below the shell, warmed by the same forces that heat Io's volcanoes.
Ganymede has mountains, valleys, craters, and cooled lava flows. Its ancient
surface resembles Earth's Moon, and it is also suspected of having a
sub-surface ocean. Callisto, the outermost Galilean moon, is pocked all over
with impact craters, indicating that its surface has changed little since
the early days of its formation.
Saturn's largest moon, enigmatic Titan, is larger than the planet
Mercury. Almost a terrestrial planet itself, Titan has a hazy nitrogen
atmosphere denser than Earth's. The
Huygens Probe executed a spectacurly successful mission in January 2005,
revealing rivers and lakebeds on the surface, and extensive details of its
atmosphere. Saturn also has many
smaller satellites made largely of
TITAN, FALSE COLOR
IMAGED BY CASSINI.
CLICK FOR MORE INFO.
water ice. The "front," or leading, side of Saturn's icy satellite
Iapetus is covered in dark material of some kind, and an equatorial
mountain range higher than Olympus Mons on Mars was recently discovered on
this 1450-km diameter moon. Icy Enceladus orbits within the densest part of
Saturn's E Ring, and may somehow be the source of that ring's fine
All of Uranus's five largest moons have extremely different
characteristics. The surface of Miranda, the smallest of these, shows
evidence of extensive geologic activity. Umbriel's surface is dark, Titania
and Ariel have trenches and faults, and Oberon's impact craters show bright
rays similar to those on Callisto.
Neptune's largest moon Triton is partly covered with nitrogen ice and
snow, and has currently active nitrogen geysers that leave sooty deposits on
the surface downwind.
IN FALSE COLOR
dust rings can be detected at close range in visible light and from
Earth in the infrared. They show up best when viewed from behind, in forward
Neptune all have rings made up of myriad particles of ice ranging in
size from dust and sand to boulders. Each particle in a ring is an
individual satellite of the planet in its own right. Ring particles interact
with each other in complex ways, affected by gravity and electrical charge.
They also interact with the thin extended atmospheres of the planets.
Saturn's magnificent ring system, as visible from Earth, spans about
280,000 km, yet its thickness is only around 200 meters! The A-ring,
measured at several points, was found to be only ten meters thick.
When two satellites occupy orbits very close to each other within a ring
system, one orbiting farther from the planet than a ring, and the other one
orbiting closer to the planet than that ring, they confine particles between
their orbits into a narrow ring, by gravitationally interacting with the
ring particles. Thus these satellites are called shepherd moons.
Jovian Planetary Data (Approximate)
|Mean distance from sun (AU)
|Light hours from sun
|Mass (x Earth)
|Radius (x Earth)
|Rotation period (hours)
|Orbit period (Earth years)
|Mean orbital velocity (km/s)
|Known natural satellites (2004)
||Broken ring arcs
Orbits are drawn approximately to scale.
Pluto omitted to accommodate scale.
Mercury and Venus are referred to as inferior
planets, not because they are any less important, but because their orbits
are closer to the sun than is Earth's orbit. They always appear close to the
sun in Earth's morning or evening sky; their apparent angle from the sun is
called elongation. The outer planets, Mars, Jupiter, Saturn, Uranus,
Neptune, and Pluto are all known as superior planets because their
orbits are farther from the sun than the Earth's.
|Planet or Moon appears in crescent phase when nearly
between observer and sun.
Phases of Illumination
Inferior planets may pass between the Earth and the sun on part of their
orbits, so they can exhibit nearly the complete range of phases from
the earth's point of view... from the dark "new" phase, to slim "crescent"
phase, to the mostly lit "gibbous" phase (approximating the fully
illuminated "full" phase when approaching the other side of the sun). Our
own Moon, of course, exhibits all the phases. Superior planets, though,
usually appear gibbous, and appear full only when at opposition (see below),
from our earthly point of view.
