different combinations and
possible
evolutionary schemes to consider! Well, the universe has a lot more
characteristics and, therefore, many more combinations to consider!
This set of notes by Nick Strobel covers: Introduction to astronomy, Naked eye astronomy (note to Jesse Helms and Sen. Exon: that means astronomy without the use of a telescope). The vocabulary terms are italicized.
I'll start by setting the stage for what we're up against when we try to understand the physical universe in the discipline we call astronomy or astrophysics. The next section will cover everything we notice by taking a look at the sky without the use of a telescope.
The movie Powers of Ten gave you some idea of the size scales involved in astronomy. Now let's try to get a feel for the time scales. I'll use the analogy of the cosmic calendar in which every second in the calendar corresponds to 475 normal years (so 24 calendar days = 1 billion regular years). Assuming that the universe is 15 billion years old, we can squeeze that huge timescale to one cosmic calendar year. Here are some important dates relevant to us humans:
Origin of the Universe--Jan. 1. Origin of our galaxy--May 1.
Solar system origin--Sept. 9. Earth Formation--Sept. 14.
Life on Earth--Sept. 30. Sexual reproduction advent--Nov. 25.
Oxygen atmosphere--Dec. 1. Cambrian explosion (600 mil years ago
when most complex organisms
appear, fish, trilobites)--Dec. 17.
Land plants & insects--Dec. 19, 20. First amphibians--Dec. 22.
First reptiles & trees--Dec. 23. First dinosaurs--Dec. 25.
KT impact, mammal age, birds--10:00 AM Dec. 30. First primates--Dec. 30.
Australopithicenes (Lucy, etc.)--10:00 PM Dec. 31. Homo habilis--11:25 PM Dec. 31.
Homo erectus--11:40 PM Dec. 31. Early Homo sapiens--11:25 PM Dec. 31.
Neanderthal man--11:57 PM Dec. 31 Cro-Magnon man--11:58:38 PM
Homo sapiens sapiens--11:58:57 PM Dec. 31. Human history--11:59:39
Ancient Greeks to now--last five seconds. Average human life span--0.15 seconds.
It is rather surprising that we've been able to discover so much about the long term evolution of the universe and the things in it, especially when you consider that we've only been seriously observing the universe for about 100 years, which is only a very slight fraction of the universe's lifetime. About 100 years ago is when photography was first used in astronomy, making truly systematic observation programs possible. How can we say that the sun will go through a red giant phase in about 5 billion years from now with confidence? Is it hubris to confidently talk about the Earth's formation process about 4.5 billion years ago? To give you an idea of the difficulties in studying long timespans consider this analogy: An alien comes to Earth to search for life and to understand how it evolved. ET has a camera and has just 15 seconds to take as many photographs as possible. Fifteen seconds is the same proportion of a human lifetime as the 100 years is to the universe's age (15 seconds/human lifetime = 100 years/universe age). ET returns home and her colleagues try to understand Earth from this 15 second period of snapshots. They won't see any important evolutionary changes. How will they determine the dominant life form? They could use a variety of criteria: 1) Size: leads them to choose whales or elephants; 2) Numbers: choose insects; 3) amount of land space controlled by one species: choose automobiles.
Suppose they somehow decide humans are dominant. They now have further
problems. There is considerable diversity among the humans (though to ET with
10 tentacles, 200 eyes, and a silicon outer shell, the humans all look alike!).
ET and colleagues try to systematically classify the humans. The humans come in
a variety of sizes. In a coarse classification scheme, they break the sizes
down into small, medium, and large. They also come in variety of optical colors
for their outer shell: red, black, brown, yellow, and white. There appears to
be 2 separate sexes (ET is both male and female). After some false starts with
theories that used hair length and eye color, they are ready to ask themselves,
``Do small, brown, female humans evolve into large, red, male humans?'' ``Do
the small stay small and the large stay large?'' ``Why is there a tendency for
small humans to be with one or two large humans?'' With the three
characteristics [size (3 divisions), color (5 divisions), and sex (2
divisions)], ET
has
different combinations and
possible
evolutionary schemes to consider! Well, the universe has a lot more
characteristics and, therefore, many more combinations to consider!
