Chapter 1 - Naked Eye Astronomy Flashcards
What is the mean diameter of earth?
13,000 km
Earth is the largest of what type of planets?
Earth is the largest terrestrial plant because of its mean diameter of 13,000 km
The others are mercury, Venus and Mars
How much smaller is earths polar diameter to its equatorial diameter ?
What does that do to the earths shape?
It is only slightly smaller by 42km
It makes the earths shape and oblate spheroid ( like the shape of a squashed beach ball when someone is sitting on it )
How much of the earths surface is converted by water?
70% of the earths surface is covered by water
Name the layers of the earth.
Atmosphere -> Crust -> Mantle -> Outer core -> Inner core
most of the crust is covered by saltwater oceans
How deep is the earths crust relatively speaking?
The earths crust is very thin with a thickness of 0-70 km
Why type of rock does oceanic crust consist of?
It consists of a darker, denser rock such as basalt.
What does the older continental crust consist of ?
It consists of low-density rocks such as Grannite and younger, thinner crust that is up to 10 km thick
What makes up 80% of the earths volume?
The silicate mantle which extends half way into the earths centre
What is the crust split up into?
A number of continental sized tectonic plates that literally float on top of the silicate mantle
What is the temp of the outer core?
~5,000k
What is the outer core made of?
It is made up of liquid iron with some nickel
What is responsible for the earths magnetic field?
It is these currents of charred particles that flow in the outer core
What is the temp of the inner core?
~5,500k ( that is about the same temp of the suns photosphere which is one of the outer layers of the sun )
What is it made of?
It is made up of nickel and iron but because of the high pressure from gravity, it prevents them from melting
What do the network lines that criss-cross an ordnance map that is a page in an atlas and a 3 dimensional globe help us find?
They help us locate an area such as a 4 figure grid reference or pinpoint a particular location in terms of its LATITUDE and LONGITUDE
Although latitude and longitude are drawn on maps as lines, what are they in real life?
They are actually angles subtended at the centre of the earth by imagery curved lines (arcs) on the earths circumference
Where is latitude measured from?
Latitude is measured North and South from the Earths equator
Where is longitude measured from?
Longitude is measured East and West from the Prime Meridian
In what year did seafarers stop using different meridians to define the zero of longitude?
They did it up to 1884 when they had a international meridian conference held in Washington D.C.
How much is the earths polar axis tilted by?
It is tilted by 23.5 degrees to the ‘vertical’
What are the consequences of acknowledging the earths correct tilt ?
It’s that observers at different latitudes in earlier years would of seen the sun at different altitudes in the sky
What is significant about the dates close to or on March 21st and September 23rd?
These are the dates when the sun lies dir fly over the equator.
These dates also correspond to the spring and autumnal equinoxes
What happened midway through the 21st of June and the 21st of December ?
The sun lies directly over the tropics of cancer (latitude 23.5 degrees) and Capricorn (23.5 degrees S)
These dates also correspond to the northern hemisphere’s summer and winter solstices
What do the Arctic and Antarctic Circles represent?
They represent the most northern (66.5 degrees N) and Southern (66.5 degrees N) latitudes from which the sun can be seen to rise and set ( if weather permits) every day of the year
What does our atmosphere provide us?
Oxygen to breathe, absorbs harmful solar UV and X-radiation, regulates our planets temperature to a mean 15C and protects us from most meteor strikes
What are some problems with our atmosphere for astronomers?
- The Sky is blue and so observations can only occur at night. This is because light is scattered by oxygen and nitrogen molecules which occurs and the shortest wave length (blue)
- Air in the atmosphere is continuously in turbulent motion -this is because air density’s rise and fall causing light to refract and so theses conditions could make it look like a star is twinkling
What is another word for glow and glare which is the problem astronomers have?
Light pollution
What is sky glow?
It is a rusty orange haze cast by the lights of near urban conurbations
What does local glare form sports grounds, street lights etc do to our eyes?
They ruin our eyes night vision or dark adaptation through light pollution
What is the sky split up into?
It is split up into 88 different areas called CONSTELLATIONS
What is significant about the stars constellations and their names
They bear no resemblance apart from a few exception like Orion’s Belt
What is an asterism ?
It is an unofficial, popular pattern of bright stars that do have a close likeness to their name
What might the stars in an asterism also belong to?
