ASTR 1230 (O'Connell) Lecture Notes
4. SOLAR SYSTEM ASTRONOMY
Saturn imaged with a 14" amateur telescope
by Damian Peach.
The Solar System consists of the Sun, 8 planets, a number of "dwarf
planets," over 160 satellites, and a thin scattering of
rocky or icy planetoids, comets, dust, and gas. The Sun is the
dominant object, being 1000 times more massive than the next largest
object (Jupiter). By terrestrial standards, the density of matter in
the Solar System is extremely low, and the planets are
separated by enormous gaps.
Other than the Sun, no solar system object is self-luminous (at
visible wavelengths), and all shine by reflected sunlight.
From the Earth, the second and third-brightest Solar System objects
are the Moon and Venus. Other than the Sun, the Moon, and some
comets, no Solar System object is resolvable with the naked eye---all
appear instead to be point sources of light. So, real exploration of
the nature of the planets and interplanetary denizens required the use
Many interesting features of the Solar System can be observed with the
naked eye, binoculars, and small telescopes, and this lecture is
aimed at exploring some of these.
B. SOLAR SYSTEM MOTIONS
For most of human history, "astronomy" consisted mostly of naked-eye
studies of the motions of solar system bodies. We will use
the Starry Night planetarium
software to simulate the appearance of the sky over many years and
illustrate the apparent motions of the Sun, Moon, and
planets. We call these "apparent" motions, because they can
be produced by motions of the Earth, which carries the observers (us),
as well as by motions of the objects themselves.
- The stars are the backdrop, or "reference frame," against which
we judge motions.
- The Sun moves about 1 degree eastward each day with
respect to the stars and takes 365.25 days to come back to the same
position against the stars.
The Sun's annual path through the stars
is always the same and is called the ecliptic. The set of
constellations through which the ecliptic passes is called the
The ecliptic path is tilted 23.5 degrees with respect to the
celestial equator, so the Sun's distance from the equator ranges from
0 degrees to 23.5 degrees throughout the year (illustrated here). This produces a
maximum range in altitude from the southern horizon at transit (i.e.
local noon) of 47 degrees. The consequent large change in the
daily solar heat input at a given latitude (see this
example) drives the "seasons".
Times when the Sun crosses the equator are called equinoxes
and times when the Sun is at maximum distance from the equator are
At the equinoxes, the hours of daytime
(when the Sun is above the horizon) and nighttime are 12 hours each
for all latitudes on Earth. The vernal and autumnal equinoxes occur
at about March 21 and September 21, respectively.
At the solstices, we have the longest periods of daytime/nighttime,
and the most extreme dependence of sunlit time on latitude. The
summer and winter solstices occur at about June 21 and December 21,
respectively. (Labeling is based on the seasons in the northern
hemisphere; seasons are, of course, reversed in the southern
- The Moon moves about 13 degrees eastward each day with
respect to the stars and takes 27.3 days to come back to the same
position against the stars. On average, the rise/set times of the
Moon advance by about 50 minutes each day. The Moon's path is tilted 5
degrees from the ecliptic.
During its monthly circuit, the Moon exhibits
drastic changes in apparent shape, from crescent to round and
back. The shapes are called phases of the Moon. The phases
repeat after a period of 29.5 days. Click here for a
- There are 5 easily visible objects in the sky other than the Sun
and Moon which exhibit significant motions with respect to the stars.
These are the brighter planets (the others were telescopic
discoveries). Although not as fast as the solar and lunar
motions. their motions are considerably more complex:
- The general motion of the planets with respect to the stars is
eastward in the sky.
- The speed of the motions depends on the planet, decreasing from
rapid to slow in the order: Mercury, Venus, Mars, Jupiter, Saturn.
- Mercury and Venus never move very far from the Sun and appear
to move back and forth in front of/behind it.
- At least once per year, each of the planets halts its eastward
motion and loops backward to the west for a brief period before
starting to move eastward again. This backward loop is
called retrograde motion.
