Earth image The Motion of Objects in the Sky

Dr. Pamela Gore
Georgia Perimeter College

Objectives

  1. Describe the motions of the Sun in the sky during the day, and during the year.
  2. Describe the motions of the Moon in the sky.
  3. Describe the motions of the stars in the sky.
  4. Explain how the North Star can be used in navigation.
  5. Describe the motions of the Earth with respect to the stars and the sun.
  6. Explain why we see different constellations at different times of the year.
  7. Describe the motion of the planets in the sky, including retrograde motion.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • SKE1. Students will describe time patterns (such as day to night and night to day) and objects (such as sun, moon, stars) in the day and night sky.
    a. Describe changes that occur in the sky during the day, as day turns into night, during the night, and as night turns into day.
    b. Classify objects according to those seen in the day sky and those seen in the night sky.
    c.
    Recognize that the Sun supplies heat and light to Earth.  
     
  • S2E2. Students will investigate the position of sun and moon to show patterns throughout the year.
    a. Investigate the position of the sun in relation to a fixed object on earth at various times of the day.
    b. Determine how the shadows change through the day by making a shadow stick or using a sundial.
    c. Relate the length of the day and night to the change in seasons (for example: Days are longer than the night in the summer.).
    d. Use observations and charts to record the shape of the moon for a period of time.
     
  • S4E1. Students will compare and contrast the physical attributes of stars, star patterns, and planets.
    a. Recognize the physical attributes of stars in the night sky such as number, size, color and patterns.
    b. Compare the similarities and differences of planets to the stars in appearance, position, and number in the night sky.
    c. Explain why the pattern of stars in a constellation stays the same, but a planet can be seen in different locations at different times.
    d. Identify how technology is used to observe distant objects in the sky.
     

  • S4E2. Students will model the position and motion of the earth in the solar system and will explain the role of relative position and motion in determining sequence of the phases of the moon.
    a. Explain the day/night cycle of the earth using a model.
    b. Explain the sequence of the phases of the moon.
    c. Demonstrate the revolution of the earth around the sun and the earth’s tilt to explain the seasonal changes.
    d. Demonstrate the relative size and order from the sun of the planets in the solar system.
     

  • S6E1d. Explain the motion of objects in the day/night sky in terms of relative position.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Like all planets and stars, the earth is approximately spherical in shape. The rotation of the earth on its axis every 24 hours produces the night-and-day cycle. To people on earth, this turning of the planet makes it seem as though the sun, moon, planets, and stars are orbiting the earth once a day.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Most objects in the solar system are in regular and predictable motion. Those motions explain such phenomena as the day, the year, phases of the moon, and eclipses.

How does the Sun move through the sky?

As we all know, the Sun rises in the east, and sets in the west as a result of the Earth's rotation on its axis. But the sun does not always rise at the same place along the horizon. In the summer, it rises much farther to the north, and it also reaches a point much higher in the sky at noon. In the winter, it rises much farther to the south, and does not rise as high in the sky at noon.

Ancient peoples recognized these facts and built stone observatories to track the movement of the sun back and forth along the horizons, and hence to mark the seasons. These sorts of structures are sometimes called horizon calendars.


Diagram showing approximate positions of the sun at sunrise, noon, and sunset at the summer solstice, equinoxes, and winter solstice.

Some of the more famous ancient observatories include:

  1. Stonehenge and similar megalithic (large stone) circles in England (such as Avebury)

     
    Stonehenge in England. Photos courtesy Pamela Gore.

  2. Newgrange in England
  3. The Sun Dagger used by the Anasazi Indians at Fajada Butte, Chaco Canyon, New Mexico
  4. Cahokia Mounds in southern Illinois
  5. The Big Horn Medicine Wheel in the Big Horn Mountains near Sheridan, Wyoming
  6. Caracol Tower at Chichen Itza, Yucatan, Mexico

Another way of marking the seasons or the time of day is by examining a shadow cast by a vertical pole or post.

When the Sun is near the horizon, just after sunrise or just before sunset, a pole casts a long shadow. At noon, when the sun is directly to the south, at its highest point in the sky, the shadow is shortest. "Noon" may actually be "noon by the Sun" rather than by the clock, because "noon by the clock" may differ a little (think about changing to Daylight Savings Time, which will throw things off a little.) After noon shadows grow longer, as the Sun descends towards the horizon.


The shortest shadows of the day occur at noon. The sun is directly south, and the shadow points to the north.

Because the shadow always points away from the Sun :

This is the principle behind the workings of the sundial. Click here to learn how to make a sundial.

Since the height of the noon sun in the sky differs at different times of the year (high in summer, low in winter), the length of the shadow will change systematically. Short in summer, long in winter. Any time the sun is low in the sky, the shadow cast by a pole will be longer. By keeping track of the position of the end of the shadow (made by the tip of the pole), you can construct a sort of "shadow calendar" that you could use to determine when the shadow is at its longest and shortest during the year, or the time of the summer and winter solstice.


The apparent path of the sun across the sky during the year.
Picture yourself as standing in the center of this diagram, on the flat circular disk with the dashed lines for north, south, east, and west. You are looking up at the dome of the sky above your head, which we call the celestial sphere. Look to the south, to see the position of the sun in the sky.
 
Image courtesy of NASA.
If this image does not appear, click the link to the NASA website. You should then be able to return to this page and see the diagram in the correct place.

Examine the diagram above (or at the NASA website) as you read through the material below and think about the questions.

The line connecting the north and south pole is called the meridian.
The sun lies on the meridian at noon.

Trace out the line that represents the path of the sun on June 21, the summer solstice.
Trace out the line that represents the path of the sun on December 21, the winter solstice.
When is the sun highest in the sky?
 
There is a third line, between these two. It represents the position of the sun at the equinoxes.

