Introduction






Charting The Heavens

The Sun is 100 times bigger than the Earth; the Sun is a spec on the Milky Way, which is a spec on the fabric of the universe.

The universe is the totality of all space, time, matter and energy (it’s everything).

A light year is a measure of how far light can travel in one year. It is a measure of distance.

Scientific method is observation, theory, prediction loop.

Science is based on fact; religion is based on faith

Constellations are groupings of stars into shapes of people, animals, objects. Useful in mapping the sky, because it lets you know what star is what, as they change through the seasons. Also helpful in navigation, such as with the north star Polaris which is part of the little dipper.

The rise in the east and set in the west of the Sun, moon and stars is due to the rotation of the Earth on its axis. As the Earth rotates around, it makes the sky appear to move from east to west. In reality, we are moving.

If the Earth stood still, a day going around noon to noon would keep the stars in the same position. Since the Earth rotates around the Sun, the position of the stars move as well. Plus, what is visible changes – we can only see the stars on the opposite side of the Sun from us, not what’s behind the Sun.

One orbit of the Sun is equal to one year.

The seasons are caused by the tilt of the Earth spreading out or centering the Sun’s heat energy on the Earth’s surface. Winter solstice has the Sun at its southernmost point o the celestial sphere.

Precession is the wobble of the Earth. It’s caused by gravity effects from the Moon and the Sun on the Earth.

Time zones are useful for commerce. Leap years keep the seasons in check, since the orbit around the Sun is not exactly 365 days.

We don’t see the completely lit hemisphere of the Moon all the time due to our angle of observation.

Lunar eclipse: Earth passes between the Sun and the Moon (during full phase). Solar eclipse: Moon passes between the Sun and the Earth.

Because the Moon is inclined 5 degrees to the ecliptic, the Moon may be above or below the Sun in the Sun-Earth line when the three are “in alignment” (during new and full moons only). Only when the alignment happens while the Moon is on the ecliptic, and all three are in alignment can an eclipse occur.

An observer on another planet in our solar system wouldn’t see a solar eclipse due to the apparent sizes of the moon of the planet and the Sun (the smaller moons can’t block out the Sun, even at further distances away). A lunar eclipse may be possible, depending on the planet’s ecliptic, and how far off the moon is from it. The circumstances for eclipses are a perfect alignment of the three, with the eclipting object able to block the light from the other object. This is difficult to obtain.

Parallax is the apparent shift in position of an object compared to background objects. It’s like closing one eye and then the other in blinking pattern while holding your finger up against a wall (that has a pattern on it, so that you can see the apparent movement).

The further objects are, the harder it is to do parallax without having a longer baseline. The apparent shifts become negligible, to indeterminable, to nonexistent without increasing the baseline.

Diameter of a far away objects is equal to the distance times (the angular diameter/57.3 degrees)

Constellations will change when you go to different observational locations due to the positions of the stars changing (they’re not next to or even close to each other).

Constellations along the ecliptic are known as the zodiac.

Annular eclipse: moon fails to completely block the Sun. Total eclipse: moon completely blocks the Sun.

Parallax of an object is inversely proportional to the distance from us (further away, less parallax). Also, smaller baseline, less parallax.

A thin crescent moon visible just before sunrise happens when the Moon is in its waning phase

If the moon were further away, the eclipses would most likely be annular rather than total.

If the moon orbited twice as fast, the number of solar eclipses would still be the same.

Today, distances to stars are measured by geometry.

Moon phases:
New (not visible)
Waxing (light on the right side, gets lighter as days progress)
Crescent (rises between sunrise and noon)
1st quarter (rises at noon, sets at midnight)
Waxing gibbous (rises between noon and sunset)
Full moon (rises at sunset, sets at sunrise)
Waning (light on the left side, gets darker as days progress)
Waning gibbous (rises between sunset and midnight)
3rd quarter (rises at midnight, sets at noon)
Crescent (rises between midnight and sunrise)
Back to new moon

The Copernican Revolution

Chinese and Islamic Dark Age astronomers made very detailed observations of the stars. Charts are still used today.

Geocentric – Earth is at the center. Most complex model to account for retrograde motion was the Ptolomaic model.

Copernicus rediscovered the heliocentric model (the Sun’s at the center of the universe). Copernican model still had circular orbits though.

A theory is a thought to explain an observation. Theories are tested through observation and prediction of the outcome. A theory can never be proven true, but can be proven false.

Copernicus’ ideas were not taken seriously until long after his death when other astronomers such as Galileo and Kepler started making them popular. Copernicus’ writings were banned in 1616, 73 years after they were first published and remained banned until the end of the 18th century.

The Copernican principle states that Earth is not in a central, specially favored position.

Galileo found four moons of Jupiter, and that Venus had phases. These discoveries helped confirm Copernicus’ views because Venus could not have phases unless it orbited the Sun instead of the Earth, and Jupiter having moons meant that there was something orbiting another celestial body other than the Earth – so the Earth wasn’t the center of everything.