Often the evidence for a particular hypothesis is indirect and will actually support other hypotheses as well. The goal is to make an observation that conclusively disproves one or more of the competing theories. Currently unresolvable questions may be resolved later with later improved observations using more sophisticated/accurate equipment. Sometimes new equipment shows that previously accepted theories/hypotheses are wrong!
Now back to the long term evolution side of the coin. We actually have a time
machine! Not the H.G. Wells variety or G. Roddenberry's Gate of Eternity but
something much simpler due to the large distances and finite speed of light
(300,000 km/sec!). It takes time for radiation from a celestial object to reach
us. Therefore, when we examine an object at a large distance from us, we see it
as it was. The farther away the object is, the longer it took the radiation to
reach, and the further back in time we observe it. The sun is 150 million
kilometers from us, so we see the sun as it was 8 1/3 minutes ago. The farthest
object you can see without a telescope is the Andromeda galaxy about
km from
us so we see it as it was a little over 2 million years
ago. By the way, we'll use a more convenient length scale than the really short
kilometer. We'll use a light year which is equal to the distance light
travels in one year (about
km). So the
Andromeda galaxy is a little over 2 million light years away from us.
To study the evolution of long-lived objects like stars (with lifetimes of millions to billions of years) or galaxies, we observe the object of interest at different distances from us so we see it at different epochs. We then see the object at various different ages or evolutionary stages.
Why a sphere? The Earth is spherical! This was known much earlier than Columbus' time. Sailors had long known that as a ship sailed away from the shore it not only diminished in apparent size, but it also appeared to sink into the water. The simplest explanation to use was that the Earth was curved (particularly, since those ships did come back without falling off some edge!). They also knew that if one traveled in a north-south direction, some stars disappeared from view while others appeared. The simplest explanation said that the Earth is round, not flat. Pythagoras noted that the shadow of the Earth falling on the Moon during a lunar eclipse was always curved and the amount of the curvature was always the same. The only object that always casts a circular shadow regardless of its orientation is a sphere. We know about this Pythagorean argument through the writings of Aristotle.
For each degree you moved south, the North Celestial Pole (NCP from here on) moved 1 degree away from the zenith toward the north and the highest point of the celestial equator's curved path in the sky moved up one degree from the southern horizon. By the time you reached Seattle (at latitude 47 degrees N) the NCP had moved 43 degrees away from the zenith so it was now 90 - 43 = 47 degrees above the horizon. The celestial equator was 43 degrees above the southern horizon and it still intercepted the horizon due east and west. Question: What path in the sky would the stars have followed as they rotated around you? To warm Rudolph's frozen nose, Santa headed down to the equator. At the equator, you would have seen the celestial equator arcing from east to the zenith to the west. The NCP would have been on your northern horizon. Question: What path in the sky would the stars have followed as they rotated around you? Continuing southward (Opus' relatives had been good this year), you would have seen the NCP disappear below the horizon and the SCP rise above the southern horizon one degree for every one degree of latitude south of the equator you went. The arc of the celestial equator would have moved to the north, but the arc still intercepted the horizon at the east/west points.
Let's make sure we understand this. No matter where you are on the Earth, you will see 1/2 of the celestial equator's arc (except for the geographic poles). Since the sky appears to rotate around us in 24 hours, anything on the celestial equator takes 12 hours to go from due east to due west. Every celestial object's diurnal (daily) motion is parallel to the celestial equator. So for us northern observers, anything south of the celestial equator takes less than 12 hours between rise and set, because most of its rotation arc around is hidden below the horizon. Anything north of the celestial equator takes more than 12 hours between rising and setting because most of its rotation arc is above the horizon. For observers in the southern hemisphere, the situation is reversed. However, remember, that everybody anywhere on the Earth sees 1/2 of the celestial equator so at the equinox, when the sun is on the equator, we see 1/2 of its rotation arc around us, and therefore we have 12 hours of daylight and 12 hours of nightime everyplace on the Earth.
