ASTR 1230 (O'Connell) Lecture Notes



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 of telescopes.

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.


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 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 Orb May

Earth Orb Nov

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).


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.



Because of its relatively large mass and proximity to Earth, the Moon has two important gravitational effects on Earth:

  1. Tides. The gravity of the Moon combined with the Sun is responsible for the tides in the ocean.

  2. 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.

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.


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 image.


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 remarkable terrain.

Plan Orbits



Planets to correct relative scale (though not separation)


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 planets: 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.

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

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).

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.

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

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.


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.

Hale-Bopp 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: 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 known(!)


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Last modified 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.