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The Rhythmic Sky

In this EBook we discuss how celestial objects move in the sky and why.
Tags Astronomy, motion, Solar System, Sun, Planets, Moon, Stars, Milky Way, observing, Earth rotation, Earth orbit, Moon orbit, seasons, axial tilt, ecliptic, right ascension, declination, pole, constellations, star names, magnitude, brightness, atmosphere, refraction, Raileigh scattering, atmospheric extinction,
Author(s) EV
First Published April 2008
This Edition - 3.0 June 2017

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Creative Commons logoby nc sa1 Attribution-NonCommercial-ShareAlike 3.0 Unported (CC BY-NC-SA 3.0)





stargazing1Image: For many millennia humans have been looking up at the sky at day or night, and wondered how to make sense of these moving celestial objects, such as the Sun, the Moon and myriads of stars. Some stars moved even slightly differently from the vast majority and we called them “wandering stars”. And then occasionally at night you see “shooting stars” and if you are really lucky, stars with a tail which the ancient Greek Aristotle called “komētēs”, or “stars with hair”.
Now in the early 21st century, we know a lot more about the Sun, Moon, stars, planets, meteors and comets, but the mystery of all these moving objects in the sky, as seen from our backyard, is not necessarily less.

In this module we are starting with the basics and want to answer the following questions:

  • Why the Sun and stars move in the sky the way they do
  • Why the Moon has phases
  • Why we have seasons
  • Why planets move differently from the stars

And also a little about

  • How we define star positions in the sky
  • Star names and constellations
  • Brightness and Magnitude
  • How the atmosphere affects what we see.



Rotation of the Earth

Let us first talk about motion in general. If I would say: “object A moves” than this would be meaningless, because how do I know A is moving? The concept of motion is a relative concept: we can only say that object A is moving with respect to another object B. There is nothing like “absolute” motion, all motions are relative, one object moves with respect to something else.
Think about you sitting on your bicycle and cycling along the road. You would probably say that you and your bike are moving with respect to the road, the houses, trees and people around you, because that is what you see. But it would be equally OK to say that everything around you, essentially the whole Earth, is moving in opposite direction with respect to you. It really is a matter of perspective, from which position you are looking.

Giordano Bruno Campo dei FioriGiordano Bruno. Wikipedia

What has that got to do with the night sky or basic astronomy?

Well ask that question to Giordano Bruno, who just over 400 years ago was burned alive at the stake in Rome, for his opinion that the Earth and not the Sun was moving. With this basic knowledge we discuss here, he might have been able to save his life. Well probably not, because his defence against the Catholic Inquisition was about religious concepts and ultimately about the conflict between religion and science.
But the point here is that motion is relative and that how you experience motion, really depends on your perspective. You could equally well say that the Earth moves around the Sun, as you can say the opposite. When we now, 400 years later, study the motions in the Solar System, the easiest perspective as seen from space, is to put the Sun in the centre and let planets and everything else orbit around it.
But is it so strange that for millennia people were convinced that the Earth was stationary? Not really when you stand in your backyard and see the Sun move through the sky during the day. Clearly the Sun is the only thing that is moving!











Motion of the Sun in the Sky

magnetic Now let us look at the Sun as it moves through the sky with respect to us as an observer. The Sun moves from east to west, but how we see that depends on where you are on Earth.

For navigation on Earth we use the concepts of North, South, East and West. When you face north, east is on your right and west is at your left. But if you turn around and now face south, east is on your left and west is at your right.

clockwisecomposite graphic based on images from and



Clockwise (red)
Counter Clockwise (blue)




Rotating earth



The Earth rotates towards the East

If you are on the Northern hemisphere, you will be facing south if you want to watch the Sun. So you see the Sun move from east to west, which is from left to right. As a rotation you can say that the Sun moves in a clockwise direction, in the same direction as the hands on a clock.

watch the sunGlobe image (edited)



But if you are on the Southern hemisphere, you have to face north to see the sun. Then east is on your right and you therefore see the Sun move from right to left or in a counter-clockwise direction.


