To ancient civilizations the sky was a source of wonder and awe. The cycles of the sky - night and day, summer and winter - were an integral part of their life. Even today these cycles still affect us - our bodies follow day and night in waking and sleeping, and the seasons still regulate part of our social lives. This course will look at the origin of the cycles we see in the sky.
Various magazines about the sky have useful home pages on the web, including Sky and Telescope and Astronomy.
The PROVISIONAL title and aims of each lecture are given below, together with links to on-line resources. This page will get added to as term progresses, so keep checking to see what is here (and hit reload to make sure you are getting the most up to date version!).
| Lecture 1 Day and Night | Lecture 2 The Moon | Lecture 3 The Seasons: Earth and Stars |
| Lecture 4 Planet motions | Lecture 5 Planetarium show! | Lecture 6 Meteors, comets and asteroids |
| Lecture 7 Impacts and satellites | Lecture 8 Stellar brightness, variable stars, star clusters | Lecture 9 Bring YOUR questions! |
Ancient people knew a great deal about the sky since knowing the seasons was literally an issue of life and death. To know when to plant and when to harvest is crucial if you are going to starve if the crops fail! And since it was a matter of life and death, then astronomy and the calender got linked into the religion of that culture, so the astronomical alignments of sites like Stonehenge are only part of its function in the ancient culture which built it. Some nice links on archeoastronomy.
But as well as the cycle of the seasons, there is the more
immediate cycle of day and night. While we are all aware of the suns
motion, where it rises in the East(ish - depending on the season), has
its maximum height above the horizon when its due south, and then sets
in the West(ish - depending on the season), that of the stars is not
so obvious. These would be more familiar to ancient people than to us,
firstly because we live a lot more of our lives indoors in a culture
which is not obviously linked to agriculture, and secondly
because we mostly live in places where there are many street lamps,
and their light drowns out all but the brightest stars. So what do we
see in the night sky ?
Stars - and more stars, the Moon, Planets, and a milky band of light
called the Milky Way. This lecture concentrates on stars, and we'll
deal with the rest later.
The stars do not appear to move relative to each other - we now know
that this is because they are so far away that their motions bring no
appreciable change in their positions on human timescales. So the same
stars in the same relative positions have been seen by all human
cultures. And all of them seemed to have played 'join the dots' and
come up with pictures (termed constellations), and stories to weave
around them. The way we 'join the dots' is mainly based on the Greeks.
More on
constellations
How do the stars APPEAR to move during the night ?
This depends on where we are on the Earth. As the Earth rotates about its
axis, so the stars APPEAR to move as if they are fixed onto a
sphere (called the celestial sphere) rotating in the opposite
direction. See the Celestial
sphere .
Extrapolate the Earths axis onto this imaginary sphere, and call these the north and south celestial poles. Extrapolate the Earths equator onto the sphere and get the celestial equator. In the Northern hemisphere there is a brightish star which happens to be rather close to the north celestial pole - this is Polaris, or the pole star. There isn't such a convienient star marking the south celestial pole. The key to what you see is then the angle which the point directly above your head (the zenith) makes with the celestial pole. If you are at the Earths north pole then your zenith is the same as the north celestial pole, so you see all the stars moving in concentric circles centered on the north celestial pole directly above you. As you move away from the north pole to, say, Durham, then there is an angle between your zenith and the north celestial pole. So you see some stars moving in circles round the north celestial pole (circumpolar stars), while others rise in the East and set in the West like the Sun. As you go down to the equator then your zenith is perpendicular to the north celestial pole and all stars rise and set, moving perpendicular to the horizon. This is hard to explain in words, so see the pictures in Reference markers on the sky . You can do the exercise we did in class looking at how the stars move during the night in this nice animated version
The starchart I gave out is here. Go and find
Ursa Major (also known as the Great Bear, the plough, the big dipper),
and polaris, the star which just happens to line up wth the Earths
rotation axis in the north. And see if you can spot Orion too.
You can make your own starchart using yoursky (Durham is at 55
degrees North and 2 degrees West, I only plotted stars of magnitude
4.0 and brighter, turned off the ecliptic and equator, deep sky
objects, constellation boundaries and star names!)
After the Sun, the Moon is the brightest object in the sky. phases of the moon are explained by the orbit of the moon around the Earth, and the times at which we see the moon are linked to its phase. There is a nice java amimation - the default point of view is top view, but its better to select 'both' and then hit animate.
