ASTR 1230 (Whittle) Lecture Notes

5. SOLAR SYSTEM ASTRONOMY

Pseudo-color infrared image of Saturn (HST)

A. INTRODUCTION

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.

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

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. The stars are the backdrop, or "reference frame," against which we judge motions.

"apparent"

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 Zodiac.
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 50 minutes each day. The Moon's path is tilted 5 degrees from the ecliptic. During this monthly circuit, it 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 photographic mosaic.
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 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.

nearly circular

Astronomical Unit

plane

365.25 days (one year)

counterclockwise

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.

Earth and 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 produced by the motion of the Moon in its orbit around Earth.
- 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 (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.
- 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 this Java demonstration.
In the figure above you are looking down on the Earth's North Pole. The Earth spins counterclockwise (in 24 hours), and the Moon orbits counterclockwise (in 27 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 90^o away (east) from the Sun in the sky and that a full Moon is 180^o away from the Sun. The Moon will be in a crescent phase when is it less than 90^o away from the Sun and a gibbous phase when between 90^o and 180^o away.
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 positions A, B, C, and D correspond to places where observers experience noon, sunset, midnight, and sunrise, respectively.
- Drawing a horizon plane at B (a flat plane, just touching the Earth's surface at the observer's position), for instance, 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).
This 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:

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

Eclipses are shadow effects in which the shadow of the Earth strikes the Moon or the shadow of the Moon strikes the Earth. There are two types: lunar eclipses and solar eclipses. Both can be dramatic and beautiful events, for properly situated observers on Earth.

More details on eclipses are given on a supplementary page

Full Moon (extract from composite exposure). Click for entire image.

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 first (1609) revealed the Moon's remarkable terrain.

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 impacts.
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 Jovian 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 regions).
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 orbital planes.
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.
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. Here is an animation showing how retrograde motion is produced.

F. OBSERVING THE 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 (and lamentable) 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 planets:

Terrestrial Planets (archetype Earth): have 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 "thin."
Jovian Planets (archetype Jupiter): are primarily made of hydrogen and helium, with a thin smattering of heavier elements. They are much larger than Earth. They have enormous atmospheres (by terrestrial standards) 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): 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.

All the planets except Uranus, Neptune, and Pluto 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. Both Venus and Mercury can be observed in daylight. Click here for sketches of the appearance of the planets in small telescopes.

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 40^o of the western or eastern horizon? Is it in a Zodiacal constellation? If yes, then it's probably Venus.]

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 demonstration.
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 here), as it orbits the Sun. Neither Venus nor Mercury have satellites.

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).

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 2005 was also close but not quite as favorable. Oppositions until 2016 will be relatively distant.
Mars' atmosphere is primarily CO₂ and is transparent. Its color is conspicuously red-pink (hence its association with the God of War), caused by iron oxide compounds = rust on its surface. Its surface 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 and controversial history over whether or not there was evidence for "canals" or other artificial features on its surface. (More details given here.) The image at the right was taken from Earth orbit by the Hubble Space Telescope (click for enlargement).
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.

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 already discussed.

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 right hand 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 62 known moons, mostly small. The four largest of these (Io, Europa, Ganymeade, Callisto) were discovered by Galileo and are known as the Galilean 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 and Galileo missions revealed astonishing differences in surface constitution among the four. In small telescopes, unfortunately, no surface details are apparent.

SATURN: Famous as the ringed planet, though all four gas giants actually have rings.

Its cloud layers are deeper within its atmosphere than are Jupiter's, so it typically shows only faint surface banding and subtle structures.
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. Spacecraft reveal hundreds of ringlets. A beautiful mosaic of the rings from the Cassini orbiter is available at this web site.
Six of Saturn's 60 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.

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.

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, asteroids) pose 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 2002.

ASTEROIDS: Also called "minor 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 diameters, was the first discovered (1801). There are now over 38,000 known(!)

Here is a snapshot plot of the location of asteroids in the inner Solar System.
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.

Assignment

Consult the Edmund Star Atlas for additional information on Solar System observations, as needed. You are not required to know all the material there.
Do Lab 2 and move on to work on Lab 3 at the earliest opportunities.

Web links

Supplement 4.1: Precession

Supplement 4.2: Eclipses

Lunar Phase Now

Lunar Info for Amateur Astronomers

Guide to Lunar Observing (A. Cidadao)

Espenak's Eclipse Home Page: everything you could possibly want to know about eclipses.

Selected Moon Images (O'Connell)

Information on the Planets ("The Nine Planets")

Information on the Planets ("Views of the Solar System")

Mars Open Directory

Selected Mars Images (O'Connell)

Java Simulator of Comet & Asteroid Orbits (UMd)

JPL Asteroid Home Page

Comet Information

American Meteor Society (includes lists of showers)

Information on the Leonid Meteor Shower

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Last modified September 2008 by rwo

Moon phase and Earth orbit drawings copyright © by Nick Strobel. Mars orbit graphic by A. Huffman. Text copyright © 2000-2008 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 130 at the University of Virginia.