Thinking about the example of you sitting on your bike, what is now moving, the Sun or the Earth? Because only the Sun moves and everything else around you seems stationary, you are inclined to say: “well clearly the Sun is moving with respect to the Earth”, the same as humans have been saying for millennia since ancient times. And it is only because of people like Bruno, Copernicus and Galileo and many others, that after a long struggle, public opinion was finally convinced that it would be more realistic to say that the Earth is rotating on its axis and that this is the reason why we see the Sun, and stars at night, move from east to west. We are, as it were, sitting on a carousel rotating in space and we therefore see all celestial objects move from east to west.


So the Earth rotates about its axis towards the east and it takes about 24 hours to complete one revolution. This means that at the equator you are moving with respect to space around Earth with a speed of more than 1600 km per hour. Here in New Zealand that speed is still some 1200 km per hour. Because we are stuck to Earth because of gravity, and everything else around us moves in the same way, we just don't notice it.






Night and Day

nightanddayImage: Sky & Telescope (edited) In the direction towards the Sun the Earth is illuminated: it is day on that side, and away from the Sun, it is dark: it is night on that side. In a period of 24 hours, the shadow side rotates around the Earth and we all experience night and day.


But if you live e.g. in Europe your day will be at the same time that people in say the Pacific, experience night. Therefore we use time zones so that the Sun is at its highest point in the sky (midday) at about 12 noon local time for everybody.





World Time Zones

Hover your mouse over a time zone of interest.


Flash module by Clocklink
For time conversion go here



eunightImage: NASA






Here you see western Europe from space on the edge between light and dark.


Question: Are the people living here awaiting sunrise or is it evening here? (Answer on next page).










Motion of the stars in the sky


For the same reason we see the stars move from east to west during the night.
When you are on the Northern hemisphere and still facing south, you see the stars move from left to right in a clockwise motion. It would actually be more interesting to turn around and face north, because if you wait long enough, you could see the stars rotate about one point in the sky: the North Celestial Pole (NCP). The stars now rotate in a counter-clockwise direction.

North Celestial Pole:

The stars rotate counter-clockwise.



Mouse over either image to see how you can find the Celestial Pole (Images generated with Skymap Pro).


When you are on the Southern hemisphere, it is again the opposite. Facing north you will see the stars move in a counter-clockwise direction and facing south, you will see the stars rotate about the South Celestial Pole (SCP) in a clockwise direction.

South Celestial Pole:

The stars rotate clockwise.

If you live close to the equator, either of the two celestial poles will be very low in the sky, or even invisible, but you will still see the stars move clockwise or counter-clockwise, depending on whether you face south or north.








Answer to question on previous page:

From the map you can see that North is up.
The Earth rotates to the East, hence western Europe is going to experience a sunrise shortly.










Rising and Setting of stars

altitudelatitudesmLet us first ask the question: “how high in the sky do we see the Celestial Pole”? Well that depends on where you are on Earth, more specifically on your latitude or angular distance from the equator.

At high latitudes (e.g. northern Europe, northern America’s, or South Africa, Australia) you will see the celestial pole quite high in the sky. It will be extreme when you are precisely on the north or south pole, when you will see the celestial pole straight above you in the zenith.

Precisely at the equator you will theoretically see both poles at the horizon, but practically you won’t see either of them.

The angle (altitude) you see the Celestial Pole above the horizon
is equal to the latitude of your position (see diagram).

circumPhoto: Gary A. Becker

Depending on where you are on Earth, you could see some stars never rise or set, but always above the horizon. These stars are called circumpolar stars. Time exposure photos can give spectacular results, showing the position of the Celestial Pole.


settingPhoto: Shigemi Numazawa





Stars that are further away from the Celestial Pole will rise and set. We don't have a special name for those, but you could call them rising-and-setting stars.











