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M1 WAPhases and Eclipses
Next to the Sun, the brightest object in the sky is the Moon. The Moon also one of the more dynamic
celestial objects, changing appearance and position every night. In a 2 to 3 paragraph essay,
answer the following questions:
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What are the phases of the Moon and why do the different phases occur?
What is the appearance of a total solar eclipse? What conditions allow a solar eclipse to occur?
What is the appearance of a total lunar eclipse? What conditions allow a lunar eclipse to occur?
Module 01 – The Night Sky
The study of astronomy covers a vast range of scales, from tiny (protons or quarks) to immense (galaxies,
galaxy clusters, the universe itself). Getting a sense of the relative sizes of the objects in question is
important for understanding the scope of what astronomers study.
Distances in astronomy are so vast they are hard to comprehend. Many times astronomers use light – the
fastest thing in the universe – to help talk about those distances. Light goes so fast, it would only take a
little more than a tenth of a second to circle Earth once. Yet it takes light over a second to make the trek
from Earth to our moon, and eight-and-a-third minutes to get from the sun to Earth.
As far away as the sun is, it’s nothing compared to the distances between stars. It would take light more
than four years to get from Earth (or the sun) to the nearest star. Astronomers often use the term “light
year” (the distance light can travel in a year – more than 63,000 times the Earth-Sun distance) as a basic
unit of measurement, and yet it still takes tens of thousands of light years to measure the width of our
Galaxy, or millions of light years to describe the gap between our galactic home and Andromeda, the
nearest major galaxy. Even then, we’ve barely scratched the surface of what the universe has to offer.
What the Ancients Knew
Let’s step back again for a moment, and think about where our modern knowledge of astronomy started.
From the earliest times, humans have looked out at the sky and noticed patterns in the positions and
motions of objects in the sky. Most obviously, the sun rises in the east, moves higher in the sky through
the morning, and then sinks to the horizon in the west. This daily motion of the sun is matched by the
nightly motion of the stars, which seem to spin around a central point in the sky.
The moon and planets make their way across the sky through a backdrop of stars, and the stars
themselves shift from month to month in a constant cycle. For ease of recognition, certain stars were
grouped together in constellations, a celestial picture for a whole culture to reference. All of these things
became important signs for early people to determine the progress of seasons, and other important
events.
Sister Moon
No one could miss the full moon, especially in the dark nights before electric lights. For those early skygazers, then, the changing shape of the moon was also an integral part of their lives. As the moon
progressed from the tiniest sliver to a half circle to a hunch-backed shape like a circle with a dark crescent
missing to the fullest of faces, those watchers would have been intimately aware that it rose later and
later in the day and set ever longer after the middle of the night. Once it passed full and began its
progress back through ever-smaller phases, they would have known to expect it to rise ever later in the
nighttime hours and set in the morning, closer and closer to noon.
Eclipses were a bit of a different story. As they are fairly difficult to predict, both lunar and the more
impressive solar eclipses would often have taken people by surprise. Since the straight-line alignment of
the sun, earth, and moon necessary for an eclipse happens every full (lunar eclipse) or new moon (solar
eclipse), you’d think they would be common. However, the fact that the plane of the moon’s orbit about
Earth is tilted relative to the plane of Earth’s around the sun provides the missing link. The shadows that
block the sun’s light and cause eclipses simply don’t fall on the relevant body (Earth’s shadow on the
moon for a lunar eclipse, moon’s shadow on Earth for a solar one) every month.
The Paths of the Wanderers
Perhaps one of the biggest mysteries to the first careful observers of the sky was the planets (a term
which comes from the Greek, meaning roughly “wanderers”). Everything in the sky could be easily
explained in the view of the time, in which the sky itself – the celestial sphere – turned around Earth. Even
the motion of the moon, which moved relative to the stars from hour to hour, could be explained simply
(basically, it moved around Earth on a different sphere). The planets, however, were different.
For the most part, as the moon does, the planets move across the background stars from west to east.
However, sometimes they appear to turn around and go back – moving east to west for a matter of days
or weeks – before returning to their normal course. The simplest explanation, which we know today to be
true, is that both Earth and the planets orbit the sun. As Earth passes a planet in its orbit, the other planet
appears for a time to be slowing and then going backwards before resuming its previous forward motion.
To help you visualize this phenomenon, think of how a car you pass on the highway seems to do the
same.