- The planets are confined to a relatively narrow band on
the sky that is roughly centered on the ecliptic but not
coincident with it.
The planets are always to be found in
the Zodiacal constellations.
C. GEOMETRY OF THE EARTH'S ORBIT
The apparent annual motion of the Sun is caused by the fact that we
are observing it from the Earth, which is a planet moving in
orbit around the Sun.
Earth's orbit is nearly circular (distance to the Sun varies
only 3.4%), with a mean radius of 150,000,000 km or 93,000,000
miles. The mean radius is defined to be the Astronomical
Its orbit lies in a plane (i.e. seen edge-on it would look like
a thin line), and it orbits the Sun in 365.25 days (one year).
Its motion is counterclockwise as seen from above the N pole
Stars visible at night are those "opposite" the Sun. See figures
above (warning! these are grossly distorted in scale!). The night
side of Earth is that opposite the Sun. So, in May, the constellation
Scorpio will be prominent in the night sky, while in November, it lies
in the direction of the Sun and therefore is not visible because of
the atmospheric glare.
The Earth's motion around the Sun is counterclockwise in the drawings
above. This produces an apparent eastward angular displacement
or "motion" of the Sun against the stellar reference frame as seen
from the Earth.
The origins of the motions of the Moon and planets are described below.
Moon seen together from a spacecraft (click for an enlargement).
D. THE MOON
The Moon is the Earth's only natural satellite. Although it has only 1/4
the diameter of Earth, it is the largest satellite with respect to its
primary of any in the Solar System except for Pluto's moon Charon.
PHASES OF THE MOON
- Lunar phases had considerable practical
consequences in pre-industrial societies that had to rely on the Moon
for nighttime illumination.
- Although it seems bright, the Moon's surface is actually very
unreflective (see the image above comparing the Earth and
Moon). Its reflectivity, or "albedo," is only about 10%.
Nonetheless, it is close enough to us to produce a large amount of
light, and it is the second brightest object in the sky after the Sun.
- The phases of the Moon are a shadow effect originating from
the facts that it is illuminated from the direction of the Sun but we view
the Moon from different perspectives as it moves in its orbit
- The Moon's sidereal orbital period around Earth with
respect to the stars is 27.3 days. This produces the daily changes
in its rise/set times.
- Because Earth moves in its orbit around the Sun, the Moon does
not return to the same position with respect to the Sun as viewed from
Earth (e.g. 180 degrees away from Sun) for 29.5 days. This is called
its synodic period and defines (roughly) our standard "month."
See illustration here
- The phases were understood as early as 500 BC by the Greeks. The
key clue is that the phase of the Moon correlates with its angular
distance from the Sun. For instance, the Moon is in its crescent
phase when it is near the Sun in the sky but full when it is opposite
the Sun. See this illustration of lunar phases viewed at sunset
throughout the month.
- The Greeks realized this implies the Moon is a solid
sphere, in orbit about the Earth, half of which is always illuminated
by the Sun. The situation is shown in the figure below and in this Java
- In the figure above you are looking down on the Earth's North
Pole. The Earth spins counterclockwise (in 24 hours), and the
Moon likewise orbits counterclockwise (in 29.5 days). The fraction of
the Moon's sunlit hemisphere which we can see from Earth determines
the lunar phase at any time.
- We see a "full," "crescent," or dark
("new") Moon depending on the angle between the Sun and Moon as
viewed from Earth. An alternative version of the diagram above, with
photographs of the Moon's appearance at each phase, is available here.
- The diagram shows that a first quarter Moon is
90o away (east) from the Sun in the sky and that a full
Moon is 180o away from the Sun. The Moon will be in a
crescent phase when is it less than 90o away from the Sun
and a gibbous phase when between 90o and
- You can also use the figure above to determine the time of
day when the Moon in a given phase will rise, transit, or set.
Use the concept of the horizon plane, and note that the
marked positions A, B, C, and D correspond to places where observers
experience noon, sunset, midnight, and sunrise, respectively.