They could belong to the same or different constellations and include the plough (urea major), Orion’s Belt, the W (in Cassiopeia) and the summer and winter triangles
What is one of Orion’s neighbouring constellations ?
It is Taurus, the bull, which boasts one of the most beautiful open cluster of stars , the Pleiades
-Orion itself contains a faint, rather fuzzy pink patch of light just below the Orion’s Belt. This is a Stella nursery of young stars, gas and dust. The Orion Nebula
What might an astronomer see in the corner of his eye during an observation ?
A meteor or a shooting star
Why can you see meteors or shooting stars ?
This is because a bright stark of light is caused by a dust particle, probably from the tail of a comet, burning up in the atmosphere
What are comets themselves ?
They are rare visitors to the inner solar system but observers might be lucky to spot one as an extended fuzzy object, possibly showing 1 or 2 tails, moving slowly against the background stars night to night
What would be even more lucky to observe than a comet etc?
A supernova as on,y 3 have ever been observed by the naked eye in the last 100 years
What would a supernova look like ?
A supernova would suddenly appear as a bright star, be visible for a few weeks and then slowly fade
What do plants move through ?
They move eastwards through an imaginary strip called the Zodiacal Band
What are the northern and southern lights and where can they be seen?
They are a dazzling display of green, yellow and red curtains and whirlpools of light in the sky
Often know as the Aurora borealis and Aurora Australis and can be seen in polar regions but on rare occasions can be observed from the mid-UK latitudes
What do many asterisms contain?
They contain stars that act as POINTERS to specific objects in the sky
What constellation is a great place to start when observing the andromeda galaxy ?
The great square of Pegasus
What does the andromeda galaxy look like with the naked eye?
A visible very faint, fuzzy patch of light
What might keep observers see?
Artificial satellite
What might artificial satellites be mistaken for?
They might be confused with aircraft that are easily identified from green and red right of way navigation lights
The aircraft engines can also be heard usually
What do we use to map stars and other objects in the sky with a network of lines that isn’t the longitude and latitude system ?
The celestial sphere
What does the prime meridian mark?
The zero on latitude
What does the celestial sphere use at its prime meridian ?
- The path taken by the sun on the celestial sphere during 1 year
- the point where the ecliptic cuts the celestial equator on its journey from south to north (the first pint of Aries )
What system do most astronomers use?
They use the equatorial coordinate system
What is the equatorial coordinate system?
It is a system in which the celestial equivalents of our latitudes and longitudes are declination and right ascension
What is declination ?
It is simply the projection of latitude onto the celestial sphere; it is measured in degrees (+ and - = north and south )
What is right ascension ?
It is measured eastwards from the first point or Aries; it is measured in hours and minutes where 1h = 15 degrees and like in time intervals, there are 60mins in 1h (so 1 min = 0.25 degrees)
Which star is closest to the celestial pole?
Polaris
What is azimuth ?
Is is a simple bearing ( in degrees ) from due north (that’s geographical and not magnetic) moving round Eastwood to the point on the observers horizon directly under the star; it ranges from 0 to 360 degrees
How is altitude found from azimuth ?
Altitude is found by the angle from the observers horizon upwards to the star or other celestial object and ranges from 0 to 90 degrees (the observers zenith)
When is azimuth and altitude used together ?
They are used together for a more observer friendly horizontal coordinate system that involves azimuth and altitude
Like the sun, which way to the stars rise?
In the east
Wha5 is the name of the highest point that the stars reach ?
The culminate
Where do the stars reach when they are due south?
The cross the observers meridian and later set in the west
What is the apparent motion of the stars called and why does it happen?
It is called diurnal motion and is simply the result of the earth rotating on its polar axis from west to east
How much does the earth rotate in a sidereal day and how long does it take?
The earth rotates 360 degrees and it takes 23h and 56mins
During a sidereal day, how much has the earth moved around the sun?
It rotates 1 degree around the sun and so needs to rotate a further 4 min to align a given point on its surface with the sun once again
How many hours is 1 solar day?
1 solar day = 24h
Most astronomers observe what instead on the sun?
Stars
What do astronomers use rather than a clock for observing stars?
They use clocks based on local sidereal time (LST)
What is the local sidereal time of an observer?
