ASTRONOMY - our Solar System, Telescopes, Planets, Satellites, Stars and Galaxies

 How has the explanation, theory and model of the Solar System evolved through time?

 Explain the usefulness of telescopes and photography - observing the Solar system and the cosmos in general with visible light.

How has astronomy changed over the centuries?

Sub-index for this page

(a) A history of models of our Solar System

(b) Our contemporary model of our Solar System and beyond

(c) Methods of astronomical observation - types of telescope

(d) More notes on the objects in our Solar System - definitions explained

(e) The physics of circular motion and the forces involved in solar and planetary systems

(f) More on natural and artificial satellites - their orbits and uses

(g) Typical learning objectives and knowledge for the contents of this page

Appendix 1. The phases of the Earth's moon

Appendix 2. The seasons of the Earth

 See also Cosmology - the Big Bang Theory of the Universe

 and the Life Cycle of Stars for more detailed notes

(a) A history of models of our Solar System

The ideas about the structure of the Solar System have changed over time, including the change from the geocentric to the heliocentric models and the discovery of new planets.

The geocentric model theory

The sun, moon, planets, and stars i.e. everything you could see, all orbited the Earth in a series of concentric circles.

So the Earth was considered to be the centre of everything in the Solar System.

An orbit is the path one object moves around another object (held in orbit by gravity).

They also considered the orbits to be perfect circles.

This model originates from ancient Greek civilisation 2000-2500 years ago and lasted for 1500-2000 years.

Ancient astronomers didn't have telescopes to make accurate observations to test out any other model.

What they observed was the Sun and moon moving across the sky in the same way every day and every night and assumed they moving in a perfect circle around the Earth.

They did observe the apparently wandering of the planets - the word planet comes from an ancient Greek meaning 'wanderer', but this did not lead to a questioning of the geocentric model

Prior to ~1500s this was the accepted model of the Universe from the time of the ancient Greeks.

The heliocentric model

From the 1500s onwards and with the help of telescopes, evidence was mounting up to indicate that the planets were orbiting the Sun - the heliocentric model, but not in perfect circles.

The Sun is now considered to be the centre of our solar system.

We now recognise that the Earth and other planets are moving around our Sun in elliptical orbits.

Initially the Sun was considered as the centre of the Universe, but of course we now know it is now only the centre of our solar system.

Astronomers such as Copernicus working in the mid-16th century, were making observations and calculations to explain the movement of the planets without the geocentric model and that a heliocentric model fitted the data better.

Copernicus published his heliocentric theory and calculations in 1543, just in time, two months before his death!

Galileo’s observations of Jupiter, using the telescope, provided evidence for the heliocentric model of the Solar System - initially it provided evidence that not everything revolved around the Earth.

Galileo, in the early 17th century, working with the newly invented telescope, found his view of the 'universe' in conflict with that of the Catholic Church, especially after discovering moons orbiting around the planet Jupiter, which meant not everything orbited the Earth and the geocentric model was flawed.

One piece of Galileo's evidence came from observing the moons moving around Jupiter with his newly invented improved telescope of 1609, he therefore observed objects moving around with Jupiter, which clearly could not be moving around planet Earth.

Initially Galileo thought four stars were surrounding Jupiter, but he figured out they were moons of Jupiter because of their movement - they moved along with Jupiter and were observed in different positions around it - therefore showing that all 'heavenly' bodies were not moving around the Earth.

The Catholic Church was not too impressed by a scientific model challenging the religious view of how our 'universe' works.

Over the following centuries more evidence for the heliocentric model increased, helped by the technological advances in astronomy eg improved telescope designs.

From the early 20th century onwards, it was recognised that our solar system is just one of billions of systems with a central star in a galaxy and that there are billions of galaxies that make up everything - the Universe!

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(b) Our contemporary model of our Solar System and beyond

The 8 major planets, minor planets and asteroids orbit the Sun in slightly elliptical orbits (our 'Solar System'), but our Sun is just one of millions-billions of stars in our galaxy (we see part of it as the 'Milky Way') and in turn the observable universe itself contains billions of other galaxies.

(a) Our Solar System - the Sun (a main sequence star) and 8 orbiting major planets, and minor planets and asteroids.