Orbit of the Moon

In the days of Giordano Bruno and long before that, the common view was that all “heavenly bodies” revolved around the Earth. In one case that was correct, the Moon indeed orbits the Earth. That is a definition of a moon (and we have many moons in the Solar System): it is an object that orbits around a planet.

The moon orbits the Earth in the same direction as the Earth rotates.


Animation: Freedman and Kaufmann

Right-click for player controls


This animation also explains the phases of the Moon as seen from Earth. We can only see the part of the Moon that is illuminated by the Sun, which most of the time is a crescent form and sometimes completely dark (new moon) or completely illuminated (full moon).

It takes the moon about one month (29.5 days) to complete one revolution around the Earth.


Because of the Moon's orbit, we see the Moon in different positions in the sky with respect to the stars. About half a month we see the Moon at night, and during the other half we see it during the day.











Why do we always see the same side of the Moon?








The Moon also rotates about its own axis just as the earth is doing. But it does so much slower. One rotation actually takes the same time as it takes the Moon the complete one orbit around the Earth. And the Moon orbits the Earth in the same direction as the Earth is rotating (towards the east if you want to call it that), and the Moon rotates about its own axis again in the same direction. All this means that we are always looking at the same side of the Moon.


Tidal locking

Tidal lockImage: Wikipedia (scaled) That is no accident and happens quite often with spherical Moons. The gravitational pull between Earth and Moon causes the Moon (and the Earth to a lesser extent) to bulge out in the direction of the Earth as well as on the opposite side of the Moon. The solid Moon resists this deformation and tends to make it constant, in the sense that the bulge always stays at the same position in the Moon’s body. It can do so by adjusting its own rotation to become synchronous with its orbital period about the Earth. This is called tidal locking.

In this animation the closer moon is in tidal locking with the central planet. The moon that is further out has its own independent rotation.
Thus both the rotation of the Moon about its own axis (the "Moon day") and its revolution around Earth (Lunar cycle) take the same time, called the Synodic month of 29.53 days.












lunationImage: Wikipedia


In this tidal locking, the Moon swings slightly back and forth and up and down about this equilibrium position. These motions are called libration. In this time lapse animation you see one complete lunar cycle.





The far side

Before the Space Age, nobody had ever seen the far side of the Moon. Only recently, since we can send orbiters around the Moon, are we able to see the far side of the Moon.

Do not call this side the "dark side" as is sometimes done, because this has nothing to do with Sunlight on the Moon.

The first photograph of the far side was taken by the Soviet Luna 3 probe in 1959. Apollo astronauts were the first humans to see the far side with their own eyes, for the first time from Apollo 8 in 1968.

apollo16 m 3021farsideImage: NASA, Apollo 16

The photo to the right shows part of the familiar near side at the left
and a large part of the heavily cratered far side to the right.














Earth's orbit around the Sun

Both Earth and Moon while orbiting each other, are moving in an orbit around the Sun.
One complete revolution takes about 365 days or one year. But we are not the only one to orbit the Sun of course. There are more planets in our Solar System.


Flashlet Heather Welch,

Right-click for player controls


Check the box "trace orbits" and click the play button

Note that the closer the planets are to the Sun, the faster they move. That is a logical consequence of Kepler's laws of orbital motion. For that reason, the outer planets take much longer to orbit the Sun.

Therefore the length of a year is different for each planet.
Earth completes one revolution in 365.25 days.
This means that Earth has a linear speed through the Solar System of about 17,110 kph.

Remember that in all these views we are looking down onto the Earth's North Pole as a convention.

We see that all planets are moving counter-clockwise around the Sun. This relates to the way the Solar System was formed. Read more about that in our EBook "Solar System".






helionsImage: It is often said that we have seasons (summer, winter and everything in between) on Earth because of the eccentric orbit of the Earth around the Sun.

This orbit is indeed eccentric. The shortest distance to the Sun is about 147,500,000 km and the longest distance is 152,500,000 km. A difference of 5 million km or about 3%.

