This document provides an overview of astronomy topics including: - The basic properties of Earth, Moon, Sun and other planets in our solar system. - How the tilt of Earth's axis causes the seasons and the celestial sphere model used to understand sky motions. - Key events like solstices, equinoxes, and lunar phases that occur as Earth orbits the Sun and the Moon orbits Earth. - The causes and viewing locations of solar and lunar eclipses when the Sun, Earth and Moon are directly aligned. - Concepts like sidereal time, precession and time zones used to measure and communicate about time and sky positions.

1. Astonishing Astronomy 101
With Doctor Bones (Don R. Mueller, Ph.D.)
Educator
Entertainer
J
U
G
G
L
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Scientist
Science
Explorer

2. Chapter 1 - Charting the Heavens
Our Planetary Neighborhood
Sun

3. The Earth
• The Earth is a planet, which
orbits a star (Sun)
• Radius: 6371 km (3909 miles)
• Mass: 6 billion trillion tons
• Actual value:
5,970,000,000,000,000,000,000,
000 kg
Use 5.97  1027 kg, instead!

4. The Moon
• The Moon is a satellite, a body
orbiting the Earth
– Rocky world, littered with
craters
• Bombarded by meteors
• Where are the Earth’s
craters?
– Smaller than the Earth
Less than 1/80 the mass
¼the diameter of Earth
– Small, so it cooled quickly!
– Cold, airless and lifeless

5. The Planets
• Wide variety of planets in the Solar System
– Rocky, hot and airless worlds
– Gas giants and ringed wonders
– Cold planets of blue methane

6. The Sun
• The Sun is a star, a ball
of gas held together by
gravity and generating
light via nuclear fusion
reactions: Converting
Hydrogen to Helium
“Burning H to form He”
• Source of all energy in
the Solar System.
• 100x wider than the
Earth and 300,000x as
massive.

7. Creating Helium via the fusion of Deuterium
and Tritium.
Deuterium
Tritium
Helium

8. The Solar System
• Planets, asteroids, comets and dust all held together by the Sun’s gravity
• Everything goes around the Sun on elliptical paths called orbits
• All orbits lie in the same plane, like peas rolling around on a dinner plate
• Too big to describe using meters – we need something more convenient

9. The Scientific Method
• The Scientific Method is the
procedure we use to construct
ideas about how the Universe
works.
– Starting with a hypothesis – a
testable idea of how something
works.
– Testing the hypothesis!
– If the test fails, modify or abandon
the hypothesis and then retest.
• Hypotheses that pass the many
years of testing become Laws or
Theories .
• A Model can be a simple
to complex description of
physical phenomena
incorporating many laws
and/or theories:
– Ex: The Celestial Sphere
– Ex: Universal Gravitation

10. The Celestial Sphere
http://www.youtube.com/watch?v=3r4x5jRca20 Hoberman Sphere
• Stars in the universe are located at
various distances from Earth, but can
be imagined as lying on a sphere, with
the Earth at its center.
• This sphere appears to rotate around
the Earth once each day, giving the
impression that stars rise and set.
• Since earliest times, humans have
sought to understand the night sky
• A useful model of the sky is called the
Celestial Sphere (CS)
• It is not real – it is simply a tool for
understanding and prediction

11. The Celestial Sphere (CS)
• Important Terms
– Zenith: The point directly overhead on the
celestial sphere (CS)
– Nadir: The point opposite the zenith on the
CS
– North or south celestial pole: The point
around which the stars appear to rotate
– Celestial Equator: An extension of the Earth’s
equator expanded out to the surface of the
CS.
– Horizon: The lower edge of the visible CS

12. Constellations and Asterisms
• The human mind is very good at
recognizing patterns – consequently
we have found and named patterns
of stars on the celestial sphere
• The names of these patterns have
their origins in mythology from all
over the globe
• Sometimes very hard to see!
• These patterns are called Constellations
– 88 internationally recognized constellations,
covering the entire sky
– Star names frequently include the name of
the constellation in which they are located
• Some popular patterns that are not
constellations – are called Asterisms
– Big Dipper
– The Teapot

13. Skywatching
• Under dark skies, you can see
thousands of stars. There are some
stars and constellations, however,
that you can only see from northern
or southern latitudes
• In the northern hemisphere,
constellations that never set (but
simply circle around the North
Celestial pole) are called
circumpolar constellations
• Skywatchers at your latitude in the
southern hemisphere never see your
circumpolar stars!

14. Finding Our Way Around
• Finding your way around the
celestial sphere (CS) is as easy
as finding your way around on
Earth
– The CS is divided by:
– Lines of declination (running
North-South)
– Lines of right ascension (running
East-West)
– Lines of declination are
comparable to lines of latitude
– Lines of right ascension are
comparable to lines of longitude.

15. The Annual Motion of the Sun
• As the Earth revolves around
(orbits) the Sun, the Sun
appears to move through 13
constellations on a belt
around the celestial sphere
called the ecliptic.
• When the sun’s glare blocks
a particular constellation
from view, we say that the
Sun is in that constellation.
• As this motion repeats itself
after one year, it is called the
Sun’s annual motion.