It is the right ascension that lies on the observers meridian at a given moment in time
This means that if a star with RA=14h 45min lies on observers meridian, the LST is 14:45
What is a stars hour angle?
It is the time (in hours and mins) since the object was last crossing the observers meridian:
-It follows that - HOUR ANGLE = LOCAL SIDEREAL TIME - RIGHT ASCENSION
What does it tell an astronomer if the hour angle is negative?
It tells an astronomer how much time must elapse before the star or other celestial object will be crossing his or her meridian (the best time to observe it) - it is like waiting for a bus
What does altitude on NCP ( or SCP ) equal?
altitude of NCP (or SCP) = observers latitude
Since Polaris is only 0.5 degrees from the NCP, then to a good approximation :
Altitude of Polaris = observers latitude
What is a polar distance sometimes called?
Co-declination
What is a useful angular measure for astronomers ?
A stars polar distance
What is a stars polar distance ?
It is the angular distance of a star from the NCP
Since the declination of the NCP is +90 degrees, it follows that:
Polar distance = 90 degrees - declination
Each small circle that a star traces during one sidereal day has a radius which is equal to ?
Radius = polar distance
What are the points called when a star crosses the local meridian?
The upper and lower TRANSIT
What does the altitude at the upper and lower transits allow people to do?
Allows us to link it equatorial and horizontal coordinates
State the equation that shows us that the altitude of a star at the upper and lower transit allows us to link equatorial and horizontal coordinates?
Altitude ( upper and lower transits )= latitude + or - ( which means do both and get two answers ) polar distance
At what point does a star culminate?
At the upper transit
What are circumpolar stars?
Stars who polar distance is so small that that they do not set it remain visible all the time
What is required for a star to be circumpolar?
It’s polar distance must be less than the altitude of the NCP (which is equal to the latitude of the observer)
Since polar distance = 90 degrees - declination then :
90 degrees - declination < latitude of observer
What are some perfect ‘targets’ of a naked eye observer?
Large constellations, full span of Milky Way, earth orbiting artificial satellites and spectacular meteor showers
What is the best time to observe celestial objects?
When they are close to culmination ( which is when they are the highest in the sky and the brightest )
What does culmination allow for stars?
It allows the clouds of some stars to be detected and more detail to be resolved in ‘extended’ objects such as nebulae and star clusters
What should the information of rising and setting times enable observers to estimate?
It allows then to estimate the ‘half-way’ time at which injects culminate
What must happen before any serious naked eye observing can take place?
Your eyes must become fully - dark adapted -
How long does it take for eyes to become dark adapted ?
It take roughly 20 - 30 minutes
The retina of the eye contains two contains 2 types of photoreceptive cells which are called?
- Rod (which are not colour sensitive)
- Cones (which are colour sensitive)
What are the rods cells in your eyes sensitive to and what are they ideal for?
- They are sensitive to changes to changes in light intensity and are over-sensitive in daylight
- They are ideal for night vision, but require time to ‘adapt’ to low light levels
If a star or nebula is insufficiently bright to stimulate the cones then what will happen?
It will not be seen if looked at directly and this is because the cones are not activated in dim light
Where are the rods in the eyes located?
They are located on the outside of the optical axis
What is averted vision?
It is looking slightly to the side of a the object
What does averted vision allow the cones to do?
It allows its light to fall onto the rods and the object to be ‘seen’
What are factors that can affect the visibility of the night sky?
The land scape (trees, buildings, etc…), the presence of cloud, amount of light pollution, the transparency of the atmosphere ( recent rain can remove dust particles ) and the seeing conditions ( relating to the steadiness of the atmosphere, which can be quantified using the I - V (ANTONIADI SCALE))
What is the most spherical shape out of the Earth, Moon and Sun?
It is the sun
What is the mean diameter of the moon?
3500km
How much smaller is the moons polar diameter than its equatorial diameter ?
It’s is smaller by 4km
How much smaller is the suns polar diameter than its equatorial diameter ?
10km
What is the suns mean diameter ?
1.4 million km
What does the full disc of the moon subtend an angle of at the human eye?
Only 0.5 degrees at the human eye
What happened early in the moons history that formed it land scapes to what it is today ?