Kepler’s three laws:
1. Orbits are ellipses with the Sun at one foci.
2. Orbits clear the same area in the same time – thus they travel faster nearer the Sun
3. p squared = AU cubed

Tyco Brahe had made the most complete and accurate set of measurements of the stars and planets than any other astronomer (and was Kepler’s boss). When he died, Kepler got his charts – decades of observations. Kepler used this information to determine the orbits of the planets without the need for epicycles.

We cannot use radar to determine the distance to the Sun directly. We use it to determine the distances to Venus, then use parallax and algebra to compute the distance to the Sun.

Before radar, astronomers used triangulation to determine the distances between the Sun the Earth and planets.

Kepler’s laws are empirical because they are derived from experiment and observation rather than theory.

Newton’s laws of motion:
1. body at rest tends to stay at rest, body in motion will continue motion (unless acted upon by an outside force)
2. F = m * a
3. For every action there is an equal and opposite reaction
Law of gravity: All objects exert a gravitational force on all other objects that is directly proportional to the mass of the objects and inversely proportional to the square of the distance between their centers.

Two modifications to Kepler’s laws by Newton: (1) The Sun is not a foci, but the center of mass between the Sun and planet is. (2) p squared does not equal au cubed. P squared equals au cubed divided by the combined mass of the two objects (orbiting and orbited) in solar units.

Because Earth is the more massive object, it creates a stronger pull on a baseball than the baseball does to the Earth. Therefore, the baseball falls to Earth instead of the Earth falling to the baseball.

A baseball thrown on the moon goes higher than on the Earth due to the moon have a weaker gravity. The pull is lower, so the baseball goes higher.

The Moon is falling, but because of the Earth rotating under it, it is in a continuous state of falling. If it were not falling, it would fly off into space.

Escape speed is the minimum speed needed for an object to move at in order to escape the gravitational pull of another object.

If the Sun’s gravity were suddenly turned off, the Earth would go flying off into space.

The first person to propose the planets revolve around the Sun was Aristarchus.

Retrograde motion is the apparent backward (westward) motion fo a planet relative to the stars.

Heliocentric model has the Sun at the center of the solar system.

Galileo used a telescope, not the naked eye, to make his observations.

Kepler never knew the true distances of the planets to the Sun, only their relative distances.

If you throw a baseball to someone, before the ball is caught, it is temporarily in orbit around Earth’s center.

Planets near opposition rise in the east.

A major flaw of Copernicus’s model was that it still had circular orbits.

Galileo’s observations of Venus demonstrated that it must orbit the Sun.

An accurate sketch of Jupiter’s orbit around the Sun would show a nearly perfect circle.

A calculation of how long it takes a planet to orbit the Sun would be closely related to Kepler’s third law of planetary distances.

An asteroid with an orbit lying entirely inside Earth’s has an orbital semimajor axis of less than 1 AU.

If Earth’s orbit around the Sun were twice as large as it is now, the orbit would take more than two times longer to traverse.

If the Sun and its mass were suddenly to disappear, Earth would fly off into space.

If one star has twice the mass of another, then the more massive star moves more slowly than the less massive star.

Inferior planets – inside Earth’s orbit (Venus and Mercury)
On other side of the Sun, superior conjunction. Between Earth and Sun, inferior conjunction
Superior planets – outside Earth’s orbit’
On other side of the Sun, Conjunction. Earth between planet and Sun, opposition

7 foundations of Copernican Revolution:
1. There is no one common center of everything - Earth is not at the center of everything
2. The center of Earth is not the center of the universe
3. All planets revolve around the Sun
4. Stars are very very far away
5. Because stars are so far away, any apparent motion is due to Earth’s movement not their movement.
6. As with 5, the Sun’s motion is because of the Earth, not the Sun, moving.
7. Heliocentric model explains away retrograde motion (which is only found in the superior planets).

The Solar System

Solar system consists of:Comets appear as long, wispy strands of light in the night sky that remain visible for periods of up to several weeks and then slowly fade from view.

  • Meteors (shooting stars) are sudden bright streaks of light that flash across the sky, usually vanishing less than a second after they first appear.
  • The asteroid belt lies between Mars and Jupiter. Most asteroids (“minor planets” orbiting the Sun) are found here. The largest (and first to be sighted) is Ceres, discovered in 1801.
  • Kuiper Belt is a region in the plane of the solar system outside the orbit of Neptune where most short-period comets are thought to originate.
  • A meteoroid is a chunk of interplanetary debris prior to encountering Earth’s atmosphere.
  • The Solar System contains 1 star (the Sun), 8 planets, 165 Moons orbiting those planets, 8 asteroids and more than 100 Kuiper belt objects larger than 300 km in diameter, tens of thousands of smaller asteroids, myriad comets a few km in diameter, and countless meteoroids less than 100 m across.