16. The Ecliptic
The ecliptic is tipped relative to
the celestial equator due to a
23.5° inclination of the Earth’s
rotational axis.

17. The Seasons I
• The Earth’s inclination is
responsible for the change
in seasons.
• In June, the Northern
Hemisphere is tilted
towards the Sun.
• In December, the Northern
Hemisphere is tilted away
from the Sun.
• Common Myths:
• Northern Summers are warmer because the Earth is closer to
the Sun than in Winter. Actually, the opposite is true.
• The tilt of the Earth’s axis brings the Northern Hemisphere
closer to the Sun in Summer and farther from the Sun in Winter.
• This accounts for only a minute fraction of the extra heating.
Please insert figure 6.3B

18. The Seasons II
• This tilt of the Earth has
two important effects:
• In Summer, the Sun
spends more time above
the horizon, the days are
longer, resulting in more
heating.
• In Summer, light from
the Sun strikes the
ground more directly,
concentrating the Sun’s
energy.
• Summers are therefore
warmer than winters.
Please insert figure 6.3 A

19. The Tilt of the Ecliptic

20. Solstices and Equinoxes

21. Precession I
• The Earth spins about its axis like a top, but the Sun’s gravity
adds a little tug.
• This tug results in the axis of the Earth rotating, or precessing,
with a 26,000 year period.

22. Precession II
• Thanks to precession,
Polaris (the North
Star) will not always
be “The North Star!”
• 6000 years ago, the
North Star was
Thuban, a star in the
constellation Draco.
• In 12,000 years, the
Earth’s axis will point
toward Vega, a bright
star in Lyra.

23. What Time Is It?
• There are many ways to
measure time on Earth.
– Sunrise to sunrise
Problem – seasons
change sunrise times.
– The time between
successive crossings of
the meridian by the
Sun (Solar Day).
Problem – inaccuracies
due to clouds.

24. Sidereal Time
• One Solar Day is 24 hours
– the sun returning to the
same spot in the sky on
successive days.
• One sidereal day is 23
hours, 56 minutes:
• A sidereal day (23 hours,
56 minutes, 4.091
seconds) corresponds to
the time taken for the
Earth to complete one
rotation relative to the
vernal equinox.

25. Length of Daylight Hours
• The number of daylight
hours a place has
depends on that place’s
latitude on the Earth
• Regions close to the
northern pole get more
daylight hours during
summer and less in
winter.
• Within the Arctic Circle
(higher than 66.5 degrees
latitude), there are some
summer days where the
Sun never sets!
• Regions close to the
equator get close to 12
hours of sunlight all year.
Please insert figure 7.3

26. Time Zones
• The globe is divided into
24 time zones, designed
such that local noon
roughly corresponds to
the time when the sun is
highest in the sky
• If it is noon on the Prime
Meridian in Greenwich,
UK, it is midnight on the
opposite side of the
world. This line is known
as the International Date
Line.
Please insert figure7.4

27. The Phases of the Moon
• As the Moon moves around the
Earth over its 29.5 day cycle (a
sidereal month), one half of its
surface is always lit by the sun
• From Earth, we see only portions
of the illuminated surface, giving
the appearance of phases of the
Moon.
• Full Moon: The Earth is between
the Moon and the Sun, so we see
all of the illuminated surface.
• New Moon: The Moon is between
the Earth and the Sun, so we see
none of the illuminated surface.

28. Phases of the Moon – the Big Picture
Please insert figure 8.2

29. Solar Eclipses
• At New Moon, the Moon is
between the Earth and the Sun.
Sometimes, the alignment is
just right, allowing the Moon to
block the light from the Sun,
creating an eclipse.

30. Solar Eclipse – the Shadow of the Moon
• In a solar eclipse, the Moon
casts a shadow on the surface
of the Earth. People within
the shadow see the eclipse,
and those outside the
shadow do not.
• The Moon’s umbra is the
darkest part of the shadow,
directly behind the body of
the Moon. Within the
umbra, the Sun appears
completely eclipsed (total
eclipse).
• The penumbra of the Moon is
the part of the shadow where
the light from the Sun is only
partially blocked (partial
eclipse).
A solar eclipse seen from space

31. Regions of visible total solar eclipses

32. Lunar Eclipses
• As the Moon passes behind the Earth, the Earth
can cast a shadow on the surface of the Moon,
creating a lunar eclipse.
• The reddish glow of a fully eclipsed Moon is light
that has been refracted through the Earth’s
atmosphere and bounced back to Earth. – It is, in
essence, the light of every sunrise and sunset on
Earth reflected off the Moon.

33. Lunar Eclipse – the Shadow of the Earth
• In a lunar eclipse, the Earth
casts a shadow on the surface
of the Moon. In its orbit, the
Moon passes through the
penumbra and umbra of the
Earth
• The penumbra of the Earth is
the part of the shadow where
the light from the Sun is only
partially blocked. The Moon
dims a little as it passes into
the penumbra (a penumbral
eclipse).
• The Earth’s umbra is the
darkest part of the shadow,
directly behind the body of
the Earth. After the Moon
moves into the umbra, its
surface becomes very dark.
This is a total lunar eclipse.