This does have an effect on the amount of heat we receive from the Sun, but does that explain the seasons?

First of all this effect can not explain why we have opposite seasons on Northern and Southern hemisphere.
Secondly, the varying distance to the Sun will have some effect in the amount of heat energy received from the Sun, but certainly cannot account for the very significant changes in temperature between summer and winter.

Note that the Earth is closest to the Sun in early January.







Axial tilt

AxialTilt 1Image:

There must be another reason and that is of course that the rotation axis of the Earth is tilted with respect to the plane in which the Earth orbits the Sun.
That plane of the Earth orbit is called the ecliptic.

The plane perpendicular to the rotation axis of the Earth itself is called the equator.
The angle between these two planes is about 23.5 degrees and is called "axial tilt" or obliquity. (Demonstrate with globe).





Animation: Freedman and Kaufmann

Right-click for player controls


The orientation of the rotation axis stays roughly the same while the Earth is orbiting the Sun. This means that in different times of the year, the Sun is high in the sky for different parts of the Earth.



250px Earth lighting winterSouthern summer, Northern winter 250px Earth lighting summerSouthern winter, Northern summer
Images: Wikipedia

This explains the changing seasons throughout each year and also why the two hemispheres have opposite seasons.
It also explains why above the arctic circles, the Sun does not set in summer and does not rise above the horizon in winter.





Equinox, Solstice



The line of intersection between ecliptic and equator points at both ends to the so called spring and autumn equinox. This equinoctial direction is an important reference for astronomical co-ordinate systems.

Perpendicular to the line of the equinoxes is the line of the summer and winter solstice. These four points along the ecliptic are indicative for the four seasons on Earth. When the Sun crosses any of these points, there is a corresponding change of season, opposite for the two hemispheres.

The names of the solstices and equinoxes, refer to the seasons on the Northern hemisphere.





Animation: Freedman and Kaufmann

Right-click for player controls


Ecliptic in the sky

The Ecliptic is the Earth's orbital plane around the Sun. So from Earth we see the sun move along a line that we call the ecliptic. The Moon and the other planets have their orbits approximately in the same plane as well, and we see those move along the same ecliptic in the sky as well.


Because of the axial tilt, the ecliptic tilts and moves up and down in the sky throughout the year. The Sun passes the celestial equator at the vernal equinox at about 21 March (northern spring) and the autumnal equinox at about 22 September (northern autumn).





Local Sun path

cs nz solImage: For our local view this means that the sun follows a much longer and higher path in the sky in summer and a lower and shorter path in winter.

The graph shows the path’s of the sun on the southern hemisphere in mid-winter (22 June) (green), spring/autumn (22 September, 21 March) (black), and mid-summer (22 December) (blue).

These seasonal effects are of course opposite on either hemisphere.




Do not confuse the ecliptic with the Milky Way that we can really see on clear nights. That shows the galactic plane and we will talk about that later.






In astrology, the ecliptic is also referred to as the Zodiac, because the Sun and planets are all there. The twelve star constellations along the Zodiac are the birth signs used in astrology. Over one year the Sun moves along all twelve constellations of the Zodiac.











Solar and Sidereal day

siderialversussolardaynewFirst we must explain the difference between the motion of the Sun and the stars.

Every day that the Earth rotates once about its axis, it also moves a bit further along its path around the Sun. It takes about 365 days or 360 degrees for a complete orbit around the Sun, so in one day the Earth will move about one degree in its orbit. This means that the Sun, as we see it from Earth, moves in the sky slightly differently from the stars.



The diagram shows that after one full rotation of the Earth, the same stars will be at the same location in the sky. But as seen from Earth, the Sun is lagging behind a little and the Earth must rotate a bit (one degree) further before the Sun is at the same position where it was the day before.