It was bombarded heavily with left over debris ( meteors and asteroids ) from the formation of the solar system
How long ago did the storm of debris adapt and Motown lava was able to sleep through the relatively thin near side crust?
4000 million years ago
What did the lava that seeped through the relatively thin crust of the moon do ?
It solidified and formed the lunar marina; mountain ranges upwards near the edges of the marina, creating deep valleys in between mountains.
How where lunar craters formed?
Lunar craters where formed by meteoroids striking the lunar surface and each impact caused a shockwave that compressed the surface material to leave a large cavity
What is the subsequent ‘rebound’ splattered material from impacts on the moon called?
Ejecta
What do we know ejecta as?
It went in all directions, creating the bright streaks we see as rays
How long does it take for the moon to revolve around the earth once?
27.3 days
How many days does it take for the moon to rotate 360 degrees on its axis ?
27.3 days
What is it called when it takes the same amount of time for main moon to rotate around its planet as it does to rotate 360 degree on its axis?
Synchronous rotation
Why does synchronous rotation occur ?
It arises from the internal tidal gravitational forces lowing down the moons period of rotation until these become tidally locked (equal) to their orbital periods
What is the moons orbital period of 27.3 days also know as?
A sidereal month
What allows us to see 59% of the moon over time over than the normal 50%
Lunar liberation
How is it possible to see under and over the moons polar regions?
From low and high altitudes from the earth
Does the moon wobble a little?
Yes
When are observers able to peek around edges of the moon’s Easter and western limbs ?
When seen from different view points at moon rise and moon set
What is liberation usually due to?
The whereabouts about where the moon is placed in its orbit
How much is the moons equator inclined to the plane of its orbit around the earth?
It’s by 1.5 degrees
How much is the plane of the moons orbit inclined at?
5.1 degrees to the epileptic
What does liberation from longitude arise from?
It arises from the caring speed in its elliptical orbits around the earth
Where do all liberation effects take place on the moon?
They all take place close to the moons limbs and so are difficult to observe in detail
What does the moon exert on the far and near sides of the earth?
Tidal forces
What happens because of tidal forces from the moon?
As the earth spins, part of the coast experiences these bulges as the twice daily high and low tides
What else’s gravitational pull contributes to tide other than the moon?
The suns gravitational pull
What happens at full and new moon with the sun, moon and earth?
They are all aligned
What happens to the tides at new and full moon ?
Particularly high and low spring tides occur
How are neap tides made?
When the moon is half full, the sun and moons tidal forces act at a right angle to one another
What are neap tides?
Tides whose variations are much lower and less dramatic
What is the rate of the earths procession per century?
1.4 degrees
What happens when the moon goes in front of the sun?
It obscures the light from the photosphere to produce a stunning total solar eclipse for observers in the dark shadow (umbra) on earth
What happens during totality ( the eclipse ) ?
Some stars can be see , the sky darkens, the temp drops, suns corona can be seen
What can happen just before and after the totality ( eclipse ) ?
Small bright spots of sunlight caused by the suns rays shining through valleys on the moon are visible ( theses are called Baileys beads and when one is visible, this is know as the diamond ring affect)
How long do the solar eclipses last for?
A few minutes
When do the solar eclipses usually take place ?
At a new moon if everything is lined up correctly
Where is the observer located if the moon obscures just part of the sun?
Located in the penumbra
What is the eclipse called if the sun only appears to have a small bright out of it?
A partial solar eclipse
What is annular eclipse ?
A special type of partial solar eclipse in which the moons silhouetted disc is slightly smaller than at other times
Why does an annular eclipse occur?
It is due to the moon being close to apogee in its elliptical orbit around the earth
What happens up to the appearance of the sun in an annular eclipse ?
The sun appears as a ring of fire instead of being totally obscured
When does a lunar eclipse occur ?
When the moon passes through the earths shadow
How many years ago did the Greeks use their geometrical skills to determine the first scale and other things?
Over 2200 years ago
Who where the first Greeks to establish the first rungs on the cosmic distance ladder?
Eratosthenes and Aristarchus of Samos
What was Eratosthenes in charge of?
The great library of Alexandria in Egypt
What did Aristarchus calculate ?
He was able to calculate the moons diameter was between 0.32 and 0.40 time that of the earth. ( it is actually 0.27 times)
How many times does the earth rotate on its axis during 1 year?
