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Astronomical Numbers
“Space is big. Really big.”
Astronomical Numbers
Key Concepts
1) Large (& small) numbers can be written
using scientific notation.
2) Units of length include kilometers,
astronomical units, and light-years.
3) Units of time include seconds and years.
4) Units of mass include kilograms.
- Hitchhiker’s Guide to the Galaxy
Scientists often use the Metric System
to express physical units.
Astronomers measure time in
seconds and years.
– Lengths in Meters
– Time in Seconds
Time for Earth to go around Sun =
1 year =
365 ¼ days =
3.2 x 107 seconds
– Masses in Kilograms
Astronomers measure mass in
kilograms.
NOTE: mass and weight are
NOT the same thing!
Example:
MASS = 1 kilogram = 1000 grams
WEIGHT = 35 ounces on Earth
WEIGHT = 6 ounces on Moon
MASS = amount of matter
WEIGHT = 13 ounces on Mars
WEIGHT = force with which
gravity pulls on matter
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Astronomers measure length in
astronomical units and light years.
1 light-year (ly) = distance traveled by light
(in a vacuum) during one year.
Distance from Earth to Sun =
150 billion meters =
150 million kilometers =
1 astronomical unit (AU)
1 light-year =
63,000 astronomical units =
9.46 x 1012 kilometers
Scientists use scientific notation
to write large (& small) numbers
1000 = 103
1,000,000,000 = 109
0.001 = 10-3
2,200,000 = 2.2 x 106
Number of people living
on Earth =
Number of stars in our
galaxy =
6.96 billion =
300 billion =
6.96 x 109
3 x 1011
(source: U.S. Census Bureau)
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Number of Oreos baked by
Nabisco =
491 billion =
4.91 x 1011
(source: Wikipedia, “Oreo”)
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Standard prefixes are used to express
large and small numbers.
103 = kilo106 = mega109 = giga-
(kilogram, kilometer)
(megawatt, megayear)
(gigabyte, gigayear)
103 = milli106 = micro109 = nano-
(millisecond, millimeter)
(microsecond, micron)
(nanosecond, nanometer)
A Brief Tour of the
Universe
100 meters
MP4023
100 meters
MP4023
EL2004
EL2004
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1000m = 1 kilometer
10 km
EL2004
EL2004
100 km
1000 km
Kansas
City
Troy
EL2004
EL2004
10,000 km
100,000 km
EL2004
Gulf of Suez
Egypt
Earth-to-Moon:
384,000 km
4
100 AU
Earth-to-Sun:
150 million km=
1AU
The AU is useful within the Solar System.
1 light year (ly)
10 ly
 Centauri
4.26 ly away
Proxima Centauri
4.22 ly away
The light-year is useful within our galaxy
(known as the Milky Way Galaxy).
Sun
1 ly = 9.461012 km = 63,235 AU
100,000 ly
Milky Way
Galaxy
Millions of light years
26,000 ly from the
Galactic Center
5
28 Billion Light Years
Imagining the Cosmos
Visible portion of the Universe
Imagining the Cosmos
Key Concepts
1) Cosmology is based on (imperfect) observations of
the universe.
2) The first known models of the universe featured a
flat earth under a domed sky.
3) From Earth, stars appear to move in circles around
Polaris, the North Star.
Cosmology is based on observation of the
universe around us.
Note on terminology:
Universe = cosmos = the whole shebang
Sun ↑
Sky (blue)
horizon
Imperfect
Imperfectand
andincomplete
incompleteobservations
observations
result
in
imperfect
result in imperfectmodels.
models.
Earth (opaque)
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The Starry Night
Cosmology version 1.0:
Domed (solid) sky over flat earth.
About 6000 stars are
visible to the naked eye
on moonless nights over
the course of a year.
Nearly 200 Billion stars
make up our Milky Way
Galaxy.
Greenfield papyrus (1050 BC)
This “Superdome” model of the cosmos was
common among early cultures.
Ancient Egypt, Babylonia, Bronze Age Greece, etc…
Ra, the sun god
Ancient astronomers were careful observers of the sky.
Gemini
Pleiades
neo-Assyrian star chart (650 BC)
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Constellations
People have seen
patterns and drawn
figures in the sky
by connecting the
bright stars.
These starry figures are the Constellations
All peoples have populated the night sky
with constellations.
On a clear, moonless night, you can see at most
3000 stars with your naked eyes.
Constellations
Starry Night Over the Rhône
Vincent van Gogh (1888)
1) It’s convenient to pretend the stars
are attached to a celestial sphere.
Distances to stars are hard to measure.
We can pretend all stars are at the same
distance, on a large celestial sphere.
Stars in a constellation are usually
not all at the same distance from us.
Position on the celestial sphere is known
even when star’s distance is unknown.
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Prime
Meridian
Latitude & Longitude
Prime Meridian:
40th Parallel
Defines Zero Longitude
Through Greenwich UK
Longitude:
Angle East / West along
Equator from Prime
Meridian
Latitude:
40º North
Latitude
Equator
83º West
Longitude
Angle North / South along
the Meridian from
Equator
Columbus, Ohio: 80 W, 40 N
Celestial Sphere: a large (imaginary)
sphere centered on the Earth.
Special locations on celestial sphere:
North Celestial Pole = point directly above
Earth’s North Pole (near the star Polaris).
South Celestial Pole = point directly above
Earth’s South Pole (no nearby star).
Celestial Equator = great circle directly
above the Earth’s Equator.
Distances on the celestial sphere
are measured in degrees,
arcminutes, and arcseconds
Sky from the
North Pole
360 degrees in a circle, 60 arcminutes in a
degree, 60 arcseconds in an arcminute.
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Celestial Navigation made simple:
The Local Sky
We only see half of the sky at any instant
At Earth’s North Pole: Polaris is
directly overhead.
Zenith
At Earth’s Equator: Polaris is
due north, on the horizon.
In Earth’s Northern Hemisphere: Polaris
is due north – height above horizon (in
degrees) equals your latitude.
W
S
N
Horizon
E
NCP
Visible Half
of the Sky
9 pm
N
Z
W
E
CEq
S
Below your
Horizon
SCP
Polaris
11 pm
1 am
40
°
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Constellations appear to travel in
counterclockwise circles around Polaris
(the North Star).
2) The celestial sphere appears to rotate
about the celestial poles (1 day cycle)
Observation: Stars, Sun, Moon, and
planets move in counterclockwise
circles around North Celestial Pole.
What causes these circular motions??
“Superdome” models could explain this motion
by saying the “dome” is the visible half of a
rotating celestial sphere, studded with stars.
visible half of
celestial sphere
hidden by
opaque flat Earth
Hypothesis (Ptolemy, 2nd century):
600 BC: The Greek philosopher
Thales is sometimes called
“the first scientist”.
Thales: the universe is made of
physical objects, which can be
explained without mythology.
Thales: the Earth is flat and
stars are stuck to a rotating
celestial sphere.
Earth is stationary; stars are attached
to a sphere that rotates around the
Earth once per day.
Hypothesis (Copernicus, 16th cent.):
Stars are stationary; Earth rotates
about its axis once per day.
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Cosmology version 1.0:
Domed (solid) sky over flat earth.
Cosmology version 2.0:
Rotating domed sky over flat earth.
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