The orbit of the Moon around the Earth is tilted slightly with respect to the plane of the orbit of the Earth around the Sun. To get an eclipse the planes of the orbits need to line up (which only happens twice per year) AND the moon phase has to be either new (for a solar eclipse) or full (for a lunar eclipse). It is a lovely coincidence that the moon and sun have the same angular diameter on the sky (the sun is much bigger than the moon, but it is also much further away), so the disk of the moon almost exactly covered the disk of the sun, allowing us to see the solar corona. There is a nice introduction to eclipses further down the page about the moon phases and eclipses. Look at the page giving details of lunar eclipses and solar eclipses.
The moons orbit is not very circular - it is an ellipse. So there are times when it is closer to the Earth than others. Only when the moon is closer than average to the Earth is its angular size big enough to completely cover the Sun. Solar eclipses at other times give an annular eclipse
The moon rotates! But its rotation period is exactly the same as its orbital period around the Earth, so we always see the same face of the moon towards the Earth. More on moon rotation. This is NOT a coincidence. It is due to the gravitational interaction between the Earth and the Moon. The effect of the moons gravity on the Earth causes tides in the sea also causes (much smaller!) tides in the rocky crust of the Earth. These will eventually bring the Earth into co--rotation with the moon. The effect of the Earths gravity on the moon is rather larger, since the Earth is larger so has more gravity. But the moon has no liquid seas, so the Earth produces tides only in the rocky crust of the moon, and the interaction of these bulges with the Earths gravitational field slowed the moons rotation, locking it with the orbit. A page with more on how tides in the moon bring it into co-rotation is here (don't be put off by the title of 'Bad Astronomy'!)
We've done what happens with the rotation of the Earth (day/night) and with the orbit of the moon (months). Now lets do what happens as the Earth orbits around the Sun (year).
The most obvious effect is the seasons - its warmer in summer than in winter and the daylight hours are longer too. This is because the Earths rotation axis is tilted by 23.5 degrees to the plane of the orbit. So there are times when the pole of our hemisphere points towards the sun (summer), and times when it points away (winter). The sun then rises earlier, sets later and has a bigger height above the horizon in summer. The greater maximum height of the sun above the horizon means that the sunlight hits the earth at a rather small angle. In winter the same sunlight makes a larger angle with the earth, so is spread over a larger area, so its cooler. Another reason its warmer in summer is that the sun rises earlier and sets later so the daylight hours are longer, so the ground has longer to heat up. More on the seasons.
But the stars also change. The motion of the Earth around the Sun makes the Sun appear to move against the background of stars. A circle is 360 degrees, while the Earths orbit is 365 days. So the Earth moves by about 1 degree per day (the angular size of the disk of the sun is 0.5 degrees). The circular orbit of the Earth is thus projected on the sky as the circular path of the Sun - the ecliptic.The ecliptic is then at an angle of 23.5 degrees to the celestial equator. Constellations (join the dots) which are on this projected path are the constellations of the zodiac. More on apparent motion of the sun.
Some more definitions. A meridian is the imaginary circle running through the point over your head (the zenith) and through the north and south celestial poles). We see the sun rise in the east(ish), make its maximum height above the horizon when its due south (and so is on the meridian) and set in the west(ish). So the time between the sun making 2 sucessive crossings (called transits) of the meridian is a solar day of 24 hours (by definition). But for the stars, the time taken for them to make 2 sucessive transits of the meridian is NOT 24 hours. The Earth moves around the sun by 1 degree per day so the sun appears to move along the ecliptic by 1 degree per day. And we know that the earth rotates on its axis in 24 hours so 360 degrees = 24 hours, so 1 degree takes 4 minutes in time. Thus the stars rise 4 minutes earlier each night - a sidereal day is 23 hours and 56 minutes long. That means a given star or constellation will rise 2 hours earlier at the end of the month than at the beginning. More on solar and sidereal time.
Equinoxes (equal nights) are where the sun crosses the celestial equator. Solstices (sun standing still) is where its at its maximum difference from the celestial equator.