The geographic poles are a special case. The celestial equator is right along the horizon and the full circle of the celestial equator is visible. Since a celestial object's diurnal path is parallel to the celestial equator, stars do not rise or set at the geographic poles. On the equinoxes the Sun moves along the horizon. At the North Pole the Sun ``rises'' on March 21st and ``sets'' on September 22. The situation is reversed for the South Pole.
Since the ecliptic is tilted 23.5 degrees with respect to the celestial equator, the sun's maximum distance from the celestial equator is 23.5 degrees. This happens at the solstices. For observers in the northern hemisphere, the farthest northern point above the celestial equator is the summer solstice, and the farthest southern point is the winter solstice. The sun reaches winter solstice around December 21 and we see the least part of its diurnal path all year-this is the shortest ``day''. The sun reaches the summer solstice around June 21 and we see the greatest part of its diurnal path all year-this is the longest ``day''.
minutes
= 120 minutes) earlier than it did before. A year later that star will rise at the same
time as it did today. Another
way to look at it is that the sun has made one full circuit of 360 degrees in a year
of
365.25 days (very close to 1 degree per day). This means that, from noon to noon,
the
Earth has to turn nearly 361 degrees, not 360 degrees, in 24 hours. This makes the
length of time for one
rotation with respect to the background stars a little less than 24 hours on the
clock.
Note that the Earth's rotation axis is always pointed toward the Celestial Poles. Currently the North Celestial Pole is very close to the star Polaris.
Imagine that at noon we have a huge arrow that is pointing at the Sun and a star directly in line behind the sun. The observer on the Earth sees the Sun at its highest point above the horizon: on the arc going through the north-zenith-south points, which is called the meridian. The observer is also experiencing local noon. If the sun were not there, the observer would also see the star on the meridian.
Now as time goes on the Earth moves in its orbit and it rotates from west to east (counterclockwise if viewed from above the north pole). One sidereal period later (23 hours 56 minutes) or one true rotation period later, the arrow is again pointing toward the star. The observer on the Earth sees the star on the meridian. The arrow is not pointing at the sun! In fact the Earth needs to rotate a little more to get the arrow lined up with the sun. The observer on the Earth sees the Sun a little bit east of the meridian. Four minutes later or one degree of further rotation aligns the arrow and Sun and we have one solar day (24 hours) since the last time the Sun was on the meridian. That night the Earth observer will see certain stars visible like those in Taurus, for example. (Notice that the Earth's rotation axis is still pointed toward Polaris.) A half of a year later Taurus will not be visible but those stars in Scorpius will be visible. (Again, notice that the Earth's rotation axis is still pointed toward Polaris.)
minutes = 60 minutes = 1 hour worth of time. Those in the next time zone east
of us (Mountain standard time) have clocks that are 1 hour ahead of ours.
There is a further complication in that the actual sun's drift against the stars is not uniform. Part of the non-uniformity is due to the fact that on top of the general eastward drift among the stars the sun is moving along the ecliptic northward or southward with respect to the celestial equator. Thus, during some periods the Sun appears to move eastward faster than during others. Apparent solar time is based on the component of the Sun's motion parallel to the celestial equator. This effect alone would account for as much as 9 minutes difference between the actual Sun and a mean Sun moving uniformily along the celestial equator. Another effect to consider is that the Earth's orbit is elliptical so when the Earth is at its closest point to the Sun (perihelion) it moves quickest. When at farthest point from the Sun (aphelion) it moves slowest. Remember that a solar day is the time between meridian passages of the Sun. At perihelion the Earth is moving rapidly so the Sun appears to move quicker eastward than at other times of the year. The Earth has to rotate through a greater angle to get the Sun back to local noon. This effect alone accounts for up to 10 min difference between the actual Sun and the mean Sun. However, the maximum and minimum of these two effects do not coincide so the combination of the two (called the Equation of time) is a bit complicated. This explains why the earliest sunset and latest sunrise is not at the winter solstice. Yet, the shortest day is at the winter solstice. Rather than resetting our clocks everyday to this variable Sun, our clocks are based on a uniformly moving Sun that moves at a rate along the celestial equator of 360 degrees/365.2564 per day. Aren't you glad that your watch keeps track of time for you?