Comparative planetology is comparing and contrasting the properties of the diverse worlds we encounter. It is used to understand better the conditions under which planets form and evolve. The goal is to better understand the origin and evolution of our planetary system.

By measuring the affect that the gravity of the planet has on other nearby objects (such as the planet’s moon or moons), we can apply Newton’s second law of motion, along with the law of gravity, to calculate the mass. However, in applying these laws, we have to be able to calculate the formulas. Part of the formulas include distance (r). Without knowing the distance, we cannot accurately calculate the mass.

List some ways in which the solar system is an “orderly” place?
1. All the planets orbit the Sun in nearly the same plane (Mercury is the only “oddball”), and go around the Sun in the same direction.
2. All of the terrestrial planets are closest to the Sun while all of the jovian planets are further away.
3. As you move out from the center, the distance between orbits gets larger and larger, as do the orbital periods.

Name some disorderly characteristics of the solar systyem
1. There is no rhyme or reason to the densities or masses. They get smaller then larger then smaller then larger. Saturn, the second biggest planet, has the lowest density of all and can float on water.
2. There’s no rhyme or reason to the rotation period. Venus rotates slowly (243 Earth days to make one rotation)
3. Venus and Neptune have negative rotations – they spin the wrong way.

Terrestrial planets are Mercury, Venus, Earth and Mars. They’re called terrestrial because they are solid, and the name comes from terra meaning land, or solid.

Jovian planets are Jupiter, Saturn, Uranus and Neptune. They’re called jovian from “Jove”, another name for Jupiter, which is the largest of the four.

Asteroids and meteoroids provide information to answering fundamental questions about the planetary environment and what the solar system was like soon after its birth.

As comets vaporize, they emit radiation, so they can be studied by spectroscopy to see what they are composed of.

We have sent dozens of unmanned space missions since the 1960s throughout the solar system. Which has increased our knowledge greatly.

We have sent spacecraft to all 8 planets, but only landed on Venus and Mars

The Galileo and Casini routes were more circuitous than Pioneer or Voyager as they used more gravity assists, including from Venus, to reach the outer solar planets.

How do you think NASA’s new policy of building less complex, smaller, and cheaper spacecraft – with shorter times between design and launch – will affect the future exploration of the outer planets? Will missions like Galileo and Casini be possible in the future?

The less complex spacecraft will be able to be launched much quicker than the more complex ones. Also, they will be less expensive, which will allow for them to be designed, built and launched within budgets. As technology advances, the instrumentation needed to make the detailed observations will become smaller and cheaper. However, by making the spacecraft less complex, with fewer instruments on them, we will miss opporotunities of study. Detailed missions like Galileo and Casini will probably not be possible until the technology catches up to the less complex designs (for example, having one small device that can simultaneously capture multiple ranges of the electromagnetic spectrum from gamma rays all the way up to the most massive radio waves)

Give three examples of how the condensation theory explain the observed features of the present-day solar system. First, start with nebular theory, then build on it.
1. All planets rotate in the same direction around the Sun.
2. Since they come from the disk of matter, they are all (basically) on the same plane.
3. Objects that form the same distance from the Sun should have basically the same composition.

The key element in the jovian planets is hydrogen. Hydrogen is the most abundant material in the entire universe. Since there’s more hydrogen than there is rocky and metallic material, there was more material to make the jovian planets – therefore, more material was used, and they became more massive.

The swirling nebula was hotter in the center where the Sun was forming, which meant that only the rocky and metallic particles could condense into planets in the inner area.

Most planets orbit the Sun in nearly the same plan that the Earth does.

The total mass of all the planets is much less than the mass of the Sun.

The jovian planets all rotate more rapidly than Earth.

Jupiter is the largest and most massive planet in the solar system.

Asteroids are similar in overall composition to the terrestrial plaents.

Comets have compositions similar to the icy moons of the jovian planets.

Not all planets have moons (Mercury and Venus have none)

Voyager 1 did not visit all four jovian planets (it swung up and out of the solar system after visiting Saturn) – Voyager 2 did visit all four planets.

Interstellar dust plays a key role in the formation of a planetary system.

A planet’s mass can most easily be determined by measuring the planet’s moons’ orbits.

If we were to construct an accurate scale model of the solar system with the Sun at one end and Neptune at the other, the planet closest to the center of the field would be Uranus.

The inner planets tend to have fewer moons than the outer planets have.

The planets that have rings also tend to have many moons.

A solar system object of rocky composition and comparable in size to a small city is most likely an asteroid.

The asteroids are mostly found between Mars and Jupiter.

The Sojourner Mars rover, part of the Mars Pathfinder mission in 1997, was able to travel over an area about the size of a soccer field.

To travel from Earth to the planet Neptune at more than 30,000 mph, the Voyager 2 spacecraft took nearly a decade.

In the leading theory of solar system formation, the planets formed from the same flattened, swirling gas cloud that formed the Sun.

The solar system is differentiated because only rocky and metallic particles could form close to the Sun.