34. Why don’t eclipses happen all the time?
• In order for an eclipse to occur, the
Moon must lie directly between the
Earth and the Sun (solar eclipse), or
the Earth must lie directly between
the Moon and the Sun (lunar eclipse).
• The orbit of the Moon around the
earth is inclined slightly to the plane
of the ecliptic (the plane in which the
Earth’s orbit lies).
Of course, most of the time,
the Moon’s shadow misses
the Earth or the Earth’s
shadow misses the Moon!

35. • For an eclipse to occur, the Moon must be crossing the ecliptic
at the same time it passes either in front of (solar eclipse) or
behind (lunar eclipse) the Earth (B & D).
• Otherwise, no eclipses are possible (A & C)
A
B
C
D

36. The Solar System
Planets:
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto (?)
Dwarf Planet

37. Beyond the Solar System - The Milky Way

38. Our Neighborhood
• The Universe is “clumpy” – galaxies tend to pull
together by gravity:
– Our immediate neighborhood is called the Local
Group, a cluster of around 4 dozen galaxies (3
million light years across): Andromeda is the
largest and the Milky Way is second largest.
– The Local Group is part of the Virgo Cluster, a
large collection of smaller clusters and groups of
galaxies.
– Superclusters: collection of larger clusters
– The Universe – simply everything!

39. • The Milky Way is a gravitationally-bound collection of
several hundred billion stars. The Sun is one of these
stars and is located ~ 24,000 light years (or 8000 parsecs)
from the center of our the Milky Way.
1 parsec = 3.26 light-years. It is defined as the length of
the adjacent side of an imaginary right triangle in space.
The two dimensions that specify this triangle are the
parallax angle (defined as 1 arc second) and the opposite
side (defined as 1 astronomical unit (AU). Given these
two measurements, along with the rules of
trigonometry, the length of the adjacent side (the parsec)
can be found.

40. Parsec unit
1 parsec =
3.26 light-years

41. Nearby Stars
Proxima-Centauri
(4.23 ly)
Alpha-Centauri A, B
(4.32 ly)
Barnard’s Star
(5.9 ly)
Sirius A, B
(8.60 ly)
Ross 128
(10.9 ly)

42. The Shape of the Earth
• In addition, he
noticed that stars
were visible in
some southern
locations, while not
visible in northern
locations.
• Again, the Earth
must be spherical
for this to happen.
• Aristotle concluded from
observations of the curved
shadow of the Earth on the
Moon during a lunar eclipse
that the Earth was spherical.

43. Distance and Size of the Moon
http://www.youtube.com/watch?v=fE5WHZW3taM
3:00 minute mark
• Aristarchus (~310-230 B.C.E.)
– Used the relative sizes of the
Moon and the Earth’s shadow
during an eclipse to estimate the
size of the Moon. See link above.
• He estimated that the Moon was
1/3 (0.33) as large as the Earth
• Not too far off! (0.27)
– Also estimated the distance to the
Moon by timing how long it took
the Moon to pass through the
Earth’s shadow during an eclipse
• Estimated a distance of 70 Earth
radii
• Pretty close! (~60 Earth radii)

44. Distance and Size of the Sun
• Aristarchus then went on to estimate
the distance of the sun, using his
calculated distance to the Moon.
• Estimated a distance of 20 times the
Earth-Moon distance.
• Using the relative distances of the
Moon and Sun, and his calculated
value for the size of the Moon, he
calculated the size of the Sun to be 7x
bigger than the Earth
• The Sun is 100x bigger!

45. Parallax Preview
• He postulated that the Earth goes
around the Sun, rather than the belief
that everything revolves around the
Earth.
• His critics claimed that if this were true,
they would see the positions of the
stars change relative to each other.
• This is called parallax
– No parallax motion was visible, so
Aristarchus must be wrong!
– Actually, there is parallax (and the
Earth does indeed go around the
Sun), but the motion was too small
for the unaided eye to see – we need
telescopes!

46. Size of the Earth
http://www.youtube.com/watch?v=G8cbIWMv0rI
• Eratosthenes (296-195 b.c.e.)
wanted to know the size of
the Earth. See link above.
• He noted that the sun could
be seen from the bottom of a
well in Syene, so the Sun must
be directly overhead
• Then he measured the angle
the Sun made with the
horizon in Alexandria (7
degrees)
• Calculated a diameter of
13,000 km is almost correct!

47. Measuring Angular Diameter
• In Astronomy, we will frequently
estimate the sizes of planets, etc.
• To do this, we measure the angle that
the object makes in the sky.
• We say that an object subtends an
angle (A) in the sky.
• For example, the moon subtends 0.5
degrees.
• The Sun also subtends 0.5 degrees,
which is why solar eclipses are so
beautiful!

48. Measuring Linear Diameter
• If we measure the
angle subtended by
an object in the sky
(A), and we know
the distance to it
(d), then we can
calculate its actual,
linear diameter (L).