The 5 planets known to the ancients (Mercury, Venus, Mars, Jupiter and Saturn) can be the next brightest objects in the sky after the sun and the moon. All the planets (except pluto) have orbits which are more or less in the same plane as the Earths orbit around the Sun - see this java animation (click on the yellow text on the black screen). So their positions projected onto the sky again project onto the same constellations as the suns motion around the sky - so the planets are also seen only along the ecliptic, projected against the zodiacal constellations. All the planets revolve around the sun in the same direction (anti-clockwise as seen from above the north pole of the Earth). These 2 facts (same plane and direction of rotation) are important for theories of how planets form. They are consistent with ideas of a disk of material rotating around the young Sun which then condenses into planets.
Understanding orbital motion is a key to understanding the motions of the solar system. Suppose we have large cannon on a mountaintop. Fire the cannon horizontally and the cannonball will go some way before hitting the ground. Use more gunpowder and fire it faster and it goes further before hitting the ground. If we could fire it fast enough then the ball could travel so far that it would never hit the ground - gravity pulls it towards the center of the earth but the Earth curves away, and the cannonball can go all around the Earth. This is an orbit - see here for more on this.
Gravity is stronger nearer the sun. Planets that are closer to the sun then need to have higher speeds in order to balance the higher pull of gravity. But they are also closer in so the distance they have to travel is shorter. The time for the inner planets to make 1 orbit is then a lot shorter than for the outer planets. Mercury's orbit is 0.39x that of the Earth's, so if it had the same orbital speed it would have a 'year' which is 0.39x that of the Earths also. But in fact the the 'year' for Mercury is 87.9 days, only 0.25x that of the Earth, because its orbital velocity is bigger to compensate for the higher gravitational pull. For saturn then the orbital period is 10,759 days (ie 29.5x that of the Earth) while its orbital distance is only 9.54x that of the Earth.
Planets shine in visible light only by reflected sunlight. So they can show phases. The inner (or inferior) planets show the full range of phases, and they can never be very far from the sun in the sky. More on phases of Venus (Mercury is similar). Inferior conjunction is earth-planet-sun. Superior conjunction is Earth-Sun-planet. Here is a nice diagram.
For the outer (or superior) planets then conjunction is Earth-sun-planet (planet furthest from Earth) and opposition is Sun-Earth-planet (planet closest to Earth (see the previous diagram. They can also show phases but only from full to gibbous - they can never be new or crescent as they can never be between the Earth and the Sun.
The brightness of a planet then depends on how much sunlight they intercept (distance from sun and size of planet), how much they reflect (composition - clouds reflect more light than rock!), how much of this can be seen from Earth (phase) and distance from Earth. Outer planets are brightest at opposition (closest Earth-planet distance and full phase). Inner planets are more complex because at their closest distance they are at minimum phase.
Because the outer planets move more slowly than the Earth then the Earth can catch them up and overtake them! So from the Earth we see them going backwards. This is called retrograde motion, and was one of the puzzles of ancient astronomy. This happens at opposition (sun-Earth-planet). More on planet motions and retrograde motion
The time taken for the earth and a planet to get back to the same relative position is called the synodic period. This is the time between successive oppositions of an outer planet or maximum elongations of an inner planet. This is more or less the same Earths orbital timescale (a year) for the very slow moving outer planets.
A bright comet is one of the most spectacular sights in the night sky. From pictures of comets and their tails it looks like they should streak across the sky in a swift blaze of glory. But they don't, they can be seen for night after night, moving slowly against the background stars.
Newton proposed that the comets orbited the Sun like the planets, but since comets tend to be rather rare this means that their orbits must be very elogated (or eccentric) rather than nearly circular. Halley looked at comet records and used these to calculate their orbits. He noticed that the orbits of the comets seen in 1531, 1607 and 1682 were very similar, and so he proposed that they were the same object and that it would reappear in 1758. It did, and so was named after him!
Comet orbits fall into two main groups. Short period comets (orbit periods less than a few hundred years) like Halley's are the exception. These have orbits which are very loosely (within 30 degrees) tied to the ecliptic. But most comets have orbits which are much much longer and come from any direction - they do NOT have to lie along the ecliptic. Their orbits indicate an maximum distance of ~1 light year (remember the nearest star is 4 light years away). and the Kuiper belt, out past the orbit of Neptune. For the Oort cloud, the gravity of passing stars can perturb their orbits and bring them plunging down towards the Sun, while for the Kuiper belt its more likely to be interactions with Neptune and Uranus. More on comets and their orbits .
