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Understanding_the_Motion_of_the_Sky

Understanding the Motion of the SkyFrom McWiki

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To locate, understand, and appreciate the astronomical objects you look at, you need a basicunderstanding of how the sky moves. This is especially true if you have a tripod with an

equatorial mount,since these are specifically designed to match the motion of the sky.

Note: this article is written from the perspective of an observer in the Northern hemisphere. Thesame concepts apply in the Southern hemisphere, except for the absence of a stationary pole star.

Contents

[hide]

1 What's the Problem?

2 What You've Probably Noticed

3 What's Going On?

4 What You See When You Look Up

5 What You See In Your Telescope

6 Timing and Movement

o

6.1 Movement in One Nighto 6.2 Movement Between Nights

7 The Planets

8 The Moon

9 Conclusion

10 Comments?

What's the Problem?

Thebeginner in our fictional articlewas disturbed by the fact that objects he was looking at kept

drifting out of his field of view, always in the same direction. He assumed there was somethingloose on his telescope. There probably was something loose, but that was not why things were

moving out of his field of view; it was because the rotation of the Earth makes objects in the sky

appear to move.

Beginners will also notice that objects aren't in the same place each night. The stars move

slowly, causing certain constellations to move around in the North, while others are visible only
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during certain seasons. Other objects, like the Moon, appear to have yet another schedule,

moving around the sky at their own pace.

What You've Probably Noticed

Even if you haven't been studying astronomy, you have probably noticed certain things throughyour life:

The Sun always rises in the East and always sets in the West, in roughly the same spot(moving North and South a bit as the seasons change).

The Moon always rises in the East and sets in the West, in about the same locations as the

Sun.

You may not have noticed that stars rise and set, but you have probably noticed:

o Different constellations are visible at different times of year.

o Certain constellations, like the Big Dipper, are almost always visible, but show up

at different places in the sky throughout the year -- sometimes upside-down or

standing on one end or the other.

What's Going On?

It's fairly simple geometry. Which we're not going into in detail here -- there are many excellent

books on planetary mechanics.

All the motions and changes you see are a result of four simple facts:

1. The Earth is spinning on its axis, one turn

every 24 hours.2. The Moon is orbiting around the Earth, one

orbit every 27.3 days.

3. The Earth and all the other planets are

orbiting the Sun, all in the same direction, and

all in roughly the same plane (i.e. it's like theyare all laid out on a large dinner plate with the

Sun at the centre). The outer planets orbit

more slowly than the inner planets.

4. The stars are stationary. (They do, in fact, move very quickly. But they are so far away thatthe motion appears to us to be too slow to perceive without special instruments.)


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What You See When You Look Up

The above facts are sufficient to explain all the motions you see when you look at the sky.

Standing on the ground, we do not perceive the motion of the Earth, either its rotation or its orbitaround the Sun -- instead, it seems to us as though we are stationary and the sky is moving

around us.

Since the Earth is rotating around its axis, the stars directly above the axis don't seem to move

much. The North Star is almost directly above the axis and seems completely stationary. As we

move away from due North, the stars appear to move, tracing circular paths around the NorthStar, with one time around the circle taking 24 hours. The North Star itself would appear directly

overhead if you were standing on the North Pole. If you are South of the North Pole, the North

Star will appear somewhere between the horizon and directly overhead. The angle above the

horizon is equal to your latitude so, in Ottawa, I see it as about 45 degrees above the horizon.

The North Star doesn't move, so it is always visible. Other stars, if they are close to the North

Star, will always be visible even though they will move around it. Such stars are calledcircumpolar. Stars sufficiently far from the North Star will be visible only part of the night,

since they will rotate below the horizon. Which stars are visible during the night hours variesduring the year as the Earth orbits the Sun.

Because the Earth is rotating toward the East (i.e. it is spinning counter-clockwise as seen from

above the North Pole) everything appears to rise in the East and set in the West: the Sun, the

stars, the Moon, and the planets.

Because the planets all lie in roughly the same plane, the Sun and Planets all followapproximately the same path through the sky. This path, called the Ecliptic, is the projection onto

the sky of the plane of the solar system. The easily visible planets like Mars, Saturn, and Jupiterare always on or very close to this line.

What You See In Your Telescope

When you look through your telescope, you are looking at a very small section of the sky, so the

apparent movement of the stars is greatly amplified. Through a typical telescope and eyepiece,

stars will move completely out of your field of view in a minute or two.

Modern telescopes include motors to rotate the telescope at the same rate at which the Earth

rotates. When everything is correctly aligned, this will result in the stars appearing to be

stationary in the telescope, permitting long observation without constant aiming adjustments.

Timing and Movement

It's interesting (or so I think anyway) to do some simple math and think about the effects of some

of the movements we see.