365 and 1/4 times
How long is one solar or synodic day?
24h
What does the earths axis of rotation remains fixed with respect to and what dons’t it?
It does the stars
It doesn’t the sun
The earths orbit around the sun and its fixed axis of rotation explain what?
The reason the sun rises and sets at different time on diffent dates during the year
Also the apparent motion of the sun on the celestial sphere and why the altitude of the sun at noon is different on different dates
Close to the spring and summer equinoxes, what happens to the days and nights?
They become around the same length
Equinoctial in Roman means equal nights
What happens to the days as spring advances?
They become longer with the times of the sunset and rise beginning gradually earlier and later in the day
When does the sun usually rise in dates close to the summer solstice in June ?
The sun rises at its earliest and sets at its latest
What times does the sun rise and set in the winter solstice around December?
It rises at the latest and sets at the earliest
What is different about the summer and winter solstice in the Southern Hemisphere ?
The opposite applies than in the north
How many days is the lunar phase cycle?
29.5 days
Why are synodic months and lunar phases different lengths?
During the time it takes for the moon to do one orbit, the earth moon system has moved through an angle of 27 degrees in its orbit around the sun
How do sundials and shadow stock tell the time?
They use an opaque object to cast a shadow of the sun
What is the sundials normally made out of and how does it work?
It is traditionally a metallic triangular gnomon whose slanting edge points to the north celestial pole (NCP)
The shadow is cast onto a horizontal plate that contains the graduated scale from which the time can be read
Since when have sundials been telling time ?
Since antiquity
When sundials are correctly aligned they indicate what?
Apparent Solar Time (AST)
What is different about how clocks tick to the sun?
Unlike clocks where they tick the same time every day, the sun can move slower some days and faster others
What did people have to do to make the time on the sun dial more constant ?
Corrections had to be made to the apparent solar time as indicated on a sundial in order to obtain the Mean Solar Time (MST) for a particular location
this was based of a imaginary mean sun that did cross the sky in a constant matter
What is the correction that had to be made to sundials know as?
This equation is know as the Equation Of Time (EOT) and it’s value ( that can be negative as well as positive ) differs for different dates of the year
What is the equation for EOT?
Equation Of Time (EOT) = Apparent Solar Time (AST) - Mean Solar Time (MST)
When the EOT is positive and negative, what do we say about the sun?
Positive - We say that the real sun is fast ( faster than the mean sun )
Negative - We say that the real sun is slow
What can a shadow stick be used for ?
Determine the time at which the sun culminates ( at local noon ) and the observers longitude
What dies a shadow stick form a sundial use?
It often uses a vertical, wooden, circular and reasonably thin stick such as 1cm diameter dowel
Other than graphs and tables, what can the EOT also be read from?
An analemma
What does the mean sun form the basis of?
Civil time keeping
How does the mean sun move?
It moves more precisely at a fixed rate across the celestial equator every day
How does the real sun move?
It moves up and down along the ecliptic at different rates and so the annual variation in the EOT is due to two factors
What are the two factors the annual variation in the EOT is due to?
The earths elliptical orbit
The tilt of the earths axis or obliquity of the ecliptic
How much does the earth rotate on its axis every min?
1 degree per min
What stopped local mean time being used by all people?
It was the rail way and its times and popularity in the second quarter of the 19th century
Why did they change the times for railways ?
This was because it was different times in different city’s because of LMT and so they agreed on one time all together
What did the 19th century see a rapid expansion in?
Number of sea vessels, exploring or conquering new territories, waging war and trading gold, silk prices and other such communities with distant land
What did King Charles ll founded in 1675?
The Royal Observatory in a Greenwich
What was the Royal Observatory meant for?
It had the intention of solving the longitude problem with ships
Who eventually solved the longitude problem with ships?
Astronomers and clockmakers
When, who and how - the longitude problem solved
In 1761, after many trials and tribulations, horologist John Harrison solved the longitude problem with the invention of the marine chronometer that kept accurate home port time aboard ships sailing in the most extreme conditions of temperature, humidity and salinity
Since the dawn of time, keen watches of the night have noticed the changing positions of five points of what in relation to the other what
Changing positions of five points of light in relation to other stars
What did the Greek call these five points of lights that moved?