Far from the Sun, a comet is merely an irregular lump a few tens of kilometers across (so not enough gravity to pull it all together into a round object). This nucleus is made up mainly of water and carbon dioxide ice, with dust embedded in it. Its not very dense - think of something like a dirty snowball. As it gets closer to the Sun it warms up, and the ices turn into a gas (sublime). The gas and dust pouring from the nucleus then forms an atmosphere around the nucleus called the coma. Then as the comet gets even closer (around aboat the distance of Mars from the Sun) then the radiation from the Sun pushes the gas and dust out into a spectacular tail - two tails in fact! Ultraviolet light from the sun can ionised gas in the coma, so there are charged ions and free electrons. These charged particles get rapidly pushed away up by the solar wind, and form a gas(or ion) tail. This emits an emission line spectrum. Small bits of dust get blown out by the much smaller pressure of the Sun's radiation so the tail is curved and we see this by the dust scattering sunlight. The tails point away from the Sun, NOT away from the direction of motion. More on comet nuclei, coma and tails
Incidentally, where the charged particles from the solar wind hits the magnetic field then they get trapped by it, and spiral down onto the magnetic poles, emitting light as they do so, forming the aurora or northern (or southern) lights.
Comets leave dust around their orbit, about the size of a grain of sand. Where these intersect with the Earths orbit then we get spectacular displays of meteorites - shooting stars! Most burn up completely on hitting the Earths atmosphere, blazing brightly for less than a second. The particles strike the earth on nearly parallel paths so appear to diverge from a point (called the radiant) as in perspective drawings.
Not all shooting stars are associated with a comet - some can be seen at any time of year implying that there is some debris in almost all directions. More on meteors and meteor showers. These are probably associated with the asteroids - minor planets mainly found in the asteroid belt, between Mars and Jupiter. These are thought to be material left over from the formation of the solar system (since they are in fairly circular orbits in the ecliptic plane like the planets) which never formed into a planet because Jupiters gravity prevented their small gravity pulling them together to form a larger object. Again most are the size of a sand grain - something the size of a marble would give a spectacular fireball.
A meteoroid is a bit of debris from a comet or asteroid which is on collision course with the Earth. When the meteoroid burns up in the Earths atmosphere it forms a meteor (or shooting star). Most meteors come from particles which are about the size of a grain of sand. A grape sized particle would give an intense fireball, and objects which are bigger still can partly survive the descent: these are the meteorites we can find on Earth.
Plainly then the Earth does get hit by debris. Sometimes, quite big bits of debris - there are impact craters on Earth. A small fraction of asteroids have orbits which cross that of the Earth, so have some possibility of collision. A collision with a large asteroid was probably responsible for the extinction of the dinosaurs! Comets too are often on Earth crossing orbits, and one probably hit a remote region in Siberia called Tunguska. We know that a comet definitely hit Jupiter! This is the famous Shoemaker-Levy 9 impact of July 1994. It can happen.
But how frequently ? Not very often at all now. Yet looking at the numerous craters on the surface of the Moon, Mercury and (to a lesser extent) Mars means that they must have been very much more common in the past. Earth and Venus have atmospheres, so weathering and erosion soon wipe out the craters, whereas on the Moon, Mercury and Mars the old surface with its many craters is preserved. For much more on the planets see The Nine Planets Impacts would have been much more frequent in the early days of the Solar system, and then declined as most of the debris got swept up by collisions with the planets. So they are now rare, though clearly possible.
When a big meteorite hits a planet then the impact can be hard enough that some of the debris is able to escape from the gravitational field of the planet. These fragments can then wander around the solar system, and eventually impact onto another planet. Seems unlikely, but there are a few meteorite which have compositions which look much more like the Moon than the asteroids, and even fewer which seem to be from Mars. One of these caused great excitement with the claim that it showed evidence for life from Mars! A nice compilation of articles from the debate which followed is here. The jury is still out as to whether this is fossilised bacteria or crystaline structures. And if it is fossilised bacteria then could these have grown from the time the meteorite spent on Earth rather than on Mars?