The movement of the planets and asteroids has been observed from visible light (reflected sunlight) for thousands of years, initially with the naked eye and from the early 16th century onwards, with telescopes.

The force of gravity

Generally speaking the gravitational field constant at the surface of a planet increases with its mass.

The gravitational constant of our Moon is 1.7 m/s² or 1.7 N/kg.

The gravitational constant of our Sun is 293 m/s² or 293 N/kg.

The strength of the gravitational pull decreases the further you are from the centre of the planet (the attractive force decreases according to an inverse square law: force 1 / distance²)

With modern techniques, the Sun, at the centre of our Solar System, can be observed by detecting emissions in various regions of the electromagnetic spectrum eg infrared, visible light, ultraviolet, X-rays and even gamma ray emissions.

Orbital paths

The orbits are not quite perfect circles, but slightly 'squashed' into an elliptical shape.

The Earth is the 3rd planet from the Sun - see above data table on the planets.

The Sun is ~150 million km away from us and sunlight takes ~8 minutes to reach us.

Mercury, Venus, Earth and Mars are the four inner planets, relatively small and consisting mainly of rock.

The gas giant planets Jupiter, Saturn, Neptune and Uranus have gases such as hydrogen, ammonia, methane and carbon dioxide their atmosphere, may have rocky cores?

(b) Our Milky Way - is our view of looking through our own galaxy

The name 'Milky Way' comes from the profusion of bright starlight from our galaxy when you look through its centre against the background of the relatively dark night sky.

Our solar system is just one small part of a galaxy - which is a massive collection of billions of stars held together by gravity.

The Milky Way rotates around the central core of the galaxy and astronomers think there is a massive black hole there.

Until relatively recently, the Milky Way galaxy, has been observed with the naked eye and then telescopes on Earth, but now it can be viewed through powerful telescopes on satellites eg the Hubble Space Telescope.

Our galaxy, and for that matter distant galaxies, can be continually observed using everything from giant radio telescopes, huge optical\visible light telescopes to gamma ray burst detectors.

(c) 'Outer space' and nebulae

Beyond the Earth and beyond our own solar system and galaxy, it is far from being a vacuum of 'emptiness'.

Interstellar space (between stars) contains huge clouds of dust and a mixture of gases, mainly hydrogen and helium gases, but traces of lots of other molecules including organic molecules.

These clouds are where stars are formed and are called nebulae. A nebula is an enormous cloud of dust and gas occupying the space between stars and acting as a nursery for new stars.

(d) The Universe is everything - see separate page Cosmology - the Big Bang Theory of the Universe

The cosmos is a term used to describe the universe seen as a well-ordered whole.

A mathematics note on distances - a sort of perspective on 'everything'!

The distance from planet Earth to the Sun is 150 million kilometres.

150 Mkm, 150 000 000 km, and in standard form 1.5 x 10⁸ km.

The distance from the Sun to the dwarf planet Pluto is 5915 million km.

5915 Mkm, 5 915 000 000 km, in standard form ~5.9 x 10⁹ km.

The diameter of our galaxy, the 'Milky Way', ~1 000 000 000 000 000 000 km, in standard form ~1.0 x 10¹⁸ km.

Some derived calculations, taking the speed of light to be 3.0 x 10⁸ m/s.

Ex. 1. How long does it take light to travel from the Sun's surface to the Earth?

speed = distance / time,   time = distance / speed

time = 1.5 x 10⁸ x 1000 / 3 x 10⁸ = 500 s, 8 minutes and 20 seconds.

Ex. 2. How long does it take light to cross from one side of our galaxy to the other?

time = (10¹⁸ x 1000) / 3 x 10⁸ = ~3.33 x 10¹² s

1 Earth year is 365.25 x 24 x 60 x 60 = 31557600 s

time = 3.33 x 10¹² / 31557600 = ~106 000 years!

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(c) Methods of astronomical observation - types of telescope

Be able to compare methods of observing the Universe using visible light, including the naked eye, photography and telescopes.

In observing the night sky, the naked eye, apart from aesthetic appreciation, has been largely replaced by photography, usually coupled to a telescope.

However, historically, stars, planes, comets, our Moon have all been successfully discovered, observed, mapped and plotted via naked eye observations and astronomical tables of data assembled.

