Lecture 2, February 5; Christopher Chyba

Homework 1 is due next Thursday (one week from today, Feb 12); there is a review session at 7:30 PM in 145 Peyton Hall next Wednesday evening.

There will be an observing session (for those whose last names start with A-F) is next Thursday, February 12, at 8 PM. Come to Peyton Hall.

Joshua Lederberg, one of the founders of the field of astrobiology,
passed away about a year ago. He also led discussions of planetary
contamination/protection (microbes on spacecraft).

We know that some microbes (especially the kinds that form spores)
can survive in a dormant state for very long times (at least a decade)
in the vacuum of space (especially if shielded from UV light from the
Sun). Will we contaminate Mars with spacecraft we're sending there,
for example?

We know that our typical spacecraft are not perfectly clean, even
though they are assembled in so-called "clean rooms".

OK, let's think about the history of science. We'll focus our
discussion on three names:
-Aristotle
-Newton
-Cecelia Payne-Gaposchkin

Aristotle understood that the Earth was spherical, using a variety
of arguments. For example, he saw that during lunar eclipses, the
shadow of the Earth on the moon is circular.

How big a sphere? Erastosthenes figured it out, by comparing the
shadow cast by a vertical stick at high noon on the summer solstice
in Syene and Alexandria in Egypt (see the textbook for a full
explanation). The Earth has a radius of about 6400 kilometers.

Ptolemy, a few centuries later, used an analogous measurement to get
the distance to the Moon:

Consider two people looking at the moon at the same time, and
suppose that for one of them, the moon is directly overhead. If they
are on different places on the Earth's surface, the direction each one
points to the moon will *not* be the same. We have a triangle whose
points are the center of the Earth, the center of the Moon, and one of
the people, and whose various angles can be measured; we can then
solve for the length of the sides (in particular, the distance from
the Earth to the Moon).

The Moon is about 60 Earth radii away from Earth (about 4 km). Its radius is about 1/4
that of the Earth.

The distance from the Earth to the Sun is one Astronomical Unit, or
AU, equal to 1.5 x 10^8 kilometers.

Venus is ~0.7 AU from the Sun, Mars is 1.5 AU away, and so on.

How far away are the stars? We measure it in light years (the
distance light travels in one year). Nearest stars are about 4
light years away. To measure this, we use parallax: the direction to
which we point to a distant star changes as the Earth goes around
the Sun. The amount of change depends on the distance to the
star.

The parallax angle for the nearest stars is small, about 0.77
arcseconds. Pretty small indeed; these measurements are hard!
Aristotle understood the principle of parallax, but the angle was
too small to measure. He concluded that because parallax wasn't
observed, we can't be moving; we are *not* going around the Sun.
Good reasoning, but wrong conclusion.

Travelling at typical spacecraft speeds, it would take tens of
thousands of years to travel to even the nearest stars.

We discussed that microbes can potentially survive a trip on a meteorite (or a spacecraft) between planets in the Solar System. However, traveling between stars would take much too long for microbes to survive. So, we would not be surprised if life on Earth is related to potential life on Mars. The origin of our life has to be in our Solar System, and not coming from other stars.

Scales of the Universe

Solar System: Our star and the planets going around it.

Our Milky Way Galaxy: The collection of ~10^11 stars of which our
Sun is one. The diameter is ~100,000 light years across, and the Sun
is ~25,000 light years from the center of our Galaxy. Thickness of the
Galaxy ~1000 light years. The Search for Extraterrestrial Intelligence
(SETI) has used radio searches to look for signals from alien
intelligences, but has explored thus far only a *tiny* fraction of the
stars in our Galaxy.

The Fermi Paradox: If extraterrestrials are exploring the galaxy,
why haven't we seen them yet? Is the fact that we haven't seen them
here yet (UFO's aside!) proof that they don't exist? To answer this
question requires making assumptions about the sociology of
extraterrestrials...

The observable Universe contains ~10^{11} galaxies.

Let's discuss Aristotle's physical understanding of the universe:

Things on Earth are composed of four elements: fire, water, earth,
and air.