- For instance, drawing a horizon plane at B (a flat line,
just touching the Earth's surface at the observer's position)
allows you to infer that a first quarter Moon will transit
at sunset; a full Moon rises at sunset; and a waning gibbous Moon is
not visible at sunset (below the horizon and not yet risen).
time lapse movie (472K) composed of still
photographs of the Moon during a 29.5 day cycle vividly illustrates
the relationship between shadowing and phases. The changes in the
apparent size of the Moon and the slight "rocking" motion (known as
libration) are caused by the fact that the lunar orbit is
not exactly circular in shape.
The Moon repeats its phases after its
synodic period of 29.5 days. This means the phases are almost, but not
quite, synchronous with our calendrical months (of 28, 30, or 31 days).
The phase of the moon on a given day of the month therefore shifts
systematically throughout the year and from one year to the next.
GRAVITATIONAL EFFECTS OF THE MOON
Because of its relatively large mass and proximity to Earth, the Moon
has two important gravitational effects on Earth:
The strong gravitational effect of the Earth on the Moon has caused
the Moon's spin period to become locked to its orbital period. That
is, it spins once on its rotation axis during one orbit around Earth.
This means that the same face of the Moon is always turned toward
Earth. If it were not for this gravitational locking, we would be
able to regularly see both hemispheres of the Moon.
- Tides. The gravity of the Moon combined with the Sun is
responsible for the tides in the ocean.
- Precession. A wobble in the orientation of the rotation axis of
the Earth that produces a long-term change in the direction of its
polar axis with respect to the stars and hence a change in the RA and
DEC of objects in the sky.
Eclipses are shadow effects.
There are two types: lunar eclipses, in which the shadow of the
Earth strikes the Moon, and solar eclipses, in which the shadow
of the Moon strikes the Earth. Both can be dramatic and beautiful
events, for properly situated observers on Earth.
Full Moon (extract from composite exposure). Click for entire
SURFACE OF THE MOON
The Moon is the only "planetary" surface which can be examined
in detail through a small telescope, and it is a fascinating
study. Galileo's small telescopes (1609) first revealed the Moon's
- The Moon has no atmosphere, so there is no obscuration of its
surface features. More importantly, these are not subject to
weathering. The Moon's surface has been shaped over 4.5 billion years
by the relentless infall of asteroids, meteoroids, and smaller
interplanetary debris. Almost all of its geology is related to
- The numerous craters (up to 150 mi diameter) are the best
indicators of impacts. Unlike on the Earth, almost none of these are
related to volcanic activity. The mountains on the Moon (drawing at
right), which range up to 25,000 feet, are also impact effects, not
the products of plate tectonics as on Earth. Similar scars from
impacts cover the other solid surfaces in the solar system (Earth
and a few outer planet satellites excepted).
- The rounded, dark grey areas (part of the "man in the Moon" face)
are called maria ("seas"), even though we now know they
contain no water. They are the products of massive impacts by
asteroidal bodies which were later filled in by dark lava. They have
smooth surfaces except for a few craters. These regions are younger
than the lighter grey, rougher "highland" regions. Ages can be
estimated by the crater density (fewer craters implies younger
- Click here for more
illustrations and descriptions of lunar topography.
E. PLANETARY ORBITS
- The planets accumulated from the flattened band of solid debris
that surrounded the Sun as it formed. Their orbits consequently lie
in almost, but not quite, the same (physical, 3D) plane.
- The picture above shows an oblique view
of the planetary orbits to scale (though the planet sizes shown are
not to scale). Here is an
edge-on plot of the orbits showing the near-coincidence of the
- The Earth's orbit defines the ecliptic path on the sky. Because
most planetary orbits are only slightly inclined with respect to
Earth's, they will always be observed in a relatively narrow band
in the sky, centered on the ecliptic. They therefore move through
the Zodiacal constellations.