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Movement in One Night

Since the Earth rotates every 24 hours, any given star must move completely around the sky in

24 hours. A complete circle around the sky is 360 degrees. 360 degrees in 24 hours is 360/24 =15 degrees per hour, or 15/60 = 0.25 degrees per minute.

You can do a lot with those figures. Some examples:

1. The angular diameter of the Moon is about 0.5 degrees. This means that a star will travela distance in the sky equivalent to the apparent width of the moon in about 2 minutes.

(Stars don't "overtake and pass" the moon, since the moon is moving too. You need to

compare to a stationary object like a chimney to see this motion.)

2. Suppose Jupiter has just risen, but it's so close to the horizon that you can't easily observeit yet. When will it be higher? Suppose we'd like to wait till it travels about 45 degrees

across the sky. That will take about 45 / 15 = 3 hours.

3. A typical amateur telescope with a typical eyepiece (say, a 200 mm Dobsonian reflector

with a 20mm Plossl eyepiece) shows you a piece of sky about 1 degree wide. So, starswill drift completely through this field of view in about 4 minutes. Or, if you centre a

star, it will drift out of your field of view in about 2 minutes.

4. Observing planets is worse. We like to use high power, with a narrower field of view.That same telescope, set up for 200x magnification, only shows about 1/2 degree of sky,

so Saturn or Jupiter will drift out of your field of view in about a minute. This is why

owners of non-motorized telescopes appreciate the wider-field eyepieces like Naglers orRadians, to keep objects in their field of view longer.

Movement Between Nights

The Earth orbits the Sun once every 365.25 days. So, at some fixed time of night (say, midnight)any given star will be in a given position one day, slightly moved from that position the next day,

and so on, returning to the same position 365.25 days later.

So, at the same time of night, a star moves its apparent position 360 / 365.25 = 0.99 degrees eachday. Let's call it 1 degree.

So if a star - say, Sirius - is just at the horizon at 9:00 PM on a given day, it will be about 30degrees higher at 9:00 PM about 30 days later. If the Big Dipper is sitting flat in the sky at

midnight on a given day (i.e. horizontal and able to hold water), at midnight 180 days later it will

be upside down, on the other side of the North Star.

The Planets

The planets appear to move across the sky, following the Ecliptic, at almost the same rate at

which the stars move. However, since the planets are also orbiting the Sun at their own rates,they do not appear completelystationary against the background stars. Instead, they will move

slowly between the stars and constellations. The planets closest to the Sun move quickly, so

Mercury and Venus will appear to change their position in the sky, relative to the stars, quite


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quickly - noticeably moving in only a few days. The outer planets move slowly, so Jupiter and

Saturn will appear to very slowly drift between the stars, leaving one constellation and entering

the next every year or two.

It was this motion across the background of stars that first drew the attention of early observers

to the planets, making it obvious that something different was going on with those points of light.

The planets can even appear to move backwardagainst the stars for brief periods, because of the

line of sight effects of the Earth and the other planet's position in their orbits. Explaining thisoccasional backward motion was one of the biggest challenges for early astronomers trying to

work out an accurate model of the sky.

The Moon

The Moon orbits the Earth every 27.3 days. 360 degrees / 27.3 days = approximately 13 degrees

per day. So at any given time of night, the moon will appear to have moved 13 degrees in the sky

from one day to the next.

The phases of the moon are caused by the angle between the Sun, the Moon, and the Earth (since

the moon "shines" only by reflecting the light of the Sun toward our eyes; it does not emit any

light of its own).

A full moon is when the Sun and Moon are on opposite sides of the Earth. We are directly

between them, so the light of the Sun is reflected straight back at us. This is why a full moon

always rises exactly at sunset, and sets exactly at sunrise. That's in theory - in practice, it varydepending on your longitude within your time zone and with the exact time the moon is 100%

full during the night of full moon; so don't set your watch by it. You'll sometimes read stories or

see movies involving a full moon rising late at night. That's an error, it can't happen that way; ifthe full moon is on the horizon, it's sunset or sunrise, give or take an hour.

After the full moon, the shrinking crescent (called waning) rises later and later in the evening,until it actually rises during daylight and is visible in the early morning. Eventually the moon is

completely invisible when it is between the Earth and the Sun, reflecting no light toward us.

Then the crescent starts to grow (waxing), visible early in the evening.

We always see almost exactly the same "face" of the moon. This does notmean the moon is not

rotating - if it was not rotating, we would see all sides of it as it orbited around the earth. We seeonly one face because the moon rotates on its axis every 27.3 days - exactly the same speed at

which it orbits the earth. So, it rotates just fast enough to always keep the same face towards us

as it orbits. This is not a coincidence - it is a result oftidal forcesbetween the Earth and the
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Moon "adjusting" the moon's rotational speed over many years until the present stable state was

reached. This is a natural effect for any large moon orbiting any large body, and allof the large

moons in our solar system (e.g. those of Jupiter and Saturn) are locked to face their planet in thesame way.