Viewed from superior planets, Earth goes through phases. Superior planets
can be seen as crescents only from the vantage point of a spacecraft that is
Conjunction, Transit, Occultation, Opposition
When two bodies appear to pass closest together in the sky, they are said
to be in conjunction. When a planet passes closest to the sun as seen
from Earth and all three bodies are approximately in a straight line, the
planet is said to be in solar conjunction. The inferior planets Venus and
Mercury can have two kinds of conjunctions with the Sun: (1) An inferior
conjunction, when the planet passes approximately between Earth and Sun
(if it passes exactly between them, moving across the Sun's face as seen
from Earth, it is said to be in transit); and (2) A superior
conjunction when Earth and the other planet are on opposite sides of the
Sun and all three bodies are again nearly in a straight line. If a planet
disappears behind the sun because the sun is exactly between the planets, it
is said to be in occultation.
Superior planets can have only superior conjunctions with the sun. At
superior conjunction the outer planet appears near its completely
illuminated full phase.
The planet is said to be at opposition to the sun when both it and
the Earth are on the same side of the sun, all three in line. (The Moon,
when full, is in opposition to the sun; the Earth is then approximately
Opposition is a good time to observe an outer planet with Earth-based
instruments, because it is at its nearest point to the Earth and it is in
its fullest phase.
Inferior planets can never be at opposition to the sun, from Earth's
point of view.
Occultations, transits, conjunctions, and oppositions offer special
opportunities for scientific observations by spacecraft. Studies of the
solar corona and tests of general relativity can be done at superior
conjunctions. Superior conjunctions also present challenges communicating
with a spacecraft nearly behind the sun, which is overwhelmingly noisy at
the same radio frequencies as those used for communications. At opposition,
such radio noise is at a minimum, presenting ideal conditions for
gravitational wave searches. These special opportunities and challenges are
further discussed in later chapters.
Chapter 2. Reference Systems
- Upon completion of this chapter you will be able to
describe the system of terrestrial coordinates, the rotation of Earth,
precession, nutation, and the revolution of Earth about the sun. You will
be able to describe how the locations of celestial objects are stated in
the coordinate systems of the celestial sphere. You will be able to
describe the use of epochs and various conventions of timekeeping.
Spatial coordinates and
timing conventions are adopted in order to consistently identify
locations and motions of an observer, of natural objects in the solar
system, and of spacecraft traversing interplanetary space or orbiting
planets or other bodies. Without these conventions it would be impossible to
navigate the solar system.
A great circle is an imaginary circle on the
surface of a sphere whose center is the center of the sphere. Great circles
that pass through both the north and south poles are called meridians, or
lines of longitude. For any point on the surface of Earth a meridian
can be defined.
The prime meridian, the starting point measuring
the east-west locations of other meridians, marks the site of the old Royal
Observatory in Greenwich, England. Longitude is expressed in degrees,
minutes, and seconds of arc from 0 to 180 degrees eastward or westward from
the prime meridian. For example, downtown Pasadena, California, is located
at 118 degrees, 8 minutes, 41 seconds of arc west of the prime meridian:
118° 8' 41" W.
starting point for measuring north-south locations on Earth is the equator,
a great circle which is everywhere equidistant from the poles. Circles in
planes parallel to the equator define north-south measurements called
parallels, or lines of latitude. Latitude is expressed as an arc
subtended between the equator and the parallel, as seen from the center of
the Earth. Downtown Pasadena is located at 34 degrees, 08 minutes, 44
seconds latitude north of the equator: 34° 08' 44" N.
| Throughout the history of navigation,
determining one's latitude on the Earth's surface has been relatively
easy. In the northern hemisphere for example, simply measuring the
height of the star Polaris above the horizon results in a fairly close
approximation of one's latitude. Measurement of longitude, however, has
been a historically siginificant
endeavor, since its determination requires accurate timekeeping.
John Harrison (1693-1776) eventually succeeded in developing a
chronometer good enough to do the trick.