As a consequence, the stars rise each day about four minutes earlier than the Sun. The period that it takes a star to return to the same position in the sky is called a Sidereal day, and for the Sun that is a Solar day. A sidereal day is thus about 4 minutes shorter than a Solar day. This is the reason why every day at the same time, new stars rise above the horizon.





Throughout a whole year we see the entire sky passing by and this explains why we see different stars and constellations at different times of the year.
A different part of the night sky is visible
from the dark side of the Earth,
throughout the year.




4 Minutes each night

360 degrees rotation takes the Earth 24 hours.
So 1 degree takes 24 / 360 hours
= 24 x 60 / 360 minutes
= 4 minutes.










Motion of the planets

starsandplanetsImage: This animation image shows Jupiter and Saturn moving with respect to the stars over a period of almost a year at two-week intervals.

In the old days planets were referred to as “wandering stars” for that reason. Astrology has been interested in the position of planets for the same reason.




planet loop animationImage: Martin Powell








The reason for this apparent motion is that the planets are very much closer to earth than any of the stars and they move, like Earth, around the Sun. When the Earth "overtakes" one of the outer planets, that planet seems to move backwards in the sky. This is called retrograde motion.




The main differences between planets and stars

Stars are very numerous. Whereas planets are few in number (5 are visible to the unaided eye).

Stars are essentially “fixed” relative to each other. Planets “wander” relative to the fixed stars. So they are not in the same location each night nor in the same position year to year.

Stars have a wide range of declination and right ascension and planets must be on (or very near) the ecliptic.

Stars produce their own light independent of the Sun’s location and they are very far away. The brightness of the planets does depend on the Sun’s location. In comparison to the stars, they are near to Earth.




The Dance of the Planets

planets today labels
The Sun and the five brightest naked-eye planets are shown here in continuous motion from January 2000 to December 2015.

The 'invisible path' along which the Sun appears to move - and which the Moon and planets follow very closely - is known as the ecliptic.

Mercury and Venus are closer to the Sun than Earth and these planets always remain close to the Sun, their positions switching between East and West of it (i.e. between evening and morning "stars").

The outer planets (i.e. from Mars outwards) move well clear of the Sun (up to 180º away) and describe looping motions at much slower speed . The further out the planet is, the slower we see it move in the sky. Note that Mars moves steadily through the constellations while Saturn seems to move at a snail's pace.

This animation has been obtained from Martin J. Powell's web site, and was produced and is kindly made available for public use by David Colarusso .







88constelsImage: The celestial sphere is divided into 88 unequal regions. These regions are what astronomers call constellations.

Historically, constellations were groupings of stars that were thought to outline the shape of something, usually with mythological significance. Their names tracing as far back as Mesopotamia, 5000 years ago.








Example: image to the right: the constellation Orion.
Mouse over to see the mythological interpretation.




In modern astronomy, the significance of constellations is no longer mythological, but practical. Astronomers use the term to describe an entire region of the sky and all the objects in that region.






Note that in the various examples we show of the famous constellation Orion, the view from the Northern hemisphere is depicted. But in the Southern hemisphere we see constellations upside-down, because we are "down under" as compared to the Northern hemisphere. Here are the two views of Orion compared.

Orion40North Orion40South
Orion from 40 degrees Latitude North (left) and from 40 degrees Latitude South (right)
Images generated with Skymap Pro.





Star names

constellationswStars are generally designated by a Greek letter and the name of the constellation in which they reside. Usually alpha is the brightest star in the constellation, beta is the second brightest, etc.

Many stars also have proper names. In this example the proper names are:

  • Alpha Virginis – Spica
  • Beta Virginis – Zavijawa
  • Gamma Virginis – Porrima
  • Epsilon Virginis – Vindemiatrix

In addition, all stars are identified by designators in various star catalogues.