Things made of water and earth tend to move in straight lines,
towards the center of the universe (i.e., the center of the Earth),
while air and fire tend to move away from the center of the universe.
The Earth itself is made of water and earth, and thus ends up as a
sphere centered on the center of the universe.

In this context, it is difficult to understand how a horizontally
thrown object moves forward... Aristotle imagined a force that air exerts behind an object, as the object flies.

Look at the sky now. Things aren't moving in straight lines at all;
observing through the night, stars move in circles around the
direction North. We now understand this as due to the rotation of
Earth around its axis, but Aristotle saw this differently: If heavenly
objects do not move like objects on Earth, they must be made of
something else. A fifth element, or "quintessence". Circular motions
are eternal, and thus unlike the Earth, material in the heavens is
eternal.

So Aristotle concludes that the laws of nature in the heavens and on
Earth are very different from one another.

The planets, Sun and Moon, have additional motions relative to the
stars (on timescales longer than the day-to-day circular motions).

Aristotle imagined the heavenly bodies moving on spheres (the
"heavenly spheres"), with the Earth at the center, with spheres
holding (in order) the Moon, Mercury, Venus, Sun, Mars, Jupiter,
Saturn, and then the stars. (They didn't know yet about Uranus,
Neptune, and Pluto). The full scheme was quite complicated, and
included a total of 55 spheres. Why so complicated? Because the
observed motion of the planets relative to the stars is complicated;
over several months, a planet can change direction several times. It
worked pretty well qualitatively, but wasn't able to make detailed,
specific predictions of where the planets would be. Ptolemy refined
the system further, and was able to do a better (but not perfect) job.

Ptolemy made the most sophisticated model of the Aristotelian
cosmology. The Earth is at the center of the universe, the planets
move on circles ("epicycles") which in turn move on larger circles
("deferents"). This does describe retrograde motion of the
planets, and was able to make pretty accurate descriptions of the
motions of the planets. In fact, to do a good job, he needed
several levels of epicycles; it was a rather cumbersome system. The
whole thing had 80 epicycles...

This will get properly resolved once Copernicus comes along, and
moves the Sun to the center of the system (in 1543). But this only makes sense
once we change the physics. Remember that Aristotle required that the
Earthly elements want to fall to the center of the Universe.

Copernicus, in the 16th century, published his idea that the Sun is
the center of the Universe, not the Earth. This was dangerous; the
Aristotelian worldview had become deeply ingrained in society, so this
was perceived as an attack on the status quo.

The Copernican system wasn't perfect; it turned out to need
epicycles as well (it used circles only, and we know now that
orbits are in fact ellipses; the epicycles that Copernicus put in
in effect try to account for the difference between circles and
ellipses). But it did explain retrograde motion naturally.

Note that Copernicus had no inherent explanation *why* it is that
the planets (including the Earth) are orbiting the Sun. Why does a
rock fall toward the center of the Earth, yet the Earth orbits the
Sun? Copernicus couldn't give a good answer.

In this heliocentric view, the Earth is no longer at the center of
the Universe; it is just another planet. It does not occupy a special
place in the Universe. The Copernican Principle, i.e., that there is
nothing special about our place in the universe, suggests (but
certainly does not prove) that life is not unique to Earth.

Galileo was the first to point a telescope at the heavens. He
found:
-The Moon has mountains, shadows, very reminiscent of Earth
(remember, Aristotle said that the heavens were made of different
stuff than the Earth)
-The Sun has spots, which move and change (again, very
non-Aristotelian; the heavens were supposed to be unchanging)
-Venus goes through phases, like the Moon! Its apparent size also
changes with time. This is easily explained in the
heliocentric/Copernican picture, but not in the
geocentric/Ptolemaic view.
-Jupiter has moons, that orbit around it. Not everything revolves
around the Earth, in contradiction to Aristotle.

He also began to put together an alternative physics. He said:
-A body in motion will continue to move in a straight line unless
acted on by a force. Notice how non-intuitive this is, given our
everyday world with all its friction everywhere.

Notes for Lecture 3