One degree of latitude equals approximately 111 km
on the Earth's surface, and by definition exactly 60 nautical miles. Because
meridians converge at the poles, the length of a degree of longitude varies
from 111 km at the equator to 0 at the poles where longitude becomes a
Terrestrial Coordinates Grid
Rotation and Revolution
"Rotation" refers to an object's spinning motion
about its own axis. "Revolution" refers the object's orbital motion around
another object. For example, Earth rotates on its own axis, producing
the 24-hour day. Earth revolves about the Sun, producing the 365-day
year. A satellite revolves around a planet.
The Earth rotates on its axis relative to the sun
every 24.0 hours mean solar time, with an inclination of 23.45 degrees from
the plane of its orbit around the sun. Mean solar time represents an average
of the variations caused by Earth's non-circular orbit. Its rotation
relative to "fixed" stars (sidereal time) is 3 minutes 56.55 seconds shorter
than the mean solar day, the equivalent of one solar day per year.
Precession of Earth's Axis
Forces associated with the rotation of Earth cause
the planet to be slightly oblate, displaying a bulge at the equator. The
moon's gravity primarily, and to a lesser degree the sun's gravity, act on
Earth's oblateness to move the axis perpendicular to the plane of Earth's
orbit. However, due to gyroscopic action, Earth's poles do not "right
themselves" to a position perpendicular to the orbital plane. Instead, they
precess at 90 degrees to the force applied. This precession causes
the axis of Earth to describe a circle having a 23.4 degree radius relative
to a fixed point in space over about 26,000 years, a slow wobble reminiscent
of the axis of a spinning top swinging around before it falls over.
Because of the precession of
the poles over 26,000 years, all the stars, and other celestial objects,
appear to shift west to east at the rate of .01 degree each year (360
degrees in 26,000 years). This apparent motion is the main reason for
astronomers as well as spacecraft operators to refer to a common epoch such
At the present time in Earth's
26,000 year precession cycle, a bright star happens to be very close, less
than a degree, from the north celestial pole. This star is called Polaris,
or the North Star.
Stars do have their own real
motion, called proper motion. In our vicinity of the galaxy, only a
few bright stars exhibit a large enough proper motion to measure over the
course of a human lifetime, so their motion does not generally enter into
spacecraft navigation. Because of their immense distance, stars can be
treated as though they are references fixed in space. (Some stars at the
center of our galaxy, though, display tremendous speeds as they orbit close
to the massive black hole located there.)
Superimposed on the
26,000-year precession is a small nodding motion with a period of 18.6 years
and an amplitude of 9.2 arc seconds. This nutation can trace its
cause to the 5 degree difference between the plane of the Moon's orbit, the
plane of the Earth's orbit, and the gravitational tug on one other.
Revolution of Earth
Earth revolves in orbit around the sun in 365 days,
6 hours, 9 minutes with reference to the stars, at a speed ranging from
29.29 to 30.29 km/s. The 6 hours, 9 minutes adds up to about an extra day
every fourth year, which is designated a leap year, with the extra day added
as February 29th. Earth's orbit is elliptical and reaches its closest
approach to the sun, a perihelion of 147,090,000 km, on about January fourth
of each year. Aphelion comes six months later at 152,100,000 km.
Because we make observations from Earth, knowledge
of Earth's natural motions is essential. As described above, our planet
rotates on its axis daily and revolves around the sun annually. Its axis
precesses and nutates. Even the "fixed" stars move about on their own.
Considering all these motions, a useful coordinate system for locating
stars, planets, and spacecraft must be pinned to a single snapshot in time.
This snapshot is called an epoch.
By convention, the epoch in use today is called
J2000.0, which refers to the mean equator and equinox of year 2000,
nominally January 1st 12:00 hours Universal Time (UT). The "J" means Julian
year, which is 365.25 days long. Only the 26,000-year precession part of the
whole precession/nutation effect is considered, defining the mean
equator and equinox for the epoch.
The last epoch in use previously was B1950.0 - the
mean equator and equinox of 1949 December 31st 22:09 UT, the "B" meaning
Besselian year, the fictitious solar year introduced by F. W. Bessell in the
nineteenth century. Equations are published for interpreting data based on
past and present epochs.
