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Unit 2 – The Moon and the Planets
ASTR 101
Prof. Dave Hanes
The Moon’s Baleful Influence (?)
Crime rates, homicides,
birth rates, acts of‘lunacy,’
traffic accidents, etc…
The Apparent Size of the Moon
Lunar Phases
Not caused by the Earth’s shadow! The phases
arise because we see the Moon from different
perspectives as it orbits the Earth, and can see
only part of the half of the Moon that is lit up by
the Sun.
Watch this very helpful video for a clear understanding:
Full Moon
For the moon to be full, it has to be on the opposite side of
the sky from the Sun (so we can see its fully lit-up face).
This means that it will be rising in the
East as the Sun is going down in the
West. It will be ‘up’ all night.
Thin Crescent
We see a thin crescent only when the Moon is close to the Sun
in the sky. (If big enough, it can be seen in the daytime sky.)
If it’s to the left (East) of the
Sun, it sets in the West shortly
after the Sun itself does (and
rises after the Sun).
If it’s to the right (West) of the Sun, it sets in the West
shortly before the Sun does (and rises before the Sun).
The Moon’s Neverchanging Face
The Moon rotates exactly once every time it goes
around the Earth.
This is a ‘resonance’ –
not a coincidence! – to
be explained later.
Any astronauts on the
front of the Moon would always see the Earth.
No Dark Side
The far side of the moon is hidden from Earth -but it is not always in darkness!
It has alternating light and dark, on a one-month
Angular Size
How big an object looks (its angular size) depends on two
 How big it really is (its linear size); and
 How far away it is.
(Even a small object
can look big if it is
close to you.)
An Amazing Coincidence
The Sun and the Moon have almost exactly the same
angular size.
The Sun is 400x as big, but also 400x farther away! This
means that when the Moon ‘gets in the way,’ we see
perfect eclipses: the Moon only just blocks off the Sun.
A Total Solar Eclipse
(Lets us see the faint, distended corona of the Sun)
Thinking About Shadows
Umbra vs Penumbra
Everything is Moving!
The dark umbral shadow sweeps across the land.
The longest possible period of totality is about 7
A Recent Total Solar Eclipse
20 March 2015
Simulation prepared by NASA:
 The big penumbral shadow. People here see a partial
 The smaller dark umbra. When this passes over you,
you experience a total eclipse. (The maximum possible
duration is ~7 minutes.)
 Many people on Earth see nothing special at all!
(an “annulus” is a
This happens when
the moon is a bit
farther away from
the Earth than
usual, so can’t
block the whole
face of the Sun.
Annular Eclipse
Lunar Eclipses
The Earth casts a big shadow, through which the Moon can
Unlike solar eclipses,
everyone can see this!
-- no need to be in a
special location.
Surprise! The Moon Does Not Vanish
(even though it passes right through the Earth’s umbra)
The Moon gets dim, and turns reddish. Why?
Sunlight is Scattered by the Earth’s Atmosphere:
Some Still Reaches the Moon
Eclipses are Rare!
(at most a few a year)
Why is there not a solar eclipse every month at the time of
New Moon, when they are ‘lined up’?
It is because the
Moon’s orbit is not in
exactly the same plane
as the Earth’s orbit
around the Sun.
To understand this fully, watch
The orbits of the Sun, Moon and Earth are like
gears in a a complex clock. The same
arrangement will recur at regular intervals.
Consequently, eclipses repeat (in very similar
form) after a period of about 54 years.
This is just a couple of human generations, so the
ancients learned to predict them!
Two Similar Eclipses
from computer simulations
Kingston, 8 April 2024
Kingston, 11 May 2078
Rare Events – But Just Wait!
The Distance and Size of the Moon
Once we know its
distance, we can
determine its true
Here’s a scale
model showing the
relative sizes (not the
real separation!)
A Crude Argument
(used by the ancient Greeks)
During a lunar eclipse, assume that
the Earth’s shadow (projected on the
Moon) is about as big as the Earth
Since they knew the size of the Earth
(from Eratosthenes), they could then work out:
 the approximate size of the moon, and thus
 deduce its approximate distance
More Precisely: Use
Parallax Measurements
Analogy: Binocular Vision Our two eyes
see two different images, giving depth
Meet Aldebaran
Starry Night
Sept 4, 10:00 PM – watch the moon from
Kingston. It occults Aldebaran as it drifts
across the field of background stars. (Do
this with your Starry Night software!)
Same date and time: from Buenos Aries, we
do not see this! Why?
Meet The Pleiades
(a very remote star cluster)
In glorious colour
More prosaically,
as photographed
by an astronomer
The Moon Occasionally Passes in Front
- for instance, 29 Oct 2004
It looks different from different places, even at the
very same instant! (a parallax effect)
Santiago, Chile
Remember that each star in the Pleiades is enormously larger than the Earth + Moon,
and that they are fantastically far away. Indeed, the star cluster is so very remote that
we look towards it in parallel directions from Kingston (K) and Santiago (S), as shown by
the arrows. (The stars are way off to the upper right , not as drawn in this figure. )
From Kingston, the Moon lies right in front of the cluster. From Santiago, we have to
shift our gaze a little to one side to see the Moon – a parallax effect.
Measuring the angle through which we shift our gaze, and knowing the distance between
K and S, we can figure out the distance to the Moon!
The Most Precise Distance Determination
Bounce radar signals or laser light off the surface (where
the Apollo astronauts left special reflectors). See how long
it takes for the return signal to reach you. This is very
Now Consider the Planets
Two categories:
Inferior planets (closer to the Sun than we
are): Mercury and Venus
Superior planets (farther out): Mars, Jupiter,
Saturn, Uranus, Neptune.
Inferior Planets
Inferior Planets
They are always fairly close to the Sun:
sometimes to the left (East) of it, visible in the evening sky;
sometimes to the right (West) of it, visible in the dawn sky.
They come and go at times as the planets orbit the Sun.
They can never be seen overhead at midnight.
Inferior planets can occasionally pass between us and the
Sun, and be seen in silhouette
Superior Planets
They can be overhead at
midnight (at “opposition”),
or around on the far side
of the sun (at “conjunction”)
- or at any intermediate
point in their orbits.
But they never pass between us and the Sun.
Motions of the Planets
Thanks to its orbit around the Earth, the Moon
drifts continuously to the East across the
background field of stars.
The superior planets move in big orbits around the
Sun, so you might expect them to behave in
roughly the same way.
Instead, but they do something quite unexpected.
‘Retrograde’ Motion
As we watch a superior planet like Mars over the space of
several weeks or months, its steady Eastward drift slows
and comes to a halt, after which it moves backwards for a
time. Eventually, it stops again, and Mars resumes its
original Eastward drift.
This is a bizarre complexity we will come back to. It is an
important clue to the structure of the Solar System.