Wanderers which the work planet derived from
The Greeks noticed that the motion of the planets was confined to a narrow ……. that was centred on the suns motion and passed through 12 constellations
Zodiacal Band
The Greeks noticed that the motion of the planets was confined to a narrow Zodiacal Band that was centred on the suns motion and passed through 12 constellations. How can this be explained?
This can be explained by the fact that all the planets orbit the sun in roughly the same place
From night to night the planets move where?
They move slowly eastwards
What did ancient astronomers notice that planets occasionally do?
The appeared to travel backwards from east to west in either a loop-the-loop or zigzag motion.
What is it called when plants appear to travel backwards from east to west in either a loop-the-loop or zigzag motion.
It is know as Retrograde Motion
Where is the best place to observe a plant?
In its orbit
Where is the best place for an inferior planet to be observed from the earth?
At a 90 degree angle from the earth to the sun ( pic in photo gallery ) which is called the greatest elongation
Where is a superior planet best observed ?
At the opposition which is when it is directly behind the earth and all 3 are in a line
What is an inferior junction?
An inferior junction is when the inferior planet is directly in the middle of the sun and earth
What can happen when a planet is at an inferior junction?
The inferior planet may undergo a transit of the suns disc
What are transits?
They are extremely rare events due to the orbital inclinations taking the planet either above or below the sun in the sky
What is occultation ?
It is an event in which a planet may temporarily obscure a distant star as it moves in front of the star for a few moments
What is the suns apparent motion due to?
It is due entirely to the orbital motion of the earth and the constant 23.5 degree tilt of the earths equator to the ecliptic
What was astronomy born out of?
The need for human survival - to be able to sow and harvest crops, pick fruits and nuts, and hunt for antelope and buffalo
What where many monuments and temples aligned specially for?
They where aligned for positions of key stars or to the rising of the sun on key dates of the year ( e.g Summer Solstice )
Why are many old monuments and temples useless for what they where meant to do for observation?
Because the earths axis of rotation is not fixed but traces out a circular path against the stars
What is precession ?
It is the relatively slow gyroscope wobbling of the earths axis
What dies procession arise from?
The gravitational pull of the moon and the sun on the earths equatorial bulge ( one complete rotation of the earths axis take ~ 26,000 years )
What where early models of the universe based on?
They where based on a geocentric ( earth centred ) systems Of Ancient Greek philosophers such as Plato and Aristotle
How long did it take for Ptolemy’s model of the solar system to gain general acceptance ?
1500 years until polish monk Nicholas Copernicus applies some mathematical modelling to the problem and advocated a Heliocentric universe
What is a Heliocentric Universe?
A sun centred universe
Who thought of a Heliocentric universe before Copernicus and why?
Aristarchus of Samos argued for it in 270 BCE because his calculations showed that the sun was bigger than the earth and could solve loads of problems and confusion
What was Copernicus reluctant to shows his model?
Some say because he was a monk, he did not wish to get in conflict with the church and others say he was worried of being ridiculed
When did Copernicus finally release his book?
On his death bead in 1543 called - on the revolutions of the heavenly spheres
When did Copernicus’s book gain acceptance ?
In the late 16th century, but astronomers were keen to reduce the disagreement between the predicated positions and true positions of the planets
What does kepler’s first law of planetary motion state?
It states that the planets move in elliptical orbits around the sun at one focus of each ellipse
What are the points called in orbit when a planet is closets and furthest from the sun?
Closet - perihelion
Furthest - aphelion
What does Kepler’s second law state ?
It states that an imaginary line from the Sun to a planet sweeps out equal areas in equal intervals of time
What does Kepler’s third law state?
It states that the square of the orbit period ( T ) of a planet is proportional to the cube of its mean distance ( r ) from the sun
How can Kepler’s third law be mathematically written?
T2 a r3
Or
T2/r3 = a constant. (All numbers are squares)
What is the only thing that will change in Kepler’s law in other systems?
The constant
Why are time periods and mean orbit radii related to the mass of a central body ?
Gravity
How did Issac Newton discover gravity?
Something to to with an apple as no one really knows
What does Newton’s law of universal gravitational state?
Newton’s law of universal gravitational states that everybody in the universe attracts every other body with a force that is directly proportionally to the product of their masses and inversely proportional to the square of their distance apart