Given an understanding of the
Earth's suite of motions -- rotation on axis, precession, nutation, and
revolution around the sun -- and given knowledge of an observer's location
in latitude and longitude, meaningful observations can be made. For example,
to measure the precise speed of a spacecraft flying to Saturn, you have to
know exactly where you are on the Earth's surface as you make the
measurement, and then subtract out the Earth's motions from that measurement
to obtain the spacecraft's speed. The same applies if you are trying to
measure the proper motion of a distant star -- or a star's subtle wobble, to
reveal a family of planets.
Chapter 2. Reference Systems
A useful construct for describing locations of objects
in the sky is the celestial sphere. The center of the earth is the center of
the celestial sphere. The figure at right illustrates that the sphere's
poles and equator are analogs of the corresponding constructs on the surface
of the Earth. We can specify precise location of objects on the celestial
sphere by giving the celestial equivalent of their latitudes and longitudes.
The point on the celestial sphere directly overhead for
an observer is the zenith. An imaginary arc passing through the
celestial poles and through the zenith is called the observer's meridian.
The nadir is the direction opposite the zenith: for example, straight
down from a spacecraft to the center of the planet.
Declination (DEC) is the celestial sphere's
equivalent of latitude and it is expressed in degrees, just like latitude.
For DEC, + and - refer to north and south. The celestial equator is 0° DEC,
and the poles are +90° and -90°.
Right ascension (RA) is the celestial equivalent of
longitude. RA can be expressed in degrees, but it is more common to specify
it in hours, minutes, and seconds: the sky appears to turn 360° in 24 hours,
and that's 15° in an hour. An hour of RA is 15° of sky rotation.
Another important feature intersecting the celestial sphere is the
ecliptic plane. This is the plane in which the Earth orbits the sun,
23.4° from the celestial equator. Looking at the ecliptic, the great circle
marking the intersection of the ecliptic plane on the celestial sphere, is
where the sun and planets appear to travel, and it's where the Sun and Moon
are during eclipses (that's where the plane and circle get their names).
The zero point for RA is one of the points where the
ecliptic circle intersects the celestial equator circle. It's defined to be
the point where the sun crosses into the northern hemisphere beginning
spring: the vernal equinox, also known as the first point of Aries.
|The equinoxes are times at which the center of the Sun is
directly above the equator, marking the beginning of spring and autumn.
The day and night would be of equal length at that time, if the Sun were
a point and not a disc, and if there were no atmospheric refraction.
Given the apparent disc of the Sun, and the Earth's atmospheric
refraction, day and night actually become equal at a point within a few
days of each equinox.
The RA and DEC of an object specify its position uniquely
on the celestial sphere just as the latitude and longitude do for an object
on the Earth's surface. For example, the very bright star Sirius has
celestial coordinates 6 hr 45 min RA and -16° 43' DEC.
The discussion gets a little more involved here, but this
section serves only to explain the old design for Deep Space Network
antennas, as well as large optical and radio telescopes, and why it all
changed not too long ago.
Before you can use RA and DEC to point to an object in the
sky, you have to know where the RA is at present for your location, since
the Earth's rotation continuously moves the fixed stars (and their RA) with
respect to your horizon. If the RA of the object happens to place it
overhead on your meridian, you're fine. But it probably isn't, so you
determine the object's hour angle (HA), which is the distance in
hours, minutes, and seconds westward along the celestial equator from the
observer's meridian to the object's RA. In effect, HA represents the RA for
a particular location and time of day. HA is zero when the object is on your
Older radio telescopes were designed with one mechanical
axis parallel to Earth's axis. To track an interplanetary spacecraft, the
telescope would point to the spacecraft's known HA and DEC, and then for the
rest of the tracking period it would simply rotate in HA about the tilted
axis (called its polar axis), as
the Earth turns. This kind of mounting is traditionally called an equatorial
mount when used for optical telescopes. It's a fine mount for a small
instrument, but unsuited to very heavy structures because the tilted polar
bearing has to sustain large asymmetric loads. These loads include not only
the whole reflector dish, but also an HA counterweight heavy enough to
balance the antenna, the DEC bearing, and its DEC counterweight! Also the
structure has to be designed specifically for its location, since the polar
bearing's angle depends on the station's latitude. This image shows the
first Deep Space Network (DSN) antenna installed at the Canberra, Australia
site, looking down along the polar bearing, which is the axis of the
antenna's large central wheel. This HA-DEC antenna is no longer in service,
nor is its sister at the DSN site at Madrid, Spain. Its counterpart at the
Goldstone, California site has been converted to a radio telescope dedicated
educational use. Click the image for an enlarged and annotated view of
its complex design.