If the explanation is so straightforward, why didn’t early people use it? The answer is that they didn’t have
any evidence to support the idea! They reasoned that if Earth was moving around the sun (an important
part of the whole premise), we would see nearer stars shifting their positions relative to further stars (the
same way close objects look to be lined up with the background differently if you look at them only with
your left eye as compared to looking only with your right eye). The problem is, the stars are all so very far
from us, the relative position shifts are much, much smaller than we can detect with the naked eye. It
wasn’t till the advent of modern telescopes we were finally able to detect these miniscule shifts.
The Beginnings of “Science”
It took consistent, careful observations to determine the astronomical patterns that early people relied
upon. Far from being unusual, these early observers were the epitome of scientific curiosity and practice.
Watch almost any small child and you will see at work the scientific process that is a deep part of human
nature. Whether it be the constant experimentation with gravity or the fascination with the reactions of
their own bodies to various conditions, children are natural scientists. They see something new and
explore it, often with the kind of single-mindedness that irritates parents, convinced that the results will not
change – until the moment when some new experience changes the rules as they understand them (like a
pebble that floats instead of sinking). Then, it’s back to the proverbial drawing board to find out why
something doesn’t fit the expected pattern.
Modern science is like a child in this way. Starting from, “that’s interesting,” or “why is that happening?”
scientists make extensive observations of the universe at work. They make sure the work they do can be
repeated, so they know it is not just a fluke or an accident, and it is tested again and again. Only then can
they say, “this is how things work,” and only until some new “that’s odd” moment starts the process again.
Module 01 – The Night Sky
Scientific Notation
Within astronomy, you will encounter many numbers that are extremely large or small compared to
mathematics that the average person will deal with in daily life. These astronomical numbers can be
difficult to write and use in calculations using normal notation. For example, the mass of Jupiter is
1,899,000,000,000,000,000,000,000,000 kilograms. Writing this number out each time in a calculation
would use up a lot of paper and you would not be able to enter this number in an average calculator,
which typically can only display eight digits. Fortunately, there is a method for writing very large or very
small numbers in a short, easy to understand form
Scientific notation uses powers of ten to break a number into two parts, a base contain nonzero values
and an exponent of ten. For example, 4,320,000 can be written as 4.32 x 106.
To convert a regular number into scientific notation, you can follow this two-step process:
1. Move the decimal point to be just after the first nonzero digit place
2. Count the number of place needed to move the decimal point. This will be your power of ten. If
you move the decimal point to the left, the power is a positive value. If you move the decimal
point to the right, the power is a negative value.
Examples:
12600 —> 1.2600, the decimal place is moved 4 places to the right, the power of ten should be 4.
then 12600 = 1.26 x 104
0.00678 —> 6.78, the decimal place is moved 3 places to the left, the power of ten should be -3.
then 0.00678 = 6.78 x 10-3
A scientific calculator has the ability to input numbers in scientific notation. These calculators will have an
“EE” or “EXP” button that will be used to enter the power of ten. For example, the mass of the Sun is
1.989 x 1030 kg. To enter this value into a calculator, you would need to input 1.989 first, then hit the “EE”
or “EXP”, then finally 30. The numbers entered after pressing “EE” or “EXP” are used as exponents
on a base of 10. If you need to enter a negative power of ten, there will be “+/-” button that you will need
to press before the number value of the exponent. To enter 1.01 x10-5, you would enter 1.01, then “EE”
or “EXP”, the “+/-” button, then finally 5.
Distance Units used in Astronomy
Given the vast scale of the Universe, common length units of kilometers and miles are generally too small
to be of good use to describe the distances of astronomical objects. Within astronomy, there are three
unique distance units.
Astronomical Unit (AU) – 150 million km or 93 million miles. The Astronomical Unit is the average
distance between the Sun and the Earth. The Astronomical Unit is commonly used for distances within
the Solar System/
Light Year (lyr) – 9.46 trillion km. A light year is the distance that a photon of light would travel in space
over the course of an entire Earth year. The light year was introduced into use by German astronomer,
Friedrich Wilhelm Bessel, who was the first to observe stellar parallax in 1838. Light years are used for
distance between stars, the lengths of galaxies, and distances between galaxies.
Parsec (pc) -3.09 x 1013 km or 3.26 lyr. A parsec is the distance at which 1 AU perpendicular to the
observer’s line of sight produces a parallax angle of one arcsecond. The unit, parsec, was coined by
astronomer, Herbert Hall Turner in 1913 and became a useful unit in the calculation of stellar distances
from parallax measurements. Today, parsecs are used for distance between stars, the lengths of
galaxies, and distances between galaxies. Parsecs are sometimes used with metric prefixes (for example,
1 kiloparsec (kpc) equals 1000 parsec).