Distant stars can be seen because they are so hot and powerful emitters of electromagnetic radiation eg visible light.

Optical telescopes have much better light gathering power than the naked eye and the lens and lens-reflecting mirror systems can produced greatly magnified images and can peer into deep space totally inaccessible to the naked eye.

Optical telescopes using refracting convex lens were the earliest types used to examine the 'universe'.

Optical telescopes using reflecting mirrors were developed later, but do employ an eyepiece lens.

Optical telescopes detect visible light and convex refracting lenses or concave reflecting mirrors are used in their construction - some telescopes use both lenses and mirrors.

To improve the quality of the image you can increase the diameter of the objective lens - the primary light gathering lens at the front end of the telescope.

The bigger the diameter of the refracting objective lens (the larger the aperture) the more light is collected to improve resolution - image quality.

The same argument applies to a larger diameter mirror in a reflecting telescope.

You also get better resolution the greater the optical quality of the lenses too - the chemical composition of the glass refracting convex lens is another important factor in image quality.

Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, high resolution = a good image, low resolution = a poor image.

The basic design of a refracting telescope

The objective lens collects and focuses the light onto the eyepiece lens.

The eyepiece lens position can be adjusted to produce a clear focussed image on the eye, photographic plate or photocell plus computer.

For mirror based reflecting telescopes, you can increase the diameter of the concave mirror to gather more light and improve the quality and resolution of the image.

Optical telescopes are limited to visible light observations, so you need other types of telescopes.

For more on detecting different EM radiations see the first section of my Life Cycle of Stars page.

Photographing the same patch of sky and comparing images from one night to another can show up whether an object is moving eg asteroid or comet or some new star appearing or an old star exploding in a massive supernovae explosion,.

So, anything that changing that reflects or emits visible light can be detected and by using long-time exposures you can detect very faint very distant objects.

The result of all these historical and continuing contemporary observations with telescopes of all kinds is to give us a pretty good picture of the observable universe, even if we don't fully understand how it all works!

Problems with observations and ways to improve matters - image quality

The most obvious problems with the use of optical telescopes is absorption of light by the Earth's atmosphere and light pollution.

The Earth's atmosphere both absorbs, refracts and scatters light from an astronomical source which reduces the quality of the image - reduces resolution.

Reminder: Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, so high resolution = a good image and low resolution = a poor image.

Light pollution comes from any light source on the Earth's surface that emits light into the sky at night eg from road traffic, office blocks, street lights etc., all of which makes it more difficult to observe dim-faint objects in the sky.

The 'twinkling' of stars, giving an unstable image, is caused by incoming light refracting several times in the Earth's atmosphere.

Air pollution eg particulates/dust can also absorb or scatter light diminishing the quality of ANY image.

All of these problems can be considerably reducing by getting a telescope to operate high in the atmosphere OR above the Earth's atmosphere completely.

Observatories using optical telescopes can be sited high up on mountains in dark places where the atmosphere is less dense (thinner), especially in remote places where there is little pollution of any nature - dust/particulates or artificial light sources.

The Hubble Space Telescope has been put into orbit around the Earth acting just like satellite with an adjustable optical telescope. Since it is above the atmosphere, many of the problems of image quality described above are greatly reduced.

For any type of telescope, the larger the size of the electromagnetic radiation collector, the greater the resolution AND the farther you can look across the universe to the most distant objects, and back in time too!

The lenses, mirrors or radio dishes etc. of the telescopes are linked to powerful light detection systems (eg. photocells rather than the naked eye), which in turn, are linked to powerful computers to generate extraordinary detailed images that can be analysed to help develop and test out astrophysics theories.

How a reflecting telescope works

A reflecting telescope uses a concave mirror

A relatively large concave mirror collects as much light as possible from distant astronomical object e.g. a star.

The collect light is reflected by a small plane mirror at ~45^o into an eyepiece or camera to record the image.

By means of a magnifying lens in the eyepiece tube you can produce a clear focussed and greatly magnified image of the star.

Advantages over convex collecting lens.

Other electromagnetic radiation telescopes

Radio telescopes are used to detect and study naturally occurring emissions from objects such as stars, galaxies, quasars and black holes.

Fortunately, not all the extraterrestrial radio waves are absorbed by the Earth's atmosphere, but some are absorbed and others are reflected back into space by the upper atmosphere - much depends on the wavelength of the radio waves.