As an example Spica is identified by the following designators:

Proper name Spica
Bayer letter a Virginis
Flamsteed number 67 Virginis
GCVS designation alpha Virginis
Tycho catalog number TYC 5547-1518-1
Hipparcos catalog number HIP 65474
PPM catalog number PPM 227262
SAO catalog number SAO 157923
HD catalog number HD 116658
Bright star number HR 5056
BD number BD -10 3672
WDS designation BUP 150





No physical connection

Animation: Freedman and Kaufmann

Right-click for player controls




The stars in a constellation often are not even near each other! We see the projected pattern of stars of varying distances. So stars that belong to any constellation have no physical connection at all.

Here is an example of the Orion constellation, made up of stars that are at greatly varying distance to us.




Thus, to say that an object (planet, Moon, cluster, nebula, etc) is “in” e.g. the constellation Scorpius is to partially locate the object on the celestial sphere, but has little to do with the physical location of that object. Astronomers use constellations as you would use place names in a road map, to identify an approximate region. This is the modern, practical way of using constellations.




mooninscorpiusImage: Sky&Telescope In this example we could say that
at 25 January "the Moon is in Scorpius".















Seasons and Patterns

seasonsconstellationsImage: In different seasons we see different parts of the night sky. As the Earth moves around the Sun, the dark side of the Earth is directed towards different constellations. This image shows the constellations of the Zodiac (along the ecliptic) that we see in different parts of the year.

In our southern sky typical Summer constellations are Hydrus, Mensa and Volans. Typical Winter Constellations are Triangulum Australe, Norma, and Circinus.



Proper motion

Stars move at high speed through space in different directions. But because they are so extremely far away, we don't see any change in relative positions between stars in a human lifetime.

But on geologic time scales, a constellation’s pattern will slowly change due to what is called “proper motion”, as is shown here for the Big Dipper. These changes are only noticeable over tens of thousands of years.






Big Dipper 150,000 BCE to 150,000 CE

This animated image is generated by SkyChart III showing the movement of stars in and around the Big Dipper over a period of 300,000 years, starting from the year 150,000 BCE. Each frame of the animation represents 1000 years.








Star Positions

celestialsphereImage: We have already looked at the Celestial Sphere with the Celestial Equator, celestial Poles, Ecliptic and and the equinoxes and solstices. The stars are mapped onto this imaginary sphere. That is convenient for defining their position in the sky.

This way of looking at the Earth and how we see the stars is a Geocentric view, similar to the old Geocentric cosmological model used for many centuries. We now know that the Sun is at the centre of the Solar system, but this “old-fashioned” geocentric view is still convenient for our discussion of the night sky and defining positions of celestial objects.

The co-ordinates of objects on the celestial sphere are expressed in Declination and Right-Ascension. These are analogous to longitude and latitude we use to identify positions on Earth.





nzonearthImage source unknown (edited)



Where we are in New Zealand we have a latitude
of about 43 degrees South and a longitude
of 172 degrees East of Greenwich.
Image source unknown (edited)




Latitude measures the number of degrees north or south of the Equator. Stars have a north-south position called the Declination which is similar to the idea of latitude. The declination of a star is the number degrees north or south of the Celestial Equator.

Polaris (near the Celestial North Pole) has a declination of almost 90º N. Stars over the equator have a declination of 0º.


Longitude measures the number of degrees east or west of the Greenwich meridian. Stars have an east-west position called the Right-Ascension which is similar to the idea of longitude. Right ascension is the angle eastward towards the star measured from the Vernal equinox. Right ascension is measured in the same direction as the rotation of the Earth and is therefore essentially a time measure. It is expressed in hours, minutes and seconds. One complete circle corresponds to 24 hours, equal to 360 degrees.

The right-ascension of a star is however independent of the rotation of the earth. Because it is connected to the Vernal Equinox, it is an angle that is fixed with respect to the stellar background. The value of Right Ascension and Declination for a star is practically constant, it only changes noticeably over ten ’s of thousands of years (as we discussed for constellations). These star co-ordinates Declination and Right-Ascension are listed in star catalogues.