- The apparent motions of the planets in the sky are determined by
orbital geometry and are a combination of the intrinsic motion of the
planets and the motion of the Earth.
- See the illustration above. All planets move in the same
direction around the Sun (counterclockwise as seen from above the
Earth's North Pole). Planets nearer the Sun move faster
in their orbits and have shorter orbital periods. (Historically, this
was an important clue to the nature of gravity as deduced by Newton.)
Right panel: As viewed from the Earth, the two planets inside
the Earth's orbit ("inferior" planets) will never appear at large
angles from the Sun. Mercury and Venus always stay within
27o and 48o, respectively, of the Sun.
Left panel: The planets outside Earth's orbit ("superior"
planets), starting with Mars, can be seen at up to 180o
from the Sun. At that point they transit at midnight and are said to
be at "opposition" with respect to the Sun. As the figure
shows, planets at opposition are also nearest the Earth then
and are therefore brightest.
These angular relationships are called the planetary
- The planetarium simulation in the image below shows the concentration
of the planetary orbits, as seen from Earth, to the ecliptic.
Time lapse exposure of a planetarium simulation
of several years
of planetary motions as seen from Earth.
- The loops in the trajectories in the image above are caused not
by the planet's motion but by the Earth's annual motion around
the Sun. Planetary motions are generally eastward, with the
"retrograde loop" caused by the Earth's motion superposed. The
retrograde motion for a superior planet is greatest at opposition.
is an animation showing how retrograde motion is produced.
Planets to correct relative scale (though
F. OBSERVING THE PLANETS
Three Kinds of Planets: What a mess! Those
who were watching the news in the summer of 2006 will know that
astronomers held a debate over the meaning of the term
"planet"---specifically whether or not Pluto and the several other
newly discovered distant objects that are similar to Pluto should be
placed in a separate category. In the end, the International
Astronomical Union voted to create a new category of "dwarf
planet" for these latter objects. All this was handled very
clumsily, and it generated needless controversy.
Including this new category, there are three types of planets:
terrestrial planets, Jovian planets, and dwarf
All the non-dwarf planets except Uranus and Neptune are easily visible
to the naked eye. With your 8-in telescopes, you can also observe
Uranus (5.5 mag) and Neptune (7.8 mag). Pluto is 14.9 mag, and is
visible only in larger telescopes. Ceres is a relatively easy target
for your telescopes (even though it is smaller than Pluto and other
ice dwarfs, it is much nearer). Venus and Mercury can be
observed in daylight.
here for sketches of the appearance of the planets in small
MERCURY: Hard to observe only because
it is always near the Sun and never very far from the horizon at
night. Surface features are too subtle to be detected in a small
telescope. Like Venus, shows phases.
VENUS: Dazzling white in the sky. Can
be astonishingly bright and is the source of more UFO reports than any
other astronomical object. [Reality check: Watch for 5 minutes; is
the "UFO" stationary with respect to the stars? Is it within about
40o of the western or eastern horizon? Is it in a Zodiacal
constellation? If yes, then it's probably Venus.]
- Terrestrial Planets (archetype Earth; Mercury, Venus, Mars):
These are relatively small planets with solid, rocky bodies and
thin or absent atmospheres. The rocky material is made of mainly of
silicon, oxygen, iron and similar elements. Even though Venus'
atmosphere is 100 times thicker than Earth's, this still counts as
- Jovian Planets (archetype Jupiter; Saturn, Uranus, Neptune):
Also called "giant" planets, these are primarily made of hydrogen and
helium, with a thin smattering of heavier elements. They are much
larger than Earth. They have enormous atmospheres
with denser but slushy interiors. They may have rocky or
icy cores, similar in size to the Earth, but they have no well defined
boundary between their atmospheres and their interiors.
- Dwarf Planets (archetypes Ceres, Pluto; dozens of
others): any other object in orbit around the Sun and massive enough
for self-gravity to make it roughly round. Pluto-like dwarfs are
primarily made of ice, with some additional rocky material.