The radio telescope can consist of a large parabolic dish or large arrays of smaller signal receivers.

The electromagnetic (mainly radio) emissions received, provide information that can be analysed to understand the structure and functioning of these various astronomical objects.

Radio telescopes are used to detect the cosmic microwave background radiation which has helped understand and test out theories of the origin of the universe.

For radio astronomy, the shortest wavelength of radio waves that can pass through the Earth's atmosphere is 100 m.

Calculate the maximum frequency of the shortest radio waves that can be detected.

(speed of 'light' = 3 x 10⁸ m/s):   v = f x λ    *    f = v / λ  = 3 x 10⁸ / 100 = 3 x 10⁶ Hz

X-ray telescopes are very good for detecting very explosive high temperature events e.g. exploding stars - supernova.

These extremely violent events emit the highest energy electromagnetic radiations.

 

The advantages of computerised telescopes

Most modern telescopes are linked to computers and can do many tasks automatically and be programmed to make a specific series of observations e.g. keep pointing at the same distant object in the sky.

Computers can help to produce clearer and sharper images and store them for later more detailed analysis.

They can collect and store huge amounts of images 24/7 and easily process the data with an amazing speed of analysis.

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(d) More notes on the objects in our solar system - definitions explained

U represents a sun (star) at the centre of a solar system.

X or Z might represent a planet.

Z might also represent the path of a large asteroid.

Y might represent a moon of plane = X

V might represent a comet.

The Sun is our star at the centre of our solar system.

A sun forms the centre of a solar system and generates its own visible light as a fraction of its enormous energy release from nuclear reactions e.g. the fusion of hydrogen into helium.

A solar system is everything that orbits our Sun (and everything that itself orbits anything orbiting the Sun e.g. moons, asteroids, comets).

Our sun is ~98% of all the mass of our solar system.

Objects orbiting the Sun are held in their circular or elliptical orbits by the force of gravity - the gravitational field force that attracts one object to another.

It decreases (gets weaker) significantly, the greater the distance between the two centres of the objects attracted to each other.

Gravity decreases proportionately to 1 / d², where d = distance between centres of the objects.

A planet is the largest type of body that orbits a star - 8 major ones orbit our Sun (a star called Sol).

A planet cannot produce its own visible light - they are seen from the reflected light of its star.

Moons are natural satellites of a planet - Earth just has the 'Moon', other planets in our solar system have many moons.

There are eight planets in our solar system - listed in the table below in order of distance from the Sun.

A true planet may be defined has having the following properties-criteria:

It must orbit a star (e.g. our Sun).

It must be big enough to have enough gravity to force it into a spherical shape.

It must be big enough that its gravity has 'pulled in' and cleared away any other objects except for its natural satellites.

Exoplanets are planets that orbit another sun in another solar system beyond ours e.g. in our own galaxy or some even greater distanced galaxies.

Dwarf planets (minor planets)

These objects do not meet the current criteria for 'major' planets - usually because they are not big enough.

A dwarf planet must be spherical and orbit a star but due to a weaker gravity field, has been unable to clear other large objects near its own orbit.

Pluto is now classed as a dwarf planet - it just isn't big enough!

Pluto has a moon called Charon that is half Pluto's radius (and 1/8th mass of Pluto) orbiting it, plus at least four other moons.

Another reason is that some asteroids are bigger than Pluto and could also be classed as minor planets!

Natural satellites - moons

A moon is a relatively large body of material that orbits a planet - they are natural satellites and they have almost circular orbits - actually very slightly elliptical.

We have one moon, Jupiter has dozens and Galileo spotted some of them in 1610 with his new telescope!

A satellite can be defined as any smaller object, due to its speed and resulting force of gravity, that orbits a more massive object

Artificial satellites

Artificial satellites are 'human-made' satellites that launched into space to orbit a planet or one of its moons and they also have almost circular orbit.

The orbit is designed so an artificial satellite can perform a particular function.

More on artificial satellites in the next section and why one object can orbit another in a steady cycle.

Asteroids

Asteroids are irregularly shaped lumps of rock-metal that orbit the Sun and are found in the asteroid belt between the orbits of Mars and Jupiter.