Star charts, Planisphere

sspn3680Example of a star chart
For amateur astronomers star charts are very useful to find stars and constellations and to become familiar with the night sky. Sky Charts are available in many forms and editions and are generally a good investment.


star disc planisphere   astrovisualsExample of a planisphere



A practical tool based on a star chart is the planisphere, that allows you to display the part of the night sky that is visible for the location at any time. The planisphere consists of two disks that rotate about a common pivot. Star charts and Planispheres are available commercially, although you can find free versions on the internet to print yourself. A good location to download templates for making your own planisphere can be found here.




There are also many computer programmes available that display the night sky at any time and any location on Earth. Such planetarium software is commercially available as well as in the form of shareware or freeware.
A good place to find such software is here.








The Milky Way

800px Milky way 2 mdThe galactic centre in the direction of Sagittarius.
The primary stars of Sagittarius are indicated in red.
Image Wikiped
Another feature in the sky, especially at very clear nights, is the Milky Way, a band of many stars. That lane across the sky shows us the Galactic Plane, the disk of the Milky Way galaxy. It shows us the galactic plane, that makes an angle of about 60 degrees with the ecliptic.

The Milky Way is a barred spiral galaxy with a diameter of at least 150,000 lightyears, although a halo of dark matter around it extends to many times further out. The disk is between 100 and 10,000 light years thick. There are at least 400 billion stars in the galaxy of which our Sun is just one.

Some parts of the Milky Way appear dark, as if there are few stars in that region. This is only apparent, it is caused by large clouds of hydrogen and stellar dust, that block the visible light. In the southern hemisphere we can see the centre of our galaxy in the constellation of Sagittarius.


milky way earth location 580x474Image:






Reconstruction of the Milky Way galaxy, based on what we have observed in many different wavelengths. Our Solar System is located somewhere in one of the outer spiral arms, the Orion arm.







Where does the name “Milky Way” come from?

The word Galaxy is related to the Greek word for milk “gala”. They called it Galaxias Kyklos (“Milky Circle). The Romans translated that into Via Lactea meaning “Milky Way”.
There are many other mythological origins for the name of our galaxy, but curiously many relate to “Milky”.















Brightness and Magnitude


We see stars in a large range of brightness, from as bright as the brightest star Sirius (apart from the Sun of course) until the faintest stars just visible with the naked eye. Beyond that we can see even fainter stars with binoculars or telescopes.

Brightness is no measure for distance to the star.
A very faint star could indeed be relatively near by, but just be less luminous than other stars. Bright stars could be far away when they have a very great luminosity.

Arctures has an apparent magnitude of -0.05 and Proxima Centauri has an apparent magnitude of 11.01. Does this mean that Arcturus is closer to us?
Not at all. Arctures is ten times further away than Proxima Centauri.


Brightness as we see it, i.e. apparent brightness, is usually measured on a magnitude scale. The magnitude of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth.
The brighter the object appears, the lower the numerical value of its magnitude.


SummerTriangleDeneb, Vega and Altair form the Summer Triangle in the northern sky. Vega is the brightest and is the reference star for the magnitude scale. Deneb is actually 4000 times brighter than Vega, but it is 100 times further away and has apparent magnitude 1.


Ancient Greek astronomers divided stars visible to the naked eye into six magnitudes. Later this system was formalised into a system that defines a typical first magnitude star as a star that is 100 times as bright as a typical sixth magnitude star.

Vega is used as the standard reference star and has magnitude 0 (zero). The modern system is no longer limited to 6 magnitudes or only to visible light. Very bright objects have a negative magnitude.

Sirius, the brightest star of the celestial sphere, has a magnitude of −1.46. The modern scale includes the Moon and the Sun; the full Moon has a magnitude of −12.6 and the Sun has a magnitude of −26.73.

The faintest stars that are observable by the naked eye have a magnitude of 6. The Hubble Space Telescope has located stars as faint as magnitude of 30 at visible wavelengths.