Ceres-like dwarfs are primarily made of rocky/metallic
material. There are probably thousands of Pluto-like dwarfs in the
outer solar system.
MARS: Undergoes large changes in distance,
and consequently apparent size & brightness, from Earth. Brightest at
opposition (once every 2.1 years); but because of its relatively
elliptical orbit, its distance at opposition can vary by a factor of
two (see diagram).
- Venus is the planet nearest Earth and has the orbital period most
closely matching Earth's. Consequently, it can "linger" near the
horizon before sunrise or after sunset, undergoing a complex set of
motions over several month's time. See our Starry Night
- Unfortunately, Venus is shrouded in dense clouds (made of
sulfuric acid droplets!), and you cannot observe its surface.
However, it shows pronounced phases, like the Moon's (see
illustration at right) as it orbits the Sun. The geometry is shown here. Neither
Venus nor Mercury have satellites.
- Click here for a Java animation of the relative motion of
Earth and Mars. At opposition, it can be brighter than Jupiter. In
August 2003, Mars was closer to Earth than at any time since 57,617 BC
(34,646,418 miles distant). The opposition in January 2010 was much
less favorable (62 million miles), but Mars was still quite bright at
-1.3 mag. Oppositions until 2016 will also be relatively distant.
- Mars' atmosphere is primarily CO2 and is
transparent. Its surface color is conspicuously red-pink
(hence its association with the God of War), caused by iron oxide
compounds = rust on its surface. Mars has been explored with ever
increasing resolution by Earthbound telescopes, orbiting spacecraft,
and lander spacecraft. Use the links below to reach the large and
beautiful set of spacecraft images of Mars.
- Mars is distant enough that even at opposition telescopes on
Earth yield relatively poor resolution (especially since they must
contend with seeing), and this led to a long controversy over whether
or not there was evidence for "canals" or other artificial features on
its surface. (More details given here.)
- However, under good conditions with an 8-in telescope, you can
easily see the polar caps (some water but mainly frozen
CO2) and contrasting red and grey markings on the surface.
Monitoring these features over several months will reveal slow
changes, including growth or shrinking of the caps with the seasons
and effects of dust storms on the surface, especially in the Martian
spring. The image at the right was taken by amateur astronomer
Antonio Cidadao with a 10-in telescope.
JUPITER: A very bright, yellowish object,
normally the fourth brightest in the sky (after the Sun, Moon, and
Venus). Celestial motion is much slower than any of the planets
SATURN: Famous as the ringed planet,
though all four gas giants actually have rings.
- Unlike the four terrestrial planets, Jupiter, Saturn, Uranus, and
Neptune are gas giants and probably have no sharp boundary between
interior and atmosphere. Through your telescopes, what you will see
is the top of their cloud layers. The banded structures in Jupiter's
atmosphere, called "belts" (dark) and "zones" (light), are multiple cloud
layers, and an 8-in telescope often reveals beautiful details. The
red spot is an oval-shaped, perpetual cyclone in the
atmosphere, about 3 times the diameter of Earth (seen near the
limb in the picture at right). Because Jupiter rotates in only
10 hours, you get to see a variety of features in just a few hours.
- Jupiter has an extensive satellite system, consisting of
63 known moons, mostly small. The four largest of these (Io, Europa,
Ganymeade, Callisto) were discovered by Galileo and are known as
satellites. They are easy to see in a small telescope, and
their relatively rapid orbital motions around Jupiter can be readily
tracked. NASA's Voyager
missions revealed astonishing differences in surface constitution
among the four. In small telescopes, unfortunately, no surface
details are apparent.
URANUS and NEPTUNE: All of the above
planets were known to naked eye astronomers. The others are
products of the telescopic age (Uranus was discovered in 1781).
Uranus and Neptune are distant enough that they show only small
blue-green disks in an 8-in telescope, without further details being
visible (they have very low contrast atmospheres even seen close up).