They vary considerably in size, from a two metres to nearly a 1000 km, but are too small to be considered as major planets - though the largest can be considered as minor/dwarf planets.

It is not impossible for a huge asteroid to collide with Earth e.g. the one of 11 to 81 km diameter that created the impact crater in Mexico that is believed to have led to the extinction of dinosaurs and the rise of the mammals like us!!!!

Meteors and meteorites

Meteors ('Shooting stars') are seen when dust particles called meteoroids come down into the Earth's atmosphere, due to the Earth's gravitational pull and 'burn-up' giving out bright visible light due to the high temperature generated by friction of the particles with the Earth's atmosphere.

Small meteor particles completely burn up in the atmosphere, but larger ones fall to Earth as meteorites, often consisting mainly of metal or mainly of stone - the largest known weighed 100, 000 kg.

Most meteors and meteorites are fragments of asteroids that have somehow broken out of the asteroid belt due to a collision and hurtle round the Sun in their own very elliptical orbits. They can also be bits of a comet.

 In the Ulster Museum, Belfast, is a meteorite of nickel and iron, perhaps once part of the metal core of a planet?

Comets

Comets are 'cosmic snowballs' of frozen gases, rock and dust that orbit the Sun.

Their orbits are often highly elliptical and some can travel from near the Sun to the outer regions of our Solar System and back!

Unlike the planet orbits, which are all in a narrow plane, comet orbits are at all sorts of angles with respect to the plane of our solar system.

When a comet's orbit brings it near the Sun, it heats up and pours out dust and gases into a huge giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of miles!

 

You should realise there is quite an 'overlap' between asteroids, meteors, meteorites and comets!

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(e) The physics of circular motion and the forces involved in solar and planetary systems

Velocity is a vector quantity, it has both size/magnitude (the speed) and direction (a reference angle).

If either the speed or direction changes, you have a change in velocity.

If you have a continually changing velocity, you have an acceleration!

SO, What velocity are we dealing with? What force are we dealing with? in terms of one object in circular motion around another object due to a gravitational field?

So, how do we explain the circular motion of the objects illustrated above!

To keep a body moving in a circle there must be a force directing it towards the centre.

This may be a moon or satellite around a planet or a planet around a star - all apply to planet Earth.

This force is called the centripetal force and produces the continuous change in direction of circular motion - which means constantly changing velocity of the moon or planet etc.

Even though the speed may be constant, the object is constantly accelerating because the direction is constantly changing via the circular path - i.e. the velocity is constantly changing (purple arrows, on the diagram).

For an object to be accelerated, it must be subjected to a force that can act on it.

Here the resultant centripetal force of gravity is acting towards the centre, so always directing the object to 'fall' towards the centre of motion (blue arrows on the diagram).

But the object is already moving, so the force of gravity causes it to change direction - speed is constant, but change in direction means acceleration is taking place.

In other words the object keeps on accelerating towards the object it is orbiting and the instantaneous velocity, at right angles to the acceleration, keeps the object moving in a circle.

SO, the actual circular path of motion is determined by the resultant centripetal force (black arrows and circle) ...

... and the circling object keeps accelerating towards what it is orbiting!

The centripetal force stops the object from going off at a tangent in a straight line.

The centripetal force will vary with the mass of the objects in question and the radius of the path the object takes around the other.

You can argue that the path of a given object in a stable constant orbit depends on its distance from the object it orbits and the strength of the gravitational field it experiences.

Summary of the 'rules'

You need to check the text below with the diagram above!

To keep an given object in a stable orbit (moving at constant speed), the faster it moves or the smaller the orbital radius, the greater the gravitational centripetal force needed. See the pattern for the planets of our solar system.

If the speed of an orbiting object changes the orbital radius must also change.

If an orbiting object initially slows down it is pulled by the gravitational centripetal force into an orbit of smaller radius and increases in speed.

Satellites at lower heights above the Earth may experience tiny friction effects from the upper atmosphere. Eventually this causes the satellite to slow down and move to a lower orbit. Ultimately the satellite may be drawn into a higher speed orbit lower in the atmosphere and burn up from the heat of friction. This is sometimes done deliberately to 'safely' remove a defunct satellite.

