Astronomy Powerpoint
Doppler Shift
Deep Space
(Any links or buttons in the powerpoint might not work)

Observations that need to be explained:

  • Sun rises & sets every day
  • Sun Rises in East & sets in West
  • Sun’s path in the sky is an arc
  • Sun is at different angular altitudes in the sky at different times.
  • The sun is never directly above (at zenith) in NY.

The Geocentric theory explains all of these phenomena. Since movement of the Earth cannot be felt early, scientists thought that the Earth was stationary and everything else moved around us.

The geocentric theory says that the Earth is at the center of the solar system/ universe.

Geocentric model with the Earth at the center.
Drawing by Phil Medina


Speed Links


Kepler's Laws






Doppler Effect



Astro Diagrams Handout

How Big is the Moon?
(Based on an activity by Chris Visco)

Seasons & Day & Night

Worksheet for shadow stick


Phases of the Moon

Jovian Moons DBQ

But the Geocentric Theory does not explain this:

Foucault Pendulum: The first good evidence of the earth's rotation was provided by a swinging pendulum. The pendulum would continue to swing in the same direction while the Earth rotated beneath it. A more recent source of evidence of rotation is the swirling pattern of weather systems seen by satellites.

The Coriolis Effect makes the Foucault Pendulum work. The rotation of the Earth puts a "spin" on the air movement which causes a curve in the travel direction. In the Northern Hemisphere the turn is towards the object’s right.

Galileo disproved the geocentric theory. He was the first to turn the telescope to the sky (he did not invent the telescope) for the purposes of Scientific observations.

The moons of Jupiter orbit an object other than Earth. Galileo noticed that the four visible moons of Jupiter appeared to orbit around Jupiter and therefore did not revolve around the Earth.

Galileo also observed the "phases of Venus" which demonstrated that Venus orbited around the sun.

The Phases of Venus as seen from Earth.
Photoshop drawing by Phil Medina

The Heliocentric Theory explains all observations made from Earth.

Heliocentric” makes the sun the center of attention rather than the earth.

Here's an article written by Benjamin Franklin about how much sense the Heliocentric Theory makes:

The Whimsical Cook And here's a Powerpoint on the same Article.

Rotation- to spin around an axis.

  • How long does it take the Earth to spin once?
  • How many degrees is a full spin?

360°÷24hours= 15°/hr
Earth spins at 15 degrees per hr

The Earth’s Rotation does not have the same speed all over

  • At the equator, rotation is 40,074 km/day or 1670 km/hr
  • At NY 31,200 km/day or 1300 km/hr
  • At the poles 0 km/day or 0 km/hr

Revolution- to orbit around an object

The Earth revolves around the sun in 365 ¼ days.

Sidereal Day vs Solar day

A day on Earth is one complete rotation. But since our planet spins through featureless outer space, how can we tell when we have gone one complete rotation? We have a choice between using the Sun as a reference point or the distant stars. It shouldn’t make much of a difference but it actually does.

When using the stars as a reference point, it takes 23 hours and 56 minutes for the Earth to rotate 360º until the star reaches the same exact spot in the sky. This method of measuring the length of a day is called a sidereal day.

However, if you measure how far the Earth has to rotate until the Sun reaches the same spot, it is no longer 360º. It is closer to 361º. This is caused by Earth’s motion around the Sun. In the 23 hours and 56 minutes that the Earth was spinning, it was also orbiting around the Sun. Since it has “stepped to the side” of the Sun while rotating, the Earth has to rotate a little more to line up with the Sun. We do not have this difference with a sidereal day because the stars are essentially infinitely far away and the distance the Earth travels in one day is negligible. The extra degree of rotation takes about four minutes, bringing the total time up to 24 hours. A solar day, as it is called, is what we use to measure the length of a true day (sunrise to sunrise) although it does not represent a 360º turn.

As the Earth revolves around the sun, different constellations are visible in the night sky.As viewed from Earth, the sun is passing through Leo (even though you can’t see the stars during the day.

The Axis of Rotation is the imaginary line that the Earth turns about.

The axis always points to the North Star. This is called Parallelism of the Earth's axis.

The imaginary surface of the Earth's yearly trip around the sun is the Plane of Earth's Orbit.


The following statement sounds crazy but you'll understand it if you read it slowly (or at least go back and re-read the last few lines)!

"The axis is tilted by 23.5° from the perpendicular to the plane of its orbit."


Name Date Key points

Summer Solstice



June 21

North Pole is tilted towards the sun

Sun is highest in the sky in NY

Longest day, shortest night

Sun is directly over the Tropic of Cancer.

Fall (autumnal) Equinox

September 23

Equal day and night

Sun is directly over the equator

Winter Solstice

December 21

North Pole is tilted away from the sun

Shortest day, longest night

Sun is directly over the Tropic of Capricorn

Spring (vernal) Equinox

March 21

Sun is directly over the Equator

Equal day and night

*Note: The seasons are NOT caused by the distance from the sun.

What if the tilt was different?

Less Tilt
If there was less of a tilt, the seasons would even out into a perpetual spring as the tilt gets closer to straight up and down. All latitudes will get closer to 12 hours of day and 12 hours of night.

More Tilt
As the poles tilt further away from the perpendicular, the seasons would get more extreme. Summers get hotter and Winters get colder. The time difference between day and night will get further out of balance.