You will need a finding chart to locate them. Their satellites are
too faint for detection in the 8-in scopes.
- Its cloud layers are deeper within its atmosphere than are Jupiter's,
so it typically shows only faint surface banding and subtle
- The rings are
orbiting chunks of rock and ice and lie exactly in the equatorial
plane of the planet. They are spectacular in small scopes, and a fair
amount of substructure, especially the dark "Cassini Division" seen in
the image at the top of this page, is visible.
hundreds of ringlets. A beautiful mosaic of the rings from the
Cassini orbiter is available at
this web site.
- Six of Saturn's 62 satellites would be visible in an 8-in telescope.
Titan, Saturn's largest moon, is the only moon to have its own
atmosphere. It is the main target of the
Cassini-Huygens Mission, now in orbit around Saturn. The Huygens
probe made a successful soft landing on Titan in January, 2005.
Images of Titan and Saturn are continually updated on the Cassini site.
G. INTERPLANETARY MATTER
Although only a trace constituent of the Solar System, the material
between the planets provides a number of interesting, even
spectacular, observational phenomena. These are all
"leftovers"---debris from the formation of the solar system. The
larger chunks (comets,
significant dangers to the Earth.
COMETS: are large chunks of ice
which evaporate when they get within several Astronomical Units of the
Sun (one AU = the distance between Sun and Earth), producing a gaseous
coma and sometimes a tail. The Solar System
contains billions of comet nuclei, but most are beyond the orbit of
Neptune. Most have
very elongated orbits and reach small distances from the Sun only
infrequently. Some, however, are gravitationally deflected by Jupiter
into orbits with shorter periods (< 100 years); these are called
"periodic" comets. Most are faint. Halley's is an exception as a
bright periodic comet. The most spectacular comets, like
Hale-Bopp (at right) are usually first-time visitors to the
inner Solar System. Click here for more information on Hale-Bopp. There are
always several faint comets available to observe in the sky; but
bright ones are rare: once a decade or so. If you are interested
in name-recognition immortality, consider searching for new comets,
since they are the only astronomical objects traditionally
named for their discoverers.
METEORS: are the incandescent trails of
tiny pieces of rocky or icy debris burning up at high altitudes in the
Earth's atmosphere. Up to about 10 per hour can be seen on dark
nights at any time of the year. Debris left behind by comets along
their orbits can produce concentrated meteor showers with
much higher rates, up to 1000's of meteors per hour in rare
instances. The Leonid shower (Nov. 17-18) was good in 1998, 1999, and
ASTEROIDS: Have also long been called
"minor planets." The largest few are now known as "dwarf planets."
Asteroids are large rocky or metallic chunks ranging from less than a
few meters to hundreds of kilometers in diameter. They move in their
own orbits around the Sun. Most orbits are concentrated between Mars
and Jupiter, but many cross the Earth's orbit. Ceres, 1000 km in
diameter, was the first discovered (1801). There are now over 231,000
a snapshot plot of the location of asteroids in the inner Solar
- Many asteroids are detectable with an 8-in telescope, but you
need updated coordinates and finding charts. Their signature is a
fairly rapid motion with respect to the background stars.
- Here is a video (266 kb) of the asteroid Eros
taken by amateur astronomer Gordon Garradd.
- Download, print, and read the notes for Lecture 4.
- Complete the Review Quiz for Week 5 on the Collab site.
- Consult the Edmund Star Atlas for additional
information on Solar System observations, as needed. You are not
required to know all the material there.
- Optional reading: Supplement 4.1:
Precession amd Supplement 4.2:
- Do Lab 2 at the earliest opportunity.
September 2015 by rwo
Moon phase and Earth orbit drawings copyright © by Nick Strobel. Mars orbit
graphic by A. Huffman. Text copyright © 2000-2015 Robert W.
O'Connell. All rights reserved. These notes are intended for the
private, noncommercial use of students enrolled in Astronomy 1230 at
the University of Virginia.