Conversely, If an object initially speeds up, it partially overcomes the centripetal gravitational force moves to a higher orbit of greater radius, but then it slows down in a larger radius stable orbit.

In positioning a satellite via a 'transporter' rocket, it must be given the correct velocity to go to the right height and with the right speed into the desired stable orbit or specified radius.

We can now apply these ideas to the three situations described below.

P = planet, m = moon

1. Circular motion - velocity & centripetal force for a moon or a satellite around a planet

 

P = planet, S = Sun

2. Circular motion - velocity & centripetal force for a moon around a planet

3. Artificial satellites - see the separate section.

 

The planets move around a star in almost circular orbits e.g. planet Earth travelling round the Sun once a year.

The same force of gravity keeps a moon orbiting a planet e.g. our Moon orbiting the Earth.

The same arguments on circular motion apply to the movements of planets around a sun, a moon around a planet and a satellite orbiting a planet.

The orbits are usually elliptical, rarely a perfect circle, but the physics is the same.

In these cases, it is the force of gravitational attraction that provides the centripetal force and it acts at right angles to the direction of motion.

You should also realise that they are moving through empty space (vacuum), so there are no forces of friction to slow the object down.

This is why the planets keep going around the Sun and the moon keeps going around the Earth.

 

The pattern in the size of a planet's orbit around a star

8 major PLANETS	Distance from Sun in Mkm	Mass relative to Earth	Size relative to Earth	Time to orbit Sun (days or years)	Axis rotation time	Average surface temperature oC
Mercury	58	0.05	0.4	88 d	58.6 d	+350
Venus	108	0.8	0.9	225 d	242 d	+480
Earth	150	1	1	365 d	24 h	+22
Mars	228	0.1	0.5	687 d	24.7 h	-23
Jupiter	778	318	11	12 y	9.8 h	-153
Saturn	1430	95	9.4	29 y	10.8 h	-185
Uranus	2870	15	4	84 y	17.3 h	-214
Neptune	4500	17	3.8	165 y	16 h	-225
Pluto (dwarf planet)	5915	0.003	0.2	248 y	153 h	-236

The closer an object is to the object it is orbiting, the stronger the gravitational force of attraction.

The stronger the gravitational force of attraction, the faster the object must move in order to avoid crashing into the object it is orbiting.

For any object in a stable orbit, the radius of the orbit must match the speed the object is travelling.

Faster moving objects must move in a smaller radius to have a stable orbit.

If the speed changes for some reason, the radius must change too.

You can see clearly that the further the planet is for the Sun (our star) the longer it takes for that planet to orbit the Sun once (relevant data highlighted in bold).

This means, the further out the planet is from the Sun, the slower it is moving AND in a larger radius circle with respect to the Sun as its centre - to move in a stable orbit.

 

Orbiting objects (planets/moons) - summary of the connection between gravity, mass and radial distance (size) of the orbit.

The gravitational field strength depends on the mass of the object creating the field.

The larger the mass of the object, the stronger the gravitational field  e.g. Jupiter > Earth > our Moon

The gravitational field strength experienced by an orbiting object, also varies with its distance from the object it is orbiting.

The stronger the force an orbiting object experiences, the greater the instantaneous velocity needed to balance it e.g. to keep it in stable orbit.

Therefore the closer a planet is to a star or a moon to a planet, the faster the orbiting object must travel to stay in orbit.

To have a stable orbit, the object must have a speed that matches the gravitational 'pull' at a particular radial distance from the object it orbits.

The smaller the radius, the faster the object must travel to have a stable orbit.

I've already mentioned the pattern in the distance of planets from our Sun and the speed they are travelling at - as measured by the length of time for one orbit of the Sun.

From the table you can see, the further than planet is from the Sun, the slower the speed it travels at.

You might think that there would be greater variation e.g. between Mercury and Neptune, but remember, the outer planets have a long way to travel in their orbit!

Because the planets are moving at different speeds, ancient astronomers had noted the varying positions of the planets against the relatively constant positions of real stars. They did not realise until later, that these 'wandering stars', as they called them, were actually what we now know as planets orbiting a star (our Sun).

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(f) More on natural and artificial satellites - their orbits and uses

You can define a satellite as a smaller object orbiting a larger object and is kept in orbit by the force of gravity.