Back to the night sky. Other things that can be see because they move are some of the many artificial satellites which orbit the Earth. We can see them only when they are lit by the Sun but we are in darkness. This means the few hours after sunset or the few hours before sunrise. The most obvious fact about them is that they move fast (and don't have flashing lights so they are not planes!) - they are generally in low Earth orbit which is ~90 minutes! Their brightness then depends on size and the biggest are Mir and the International Space Station. The Shuttle (when its up) is also easily visible, and predictions for times, positions and brightness (as seen from Durham) are here. But one can also see bright flashes from the Iridium communications satellites when their antennae reflect the Sun - these are called Iridium flashes. A list of predicted times to see these Iridium flashes from Durham is given here
When you look at the stars they have different apparent brightnesses. From the Evolving Universe part of this course you now know its because the stars have different intrinsic luminosities and they are at different distances from us. The Greeks are responsible for a vile way to talk about the apparent brightness of an object - magnitudes. They classified stars into first class, second class, third class all the way down to sixth class (ones which are only just visible to the naked eye on a clear dark night), with the first class ones being the brightest. Each successive magnitude is about 2.5x fainter than the previous one. Sadly this system is still in use! Its now defined so that two stars which differ by a factor of 100 in brightness have magnitudes which are different by 5 units (with the fainter star having the bigger magnitude!) On these definitions then we can relate any brightness to a magnitude. Rigel (blue supergiant bottom right star in Orion) has a magnitude of about zero. Sirius (the brightest star) is about 4x brighter and has a magnitude of -1.5. The Sun has a magnitude of -26.85, while faintest galaxies which can be seen with big telescopes have magnitudes of ~30, i.e. 10 billion times fainter than the faintest stars we can see with our eyes. With just binoculars or a small telesope you can see objects that are 100x fainter (or 5 magnitudes bigger) - and then you see all these fuzzy objects which are not single stars. They first got catalogued by people looking for comets (faint fuzzy things!), and many are still called after the number one of these comet hunters - Messier - gave them. There are open clusters and globular clusters (both of which are very important in giving us a sample of stars of different masses but more or less the same age, chemical composition and, most importantly, distance from us - we can use these to test our models of stellar evolution). Other fuzzy objects are star forming regions - diffuse clouds of gas and dust lit up by the light of the young stars forming deep within the cloud. Yet others are associated with the end points of stellar evolution planetary nebulae and supernovae remnants. And of course there are the external galaxies Browse the whole Messier fuzzy object catalogue here
Starmaps generally use the size of the dot as a measure of stellar brightness. And bright stars are much easier to see than faint ones. So often the shapes we pick out for stars are not the full constellation but just the bright stars - for example just picking out the Plough rather than all the stars in the constellation of Ursa Major (Great Bear). Groups of stars like this which are NOT constellations are called asterisms. Other examples are the 'summer triangle' made by the bright stars Vega (in Lyra) Deneb (in Cygnus) and Altair (in Aquilus).
Now we have a way to classify brightness we can look and see what we can see. And the most obvious thing is that stars twinkle - this is because of the Earth's atmosphere and has nothing to do with the star! But not all stars have constant brightness - some are intrinsically variable. There are dramatically explosively variable stars - supernovae. Type II supernovae mark the end of the life of a massive star, Type I mark the end of a white dwarf, where accretion of material from a companion star finally pushes the mass over the limit where electron degeneracy pressure can hold it agains gravity. Supernovae are rare - we expect only one every hundred years in a galaxy like ours. The last was in 1680 so we are overdue for one. Supernovae are incredibly bright - 10 billion times brighter than the Sun at the peak, so they can be seen to very great distances. Images of recent supernovae in other galaxies can be found here - these are the ones which are used to get distances to remote galaxies and so track the expansion of the Universe. These explosions completely rip apart the white dwarf, and so are very different from the much less catastrophic novae. Matter accreting from a companion onto a white dwarf builds up on the surface until the density is so large that nuclear fusion of the hydrogen occurs. The explosion causes the star to become 10,000x brighter (10 magnitudes) within a few days, then dimming over timescales of a few months. But the amount of matter blown away in the explosion is tiny so accretion onto the white dwarf soon resumes.
Then there are more restrained variable stars. Stars on the main sequence are generally stable because of hydrostatic equilibrium, but as they evolve then they can pulse. The large envelope that forms in a red giant or supergiant star is not well coupled to the energy producing core. Suppose that the envelope expands a bit, then it cools, then it contracts... These pulses can be irregular as in the Mira (or long period) variables, or regular as in the Cepheids and RR Lyra stars.
Some variables do not intrinsically change in brightness! Stars in binary systems can eclipse each other giving periodic changes in apparent brightness. These eclipsing binaries are the ones which are very useful to get a fairly direct measure of star masses and radii - the most famous one of these is Algol, in Perseus. A nice compilation of many sorts of variable stars is given here