Conclusion

The sky is an active place, with stars whirling around the North pole, and the Sun and planets

tracing an invisible path through the sky at varying speeds. While everything appears stationary

to a casual glance upward, your telescope will magnify the motion enough to actually see it.

Understanding the motion of the sky is important to help you plan your observing sessions, know

where to find interesting objects, and understand the changes you'll encounter throughout theyear.

Back to Astronomy Writings Up to Richard's AstronomySection

Comments?

7 commentsPost comment

T.A.

(8 September 2008, 05:46)

ReplyGreat site! Lots of information to share with my 5th grade class!

Paul

(14 September 2008, 22:08)Reply

Fantastic articles explaining how to set up your telescope, the whys and wherefores etc.

Recommended!

JXB7076(30 March 2009, 22:44)

Reply

This was very informative. We have a tendency of taking our limited knowlege of science and

thinking that we know what's gong one in the skies, until article like these come a long andreminds us that our true knowlege represent less than 1% of what's really going on. I learned a

lot. Thanks for sharing the wisdom!

E.C.(30 November 2009, 01:11)

Reply

Thank you--this was a very clear explanation of the apparent motion of objects in the sky.

oeja
http://www.themcdonalds.net/richard/index.php?title=Astronomy_Writingshttp://www.themcdonalds.net/richard/index.php?title=Astronomy_Writingshttp://www.themcdonalds.net/richard/index.php?title=Astronomyhttp://www.themcdonalds.net/richard/index.php?title=Astronomyhttp://www.themcdonalds.net/richard/index.php?title=Astronomyhttp://www.themcdonalds.net/richard/index.php?title=Astronomyhttp://www.themcdonalds.net/richard/index.php?title=Astronomyhttp://www.themcdonalds.net/richard/index.php?title=Astronomy_Writings


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(20 July 2011, 04:22)

Reply

Thanks for such a nice stuff

Robin(9 November 2011, 05:07)

ReplyThanks for the info, it helped to explain alot.

Caroline(11 November 2011, 18:29)

ReplyThis is so clearly put. Thanks. It helped to get a hold on what was going on. Turned to this after

first seeing Jupiter in my new telescope moving fast out of field of view - this helped me explain

to 10 year old daughter!!

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The Motion of Objects in the Sky

Dr. Pamela GoreGeorgia Perimeter College

http://facstaff.gpc.edu/~pgore/Earth&Space/GPS/motion_

of_objects_in_sky.html-30/10/2014

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.
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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 varioustimes 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 canbe 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 systemand 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 earths tilt to

explain the seasonal changes.

d. Demonstrate the relative size and order from the sun of the planets in the solarsystem.

S6E1d. Explain the motion of objects in the day/night sky in terms of relativeposition.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:


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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 peopleon 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 EducationStandards:

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, andeclipses.

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'srotation 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 highin the sky at noon.

Ancient peoples recognized these facts and built stone observatories to track the movementof the sun back and forth along the horizons, and hence to mark the seasons. These sorts ofstructures are sometimes calledhorizon calendars.

Diagram showing approximate positions of the sun at sunrise, noon, and sunset at thesummer 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)
http://www.hao.ucar.edu/public/education/archeoslides/slide_4.htmlhttp://www.hao.ucar.edu/public/education/archeoslides/slide_4.htmlhttp://www.hao.ucar.edu/public/education/archeoslides/slide_4.htmlhttp://sunearthday.nasa.gov/2005/locations/stonehenge.htmhttp://sunearthday.nasa.gov/2005/locations/stonehenge.htmhttp://www.hao.ucar.edu/public/education/archeoslides/slide_4.html


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Stonehenge in England. Photos courtesy Pamela Gore.

2. Newgrangein England

3. TheSun Dagger used by the Anasazi Indians at Fajada Butte, Chaco Canyon, NewMexico

4. Cahokia Mounds in southern Illinois

5. The Big Horn Medicine Wheel in the Big Horn Mountains near Sheridan, Wyoming6.

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 avertical 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 SavingsTime, which will throw things off a little.) After noon shadows grow longer, as the Sundescends towards the horizon.
http://sunearthday.nasa.gov/2005/locations/newgrange.htmhttp://sunearthday.nasa.gov/2005/locations/newgrange.htmhttp://library.thinkquest.org/C0118421/dagger.htmlhttp://library.thinkquest.org/C0118421/dagger.htmlhttp://library.thinkquest.org/C0118421/dagger.htmlhttp://library.thinkquest.org/C0118421/dagger.htmlhttp://sunearthday.nasa.gov/2005/locations/newgrange.htm


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

Because the shadow always points away from the Sun :

At sunrise, with the Sun in the east, it points to the west.