Absolute magnitude uses the same scale but measures the actual brightness (usually called luminosity) of a star. Without additional information, we cannot tell what the absolute magnitude of a star is. Astronomers can calculate the distance to a star when they know both the apparent and absolute magnitude. See our separate Ebook "Stellar Distance" for more details.






Effect of the Atmosphere

AtmosphereLayersAtmosphere profile
Image Wikipedia

The atmosphere is really very thin. Imagine that you have a globe of the Earth with a diameter of 30 cm.
The atmosphere of 480 km is then less than half a millimetre at that scale.

The Earth's atmosphere is a thin layer of gases that surrounds the Earth. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases. The Earth's atmosphere is about 480 km thick, but most of the atmosphere (about 80%) is within 16 km of the surface of the Earth. There is no exact place where the atmosphere ends; it just gets thinner and thinner, until it merges with inter-planetary space.

This thin gaseous layer insulates the Earth from extreme temperatures; it keeps heat inside the atmosphere and it also blocks the Earth from much of the Sun's incoming high-energy radiation.


Absorption and scattering

The nearer a star is to the horizon the fainter the star appears because of atmospheric extinction. This is caused by a combination of absorption (by water and ozone) and scattering.

Rayleigh scattering affects blue light more than red so that when we see an object lower towards the horizon, there is a corresponding reddening of the object. This scattering is also the reason why we see the sky during the day as blue. The blue component of sunlight scatters most, whereas the yellow and red colours of the sunlight are more passing straight through the atmosphere. So we see the sky as blue and interpret sunlight as being predominantly yellowish.

Atmospheric extinction is the reason that the best spot in the sky to observe astronomical objects is the zenith which is directly overhead. Star light travels through less atmosphere at the zenith than in any other direction. Extinction is worst at or near the horizon.

For example, you can look directly at a Sun set because the Sun's light has maximum atmospheric absorption at the horizon. When the Sun is high in the sky it is painful and dangerous to look at the Sun without optical protection.

400px Top of AtmosphereRayleigh scattering causes the atmosphere
to appear blue.
Image: Wikipedia
As stellar light passes through the atmosphere, it is refracted (bent) just as through a lens. This bending results from the increase in the atmosphere’s density as the light ray travels downward toward the observer. Thus refraction makes an object appear higher in the sky than it actually is.

 refractionRefraction does make a star appear
higher in the sky than it actually is.

The resolution of a telescope is significantly limited by atmospheric turbulence. This is caused by irregular motion of air in the atmosphere and happens in and around an observatory dome and in the free atmosphere. It depends largely on meteorological conditions.

vt photo 02 tpmVenus in
turbulent sky.

We are all familiar with the distortion of a setting Sun or Moon. These bodies average about 30 arc minutes in size. The image to the right illustrates the flattening effect by refraction as well as the reddening closer to the horizon. It also includes the effects of atmospheric turbulence.


sun setting w Brown PelicanImage:



Because of all these atmospheric limitations, we prefer to observe objects above a minimum altitude, usually 20 or 30 degrees above the horizon. Professional observatories are built at high altitudes above sea level to minimise these atmospheric effects. Better still, we now have many astronomical observatories in space where there are no atmospheric limitations at all. But a lot more technical problems.

distantkecksKeck Observatory, Mauna Kea is at 4,200 m above sea level.
Image: (edited)






OASObservatory 1In this module we have talked about how and why celestial objects move in the sky and how that depends on where we are on Earth. We also have discussed some practical aspects of observational astronomy, such as star positions, constellations and about magnitude and the influence of the atmosphere.

After this introduction it might be a good idea to go outside on a clear night and to possibly feel a little bit more comfortable about what you see, knowing how things move and why. Get yourself some star charts and try to find some popular constellations and stars, and try to find the Celestial Pole. Maybe you can also identify some planets and then realise where the ecliptic is.

As a beginner amateur astronomer it is even better to visit the nearest observatory where experienced star gazers can help you to become familiar with the night sky.

Happy star gazing !




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