Natural satellites are those objects that occur naturally in a 'solar system' orbiting a planet.

e.g. our own moon orbiting the Earth, the many moons of Jupiter.

Artificial satellites are those that we, with our technology, put into orbit around our moon, the Earth or our other fellow planets e.g. Mercury, Venus and Jupiter.

When satellites are put into orbit they are given just the right amount of horizontal velocity so that the resultant centripetal force of gravity keeps the satellite accelerating in its a circular orbit.

Reminder: Acceleration and velocity are vector quantities.

The speed of a satellite may be constant, but it is constant changing in direction, so the velocity is constantly changing and change in velocity is acceleration (in this context).

You can vary this horizontal velocity to position satellites at different distances above the Earth's surface.

Most satellites we launch into space are those that orbit our own planet Earth - diagram on right of geocentric orbits - an orbit of anything orbiting the Earth (over 2000 artificial satellites currently orbiting the Earth).

 

Satellites in polar orbit  (orbit 1 on diagram, actually should be lower than orbit 2)

Satellite orbit 1 passes over (or near) both poles, one after the other in its cycle, and is said to have a polar orbit. It circles around the Earth at about 90^o to the equator.

'Polar satellites' have relatively low orbits and the Earth rotates under them.

The lower orbit means they have to have a faster speed (physics above) than geostationary satellites (below) to stay in orbit which may take as little as a few hours.

Because the orbit time is short and the Earth rotates within the orbit, they can monitor the whole surface of the Earth many times in every 24 hours.

Therefore satellites in polar orbit are used for mapping the landscape, military surveillance (spying!) and producing weather charts for weather forecasting.

Satellites in a geostationary orbit (geosynchronous satellites)  (orbit 2 on diagram)

Satellite 2 travels around in the same direction as the Earth's spin and at the same angular speed are moving in a geostationary (geosynchronous) orbit.

Geostationary satellites have a high orbit that takes ~24 hours to go round the Earth (e.g. at a height of 35786 km above the Earth's surface).

This means they keep a constant position above the Earth's surface because they take exactly one day (23 hours 56 minutes) to complete one orbit.

These geosynchronous satellites are very useful for continuous communications e.g. radio, telephone and TV because you can point transmitters and receivers in their direction and they are stationary relative to each other. Also, the signals can transmitted all around the globe in a fraction of a second ~speed of light.

They are also used for astronomy e.g. the Hubble space telescope producing amazing images of distant star clouds and galaxies - technically it isn't in a geostationary orbit.

On a larger scale, the manned international space station is effectively a giant satellite performing all sorts of scientific functions.

Appendix 1. The phases of the Earth's moon

In sequence as the Moon orbits the Earth in 28 days. Phases in sequence:

1. new moon,  2. waxing crescent moon,  3.first quarter moon,  4. waxing gibbous moon

5. full moon,  6. waning gibbous moon,  7. third quarter moon,  8. waning crescent moon

Appendix 2. The seasons of the Earth

The different seasons on Earth, occur because its spin axis is tilted ~21o

Position 1 corresponds to the autumn equinox in the northern hemisphere.

Equal hours of night and daylight.

Spring equinox in the southern hemisphere.

Position 2. corresponds to mid-winter in the northern hemisphere.

Spin axis tilted away from the sun, so reducing the intensity of sunlight and shorter length of daylight.

Mid-summer in the southern hemisphere, spin axis tilted towards the sun.

Position 3. corresponds to the spring equinox in the northern hemisphere.

Equal hours of night and daylight.

Autumn equinox in the southern hemisphere.

Position 4. corresponds to mid-summer in the northern hemisphere.

Spin axis tilted towards the sun, so increasing the intensity of sunlight and longer length of daylight.

Mid-winter in the southern hemisphere, spin axis tilted away from the sun.

 

Apart from the Earth's rotation giving the rising and setting sun and the arced path in between, the position of the sun in the sky is also influenced by the tilt of the Earth's spin axis.

Path 1 of the sun represents mid-winter in the northern hemisphere, shortest daylight time.

Path 2 of the sun represents its path at the equinoxes.

Path 3 of the sun represents mid-summer in the northern hemisphere, longest daylight time.

See also Cosmology - the Big Bang Theory of the Universe and the Life Cycle of Stars for more detailed notes

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