At noon, with the Sun in the south, it points north At sunset, with the Sun in the west, it points to the east.

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

Since the height of the noon sun in the sky differs at different times of the year (high insummer, 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 thepole), 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.
http://www-istp.gsfc.nasa.gov/stargaze/Sundial.htmhttp://www-istp.gsfc.nasa.gov/stargaze/Sundial.htmhttp://www-istp.gsfc.nasa.gov/stargaze/Sundial.htmhttp://www-istp.gsfc.nasa.gov/stargaze/Sundial.htmhttp://www-istp.gsfc.nasa.gov/stargaze/Sundial.htmhttp://www-istp.gsfc.nasa.gov/stargaze/Sundial.htm


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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 thedashed 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 positionof the sun in the sky.

Image courtesy ofNASA.

If this image does not appear, click the link to the NASA website. You should thenbe 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 materialbelow 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 wintersolstice.

When is the sun highest in the sky?

There is a third line, between these two. It represents the position of the sun at theequinoxes.

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 furthestto 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 THEWEST?

For more information, seecourse 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 westdaily, but its position in the skymoves 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, whilelooking 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.
http://www-istp.gsfc.nasa.gov/stargaze/Sunangle.htmhttp://www-istp.gsfc.nasa.gov/stargaze/Sunangle.htmhttp://www-istp.gsfc.nasa.gov/stargaze/Sunangle.htmhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/seasons.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/seasons.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/seasons.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/moonphases.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/moonphases.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/moonphases.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/seasons.htmlhttp://www-istp.gsfc.nasa.gov/stargaze/Sunangle.htm


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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 theEarth.

The length of time from New Moon to New Moonis called the LUNAR MONTHorSYNODIC 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 takesLONGER (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 degreesover the course of the month. The Moon will have gone around the Earth once with respectto 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 periodis 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 moreinformation, seecourse 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 toremain 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, youwould see that the stars appear to rotate around the North Star. For example, seephotos at

this web site,andhere.See animation of star movement here.As you can see, the stars rotate counterclockwisearound 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 positionchanged?

The North Star and Latitude

Polaris always seems to stay in one point in the night sky. The other stars seem to rotatearound it. This is a result of the Earth's rotation on its axis. We can use the stars todetermine 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 higherabove the horizon.
http://facstaff.gpc.edu/~pgore/Earth&Space/GPS/eclipses.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/eclipses.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/eclipses.htmlhttp://www.danheller.com/star-trails.htmlhttp://www.danheller.com/star-trails.htmlhttp://www.danheller.com/star-trails.htmlhttp://www.danheller.com/star-trails.htmlhttp://www.polaris.iastate.edu/NorthStar/Unit1/unit1_sub2.htmhttp://www.polaris.iastate.edu/NorthStar/Unit1/unit1_sub2.htmhttp://www.polaris.iastate.edu/NorthStar/Unit1/unit1_sub2.htmhttp://www.goals.com/ClassRm/SailSci/latitude.htmhttp://www.goals.com/ClassRm/SailSci/latitude.htmhttp://www.goals.com/ClassRm/SailSci/latitude.htmhttp://www.polaris.iastate.edu/NorthStar/Unit1/unit1_sub2.htmhttp://www.danheller.com/star-trails.htmlhttp://www.danheller.com/star-trails.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/eclipses.html


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

Daily motions of the Earth with respect to the stars and thesun; or

Why we see different constellations at different times of theyear

If we point a telescope at a star on the meridian, where will it be 24 hours later? If a day is24 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 sunEVERY DAY.

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

The Sun seems to shift against the background of stars as the days go past. The sun will notbe 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 thewinter. 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 seeOrion. Six months later, in summer, we have moved around to the other side of the sun.

See diagram inSeasons course notes.So in summer, in order to look toward Orion, we have to look in the direction of the Sun.
http://facstaff.gpc.edu/~pgore/Earth&Space/GPS/seasons.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/seasons.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/seasons.htmlhttp://facstaff.gpc.edu/~pgore/Earth&Space/GPS/seasons.html


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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

appearfarther east each night. Occasionally, however, they seem to slow down andmove backwards (westward) for a month or two. This reversal in direction is calledRETROGRADE 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 solarsystem.

Return to Earth & Space Science page

Return to Georgia Geoscience Online

Page created byPamela J.W. Gore

Georgia Perimeter College,Clarkston, GA

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

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http://flintriverastronomy.org/Articles/MEASURING%20TIME%20AND%20MOVEMENT%20IN%20THE%20NIGHT%20SKY.pdf-31 Oktober 2014
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