Now, let's look at the place that the three "sun paths" intersect the horizon (the flat circular disk). From this diagram, at which time of year does the sun rise along the horizon furthest to the north?
From this diagram, at which time of year does the sun rise along the horizon furthest to the south?
At which time(s) of year does the sun rise EXACTLY IN THE EAST and set EXACTLY IN THE WEST?

For more information, see course notes on the seasons.


How does the Moon appear to move and change in the sky?

The Moon rises in the east and sets in the west daily, but its position in the sky moves EASTWARD by about 13 degrees per day. (360 degrees divided by 27.32 days = 13.177 degrees per day).

Thirteen degrees is about the width of your fist, held at arm's length, while looking at the sky. So the moon will appear to move one "fist" to the east, each day.

The Moon rises and sets almost 1 hour later each night.

For more information on the moon, see Phases of the Moon.

The Moon revolves around the Earth in the same direction that the Earth rotates.

To circle the Earth once (relative to the stars) takes 27.32 days. This is called the SIDEREAL PERIOD of the Moon (or the sidereal month), or the orbital period of the Moon around the Earth.

The length of time from New Moon to New Moon is called the LUNAR MONTH or SYNODIC PERIOD of the Moon. It is 29.53 days.

The Moon takes 27.32 days to orbit the Earth (with respect to the stars), but it takes LONGER (29.53 days) to go through a cycle of phases. WHY?

This is because the Earth-Moon system has moved around the sun by about 27 degrees over the course of the month. The Moon will have gone around the Earth once with respect to the stars, but it needs to travel further to line back up the same way with the sun.

The same side of the Moon always faces the Earth. WHY? The Moon's orbital period is equal to its rotational period.

In other words, the Moon turns on its axis at the same rate as it revolves around the Earth.

The Moon is also involved with the Sun in the phenomenon known as an eclipse. For more information, see course notes on eclipses.


How do the stars appear to move in the sky?

The stars rise in the east and set in the west as a result of the Earth's rotation on its axis. The star that lies almost perfectly above the Earth's rotational axis, however, appears to remain stationary in the sky. That star is known as the North Star, or Polaris.

If you were to take a time exposure photograph of the sky around the North Star, you would see that the stars appear to rotate around the North Star. For example, see photos at this web site, and here.
See animation of star movement here. As you can see, the stars rotate counterclockwise around the North Star over the night, as a result of the Earth's rotation. To best observe this, find the Big Dipper, and note the way it is oriented in the sky (standing on the handle? pouring water? handle on top?). Then check back several hours later. How has the position changed?

The North Star and Latitude

Polaris always seems to stay in one point in the night sky. The other stars seem to rotate around it. This is a result of the Earth's rotation on its axis. We can use the stars to determine our latitude in the Northern Hemisphere.

At the North Pole, Polaris is straight up (at the zenith). 90 degrees up. At the equator, Polaris is on the horizon. 0 degrees up.

For every degree of latitude that you move toward the N pole, Polaris shifts 1 degree higher above the horizon.

Therefore, the altitude (in degrees) of Polaris above the horizon (as measured with a protractor) = your latitude.

Daily motions of the Earth with respect to the stars and the sun; or
Why we see different constellations at different times of the year

If we point a telescope at a star on the meridian, where will it be 24 hours later? If a day is 24 hours long, it should be in the same spot, BUT IT IS NOT!

How long does it take for the star to return to the same spot?
23 hr 56 min 4 sec

This is the time for the Earth to rotate once on its axis with respect to the stars.

It is called the sidereal day. It is about 4 minutes shorter than a 24 hour day.

So why do clocks keep 24 hour time? From noon to noon is 24 hours. This is the solar day.

THE SOLAR DAY IS NOT EQUAL TO THE SIDEREAL DAY.

Reason: As the Earth turns on its axis, it also revolves part of the way around the sun, travelling about 3.2 million km or 2 million miles on its year-long journey around the sun EVERY DAY.

This is an angle of 0.986 degrees per day - almost 1 degree. (360 degrees divided by 365 days).

The Sun seems to shift against the background of stars as the days go past. The sun will not be in the same place on the celestial sphere after 1 rotation.

THERE IS A 4 MINUTE DIFFERENCE.

The stars are AHEAD by 4 minutes each day. (After 2 days = 8 min, after 3 days = 12 min, ..., after 365 days = 24 hours).

Hence, after 1 year (365 days), the stars are back in the same place again.

The stars rise and set 4 minutes earlier each day. This is why we see different constellations at different times of the year.

Example - Familiarize yourself with the constellation of Orion. When are you most likely to see it in the sky in the evening? Summer or Winter? You are most likely to see Orion in the winter. It will be below the horizon at night in the summer, and up above the horizon in the daytime. In other words, in winter, we look out into space away from the sun and see Orion. Six months later, in summer, we have moved around to the other side of the sun.
See diagram in Seasons course notes.
So in summer, in order to look toward Orion, we have to look in the direction of the Sun. Since we are looking toward the Sun that means Orion would be in the sky during the day. So we don't see it in the summer.


Planetary Motion

The ancient Greeks were aware of 5 bodies in the sky that did not behave in a regular, predictable pattern. They were called "wanderers", planetes.

These planets are all on (or very close to) the ecliptic.

Planets rise in the east and set in the west (due to the Earth's rotation), but normally appear farther east each night. Occasionally, however, they seem to slow down and move backwards (westward) for a month or two. This reversal in direction is called RETROGRADE MOTION.

This motion is easily explained by a model in which the Earth and the planets orbit the sun at different distances and at different speeds (faster closest to the sun). However, it was a problem for the early astronomers who did not use a sun-centered model for the solar system.


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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 30, 2005
Updated January 28, 2007
Links updated September 22, 2008