Why are the synodic and sidereal periods not equal to each other? For a reason similar to the reason why the solar day and sidereal day are not the same. Remember that a solar day was slightly longer than a sidereal day because of the sun's apparent motion around the Earth (caused by the Earth's motion around the Sun). The Moon's synodic period is longer than its sidereal period because of its motion around the Earth. At new moon, the Sun and Moon are seen from the Earth against the same background stars. One sidereal period later, the Moon has returned to the same place in its orbit and to the same place among the stars, but the in the meantime, the Sun has been moving eastward, so the Moon has not yet caught up to the Sun. The Moon must travel a little over two more days to reach the Sun and establish the new moon geometry again.
The modern model has the moon going around the Earth with the Sun far away. At different positions in its orbit we see different phases all depending on the relative positions of the Earth-Moon-Sun. Another possible model was presented by the highly-esteemed Harvard graduates. They proposed that the dark part of the moon is the result of portions of the moon lying in the shadow of the Earth. Question: If the Harvard model was true, what would be the difference in Moon rise time and the sun rise time for a New Moon or first quarter phase? What would be the angular separation between the Moon and the Sun for a New Moon or first quarter phase in the Harvard model?
These tidal bulges are always along the Earth-Moon line and the Earth rotates beneath the tidal bulge. When the part of the Earth where you are located sweeps under the bulges, you will notice a high tide; when it passes under one of the depressions, you experience a low tide. An ideal coast should experience the rise and fall of the tides twice a day. In reality, the tidal cycle also depends on the latitude of the site, the shape of the shore, winds, etc.
The Sun's gravity also produces tides that are about half as strong as the Moon's and produces its own pair of tidal bulges. They combine with the lunar tides. At new and full moon, the Sun and Moon produce tidal bulges that add together to produce extreme tides. These are called spring tides (the waters really spring up!). When the Moon and Sun are at right angles to each other (1st & 3rd quarter), the solar tides reduce the lunar tides and we have neap tides.
Several people have asked me for references about the evidence for the slowing down of the Earth's rotation so here's a list:
Sometimes the umbra does not reach the Earth at all (only the penumbra) even though the Moon is on the ecliptic and it is exactly in New Moon phase. We see a bright ring around the Moon when it is lined up with the Sun-an annular eclipse (because of the annulus of light around the Moon). Question: Why would the umbra not touch the Earth? What does the fact that we sometimes observe annular eclipses and sometimes total solar eclipses indicate about the shape of the Moon's orbit?
There are other celestial objects that drift eastward with respect to the stars. They are the planets (Greek for ``wanderers''). There is much to be learned from observing the planetary motions with the naked eye (no telescope). There are 5 planets visible without a telescope, Mercury, Venus, Mars, Jupiter, and Saturn (6 if you include Uranus for those with sharp eyes!). All of them move within 7 degrees of the ecliptic. Question: What does that imply about their orbital planes? What would an edge-on view of our solar system look like? Two of the planets (Mercury and Venus) are never far from the Sun. Venus can get about 48 degrees from the Sun, while Mercury can only manage a 27.5 degrees separation from the Sun. When Venus and/or Mercury are east of the Sun, we'll see them as an ``evening star'' even though they are not stars at all. When either of them is west of the Sun they are called a ``morning star''. Planets produce no visible light of their own; we see them by reflected sunlight. True stars produce their own visible light. Venus can be the brightest of all the planets, sometimes getting so bright that it can create a shadow! Mercury and Venus are never visible at around midnight (or opposite the Sun), the other planets can be visible then. Sometimes a strange thing happens--a planet will slow down its eastward drift among the stars, halt, and then back up and head westward for a few weeks or months (retrograde motion), then halt and move eastward again. The planet executes a loop against the stars! What causes that? The answer to that question involved a long process of cultural evolution, political strife, and paradigm shifts. We'll investigate the question when we look at geocentric (Earth-centered) models of the universe and heliocentric (Sun-centered) models of the universe.
last update: 05 Dec 95
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