Kepler’s Laws

1) All planets travel in elliptical orbits with the sun at one focus.

Note: The Earth's orbit is so close to a perfect circle that it can't be distinguished with the human eye.

Here's a highly exagerated drawing of Earth's Elliptical orbit:

Photoshop drawing by Phil Medina

2) Each planet travels in such a way that a line joining the planet and the sun sweeps equal areas in equal times.

Also called the law of equal areas. Each of the triangles in the diagram below will have the same area. Also, the time it takes for the planet to go from one position to the next will be equal (in this example a month).

3) Keppler's Law of Harmonic Motion- Relationship between distance and period:

P2 = D3 ("P squared = D cubed")

Where P is the Period in Earth Years
And D is the distance in “AU’s”

An “AU” is an astronomical unit, which is the average distance from the sun to the Earth.

It is called the Law of Harmonic Motion because it describes how the planets move in harmony with each other- fastest in the center and gradually moving slower towards the outer solar system.

The Moon

The moon revolves around the Earth once a month (moonth).

Moon rises 50 minutes later every day.

It can be seen during the day just about as much as the night.


Eclipse means "gets dark."

A lunar eclipse occurs when the Earth blocks out the Moon's sunlight. The moon "gets dark" as it goes into the Earth's shadow.

Lunar eclipses happen during a Full Moon.

Lunar Eclipse Photoshop drawing by Phil Medina

A solar eclipse is when sun "gets dark" because the moon blocks sunlight from reaching the Earth.

Solar Eclipses happen during a New Moon.

Solar Eclipse Photoshop drawing by Phil Medina

Eclipses do not occur every time there is a full or new moon because the moon's orbit around the Earth is inclined.

Tilt of the Moon's orbit Photoshop drawing by Phil Medina


The strength of gravity depends on how close two objects are and their masses.

Where m1 and m2 are the masses of the objects and d is the distance between them.

A Spring Tide is caused by a full moon and a new moon. The water “springs” from its highest level to its lowest for the month.

A Neap Tide is caused by the half moon phases. The high tide is not so high and the low is not so low.


  • Chunk of ice and dirt following an elongated elliptical orbit.
  • Has a tail only when it comes close to the sun.
  • The heat melts it and the solar wind sweeps the tail away.
  • The tail always points away from the sun.


Meteor Showers result mainly from debris from the orbit of the comet.

If the Earth crosses the path, the debris will fall to Earth as a visible meteor or “shooting star.”

If the rock is big enough it will survive the fiery entry into the atmosphere and hit the surface as a meteorite.

For more details on meteors and meteor showers Click Here.


Circumpolar Stars

As the Earth turns once a day, the stars appear to move across the sky in large arcs with the North Star at the middle of the arc. If a camera was left open for several hours. The stars will create streaks in the picture forming a bulls-eye around the North Star.

The stars that make a full circle without going below the horizon are called circumpolar stars and are part of circumpolar constellations. These stars are visible all night, every night of the year. In NY the Big Dipper is one of a few such circumpolar constellations.

Stellar Fusion

Stars get their energy by converting matter into energy. According to Einstein's famous equation (E=MC2) the amount of energy (E) equals the mass of the matter converted (M) times the speed of light squared (C2). As a result, a small amount of energy will give off a tremendous amount of energy. Stars are fueled by nuclear fusion which is a process of taking small atoms and fusing them together into larger atoms. During the process, some of the matter is converted into energy.

The Life of a Star

  • The star begins as a c loud of gas in space.
  • Due to gravity of every atom in the gas, the cloud will contract.
  • As it contracts, it heats up.
  • When the heat gets intense enough, atome will begin the process of fusion.

The star is born and will continues to fuse its atoms until it runs out of fuel. At this point the star can go on two different paths depending on the size of the star:

If the star is 7x or sun or smaller If the star is 7x or sun or larger
  • Swell up to a red giant
  • Shrink to a white dwarf
  • Slowly cool and die
  • Swell up to a red giant
  • Shrink collapse and explode as a supernova
  • Dies as a rapidly spinning core
  • Or better yet...
  • A black hole!

Doppler Effect

The Visible Spectrum- white light splits into the colors of the rainbow when passed through a prism.

Red Orange Yellow Green Blue Indigo Violet


There’s more to the spectrum than what you can see.

When an element is heated, it gives off light. As seen through a prism, the light will split into the basic colors that make that light (just like the rainbow). Each element has its own unique light with its own pattern of colors sometimes called the “fingerprint” of an element.

A simulated spectrum of Hydrogen

When sound is made by a stationary object, you hear a certain frequency.

If you could see it, it would look like this...

But when the sound moves towards you, it sounds higher in pitch and would look like this...

The faster the object moves, the higher the pitch.

When the sound moves away, it sounds lower.

The same shifting happens with the light from a moving star.

When the star is moving closer the spectrum gets shifted to the blue end of the rainbow.

Stationary Hydrogen

Hydrogen from an approaching star

And when a star moves away from us...

Stationary Hydrogen

Hydrogen from a star moving away

Since the universe is expanding...
All distant objects have a red shift.

The farther the object, the more extreme the red shift.

Astronomers use this relationship between distance and speed to measure the distance to distant stars and galaxies.

Over fifteen years of
Medina On-Line