A simpler system was needed for larger Deep Space
Network antennas. The solution is an azimuth-elevation configuration.
The design permits mechanical loads to be symmetric, resulting in less
cumbersome, less expensive hardware that is easier to maintain. It locates a
point in the sky by elevation (EL) in degrees above the horizon, and azimuth
(AZ) in degrees clockwise (eastward) from true north. These coordinates are
derived from published RA and DEC by computer programs. This computerization
was the key that permitted the complex mechanical structures to be
In an AZ-EL system anywhere on Earth, east is 90 degrees
AZ, and halfway up in EL or altitude (ALT) would be 45 degrees. AZ-EL and
ALT-AZ are simply different names for the same
reference system, ALTitude being the same measurement as ELevation.
The image at right shows a DSN antenna at Goldstone that
has a 70-meter aperture, over twice that of the Australian HA-DEC antenna
shown above. In the picture it is pointing to an EL around 10°. The EL
bearing is located at the apex of the triangular support visible near the
middle right of the image. The whole structure rotates in AZ clockwise or
counterclockwise atop the large cylindrical concrete pedestal. It is
pointing generally east in the image (around 90° azimuth), probably
beginning to track a distant spacecraft as it rises over the desert horizon.
All newly designed radio telescopes use the AZ-EL system.
To complete our survey of mounting schemes for DSN antennas
(including steerable non-DSN radio telescopes as well), we need to describe
the X-Y mount. The X-Y mount has two perpendicular axes. By examining the
image of DSS16 here (click the image for a larger view) you can see that it
cannot directly swivel in azimuth as can the AZ-EL (ALT-AZ) -mounted
antenna. But the X-Y mount has an advantage over AZ-EL.
The advantage is a matter of keyholes. To compare, picture an
AZ-EL antenna like the 70-m DSS in the section above. If a spacecraft were
to pass directly overhead, the AZ-EL antenna would rise in elevation until
it reached its straight-up maximum near 90°. But then the antenna would have
to whip around rapidly in azimuth as the spacecraft is first on the east
side of the antenna, and then a moment later is on the west. The antennas'
slew rate isn't fast enough to track that way, so there would be an
interruption in tracking until acquiring on the other side. (AZ-EL antennas
in the DSN aren't designed to bend over backwards, or "plunge" in
For an HA-DEC antenna, the keyhole is large and, in the northern
hemisphere, is centered near the north star. To track through that area the
antenna would have to whip around prohibitively fast in hour angle.
The X-Y antenna is mechanically similar to the old HA-DEC antenna, but
with its "polar" axis laid down horizontally, and not necessarily aligned to
a cardinal direction. The X-Y antenna is situated so that its keyholes (two
of them) are at the eastern and western horizon. This leaves the whole sky
open for tracking spacecraft without needing impossibly high angular rates
around either axis... it can bend over backwards and every which way. The
X-Y's were first built for tracking Earth-orbiting spacecraft that require
high angular rates and overhead passes. Earth-orbiters usually have an
inclination that avoids the east and west keyholes, as well. Interplanetary
spacecraft typically do not pass overhead, but rather stay near the ecliptic
plane in most cases. Of course X-Y's can be used with interplanetary
spacecraft also, but in the DSN they are only equipped with a 26-m aperture,
smaller than most other DSN stations, and thus not useful for most