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Discovering the Universe
• Astronomy and human culture have always been intertwined
• Astronomical events have defined the cycles of human life
• They have inspired great religion stories
• The scientific revolution started with efforts to explain planet
motion in the sky
• And now science has given us the modern lifestyle we enjoy
• Lots of things we use every day come from space programs
(for some examples, visit here)
• And in fact, we came from the sky…
• …we are star stuff…
• …we are made of elements made by stars…
• …and just as we came from the sky…
• …much of our civilization has come from our drive to explain
what we see in the sky
• Among the things we see in the sky are the patterns of stars associated with constellations
• A natural human tendency is to try to explain patterns
• Among the things we see in the sky are the patterns of stars associated with constellations
• A natural human tendency is to try to explain patterns…do you see a pattern here?
• Sometimes we see patterns even when there’s nothing there
• Among the things we see in the sky are the patterns of stars associated with constellations
• A natural human tendency is to try to explain patterns
• Sometimes we see patterns even when there’s nothing there
• When there is something there, we sometimes identify it as something that it isn’t
• Among the things we see in the sky are the patterns of stars associated with constellations
• A natural human tendency is to try to explain patterns
• Sometimes we see patterns even when there’s nothing there
• When there is something there, we sometimes identify it as something that it isn’t
• This is called “pareidolia”
• So patterns of stars have been explained as being somehow associated with – or even
actually being – some mythological character or other
• So patterns of stars have been explained as being somehow associated with – or even
actually being – some mythological character or other
• The pattern resembling a character like Orion is what most people mean when they use the
word “constellation”
• But today’s astronomers use the word “constellation” somewhat differently
• Their constellations are regions of the sky containing the traditional constellation patterns—88 of them in all
• The modern constellations completely cover the sky, like continents and oceans completely cover the surface of the Earth
• Back to the traditional meaning (which most people mean when they use the word)
…The stars in a constellation are typically different distances from Earth
• But they don’t appear to be
• They appear to be fixed to the inside surface of a sphere surrounding us (a small
section of which is visible above)
• The modern constellations completely cover the surface of this “celestial sphere”
• Most ancient people thought the celestial sphere was real, because it looks that way
• We know now that the celestial sphere doesn’t really exist, but we use the concept anyway…
The celestial sphere has features strongly linked to Earth, which is at its center
The Celestial Sphere
It has a north celestial pole directly above Earth's north pole
The Celestial Sphere
It has a celestial equator, which is an extension of Earth's equator
The Celestial Sphere
The ecliptic is the apparent path of the Sun against the background of the stars
The Celestial Sphere
Celestial coordinates are analogous to latitude and longitude
The Celestial Sphere
Declination (Dec) ~ latitude
Dec = 0° at the celestial equator
The Celestial Sphere
Right ascension (RA) ~ longitude
RA = 0 h in Pisces, which is where the Sun is at the Spring equinox
The Celestial Sphere
Stars, but not planets, always have the same Dec and RA
Of course, over very long times, a star’s Dec and RA will change
The Celestial Sphere
• The motion of the stars in the sky could be because the celestial sphere is turning or
because the Earth is rotating
• We now know that it’s the Earth rotating (a Foucault pendulum shows this, for example)
• But ancient people thought it was the rotation of the celestial sphere because they had no
evidence otherwise, and it looks and feels that way
• Let’s consider something else ancient people believed…
• Astrology
• Astrology is the belief that celestial bodies can somehow influence earthly events and
human destinies
• To understand astrology, you need to understand “sidereal” rotation, why there are seasons,
and how our calendar works…
Sidereal Day vs Solar Day
• A “sidereal day” is the length of time it takes for
the Earth (or any celestial body) to turn once
around its axis “relative to the stars”
• Such a full 360° rotation relative to the stars is
called a “sidereal rotation”
• A “solar day” is the length of time it takes for the
Earth to turn around far enough that the Sun is at
the same location in the sky – say at noon
• The Earth has to turn a little bit farther than one
sidereal rotation to put the Sun in the same place
in the sky
• So the solar day on Earth (exactly 24 h) is about
4 minutes longer than the sidereal day (23h 56’)
• Any type of rotational motion – spinning, orbiting,
etc – can have sidereal periods different from
other types of periods, as in the case of Earth’s
sidereal vs solar day
• And this has implications for astrology, as you will
see…
• Here are the Sun, the Earth, and the ecliptic, the path the Sun takes through the constellations on the celestial sphere
• Do you recognize these constellations?
• They are the constellations of the zodiac, the astrological signs
• According to astrology, the sign the Sun and the planets are in when you are born is supposed to affect your fate
• But is this possible, according to our best current scientific understanding?
• Does astrology work?
• One strike against it is that your sign is likely not what you thought it was
• How can this be?
• It is due to the difference between “sidereal years” and “tropical years”
• The standard calendar is based on the seasons…
• First…Why do you think we have seasons?
• Seasons are caused by the tilt of the Earth, not by how close the Earth is to the Sun
• It’s summer when the rays of the Sun hit the Earth’s surface more directly, and it’s winter when the rays hit more at a glancing angle
• The solstices and the equinoxes define the first day of each season
• Our calendar is based on the “tropical year”, which is measured from one vernal (Spring) equinox to the next
• But the problem with this is that like any spinning body – including this top – the Earth precesses
• This makes a difference between the length of a tropical year and a solar year
• Notice the direction of precession for the Earth
• The tropical year of our calendar measures from one vernal equinox to the next
• But the Earth precesses a little during each sidereal year
• The result is that a tropical year is about 20 minutes shorter than a sidereal year
• And also that the vernal equinox position shifts against the background constellations – against the zodiac
• Compound that 20 min/year over 2000 years, and you find that in 2000 years, the Earth has precessed the equivalent of ~1 month
• The result is that the Sun is not in the same constellation as it used to be
• So what is your sign, and where is the Sun supposed to be on your birthday?
• Where is it really? We’ll find out on the next slide…
Adjusted birthday ranges for the various Sun signs
Capricorn - Jan 20 to Feb 16
Aquarius - Feb 16 to Mar 11
Pisces - Mar 11 to Apr 18
Aries - Apr 18 to May 13
Taurus - May 13 to Jun 21
Gemini - Jun 21 to Jul 20
Cancer - Jul 20 to Aug 10
Leo - Aug 10 to Sep 16
Virgo - Sep 16 to Oct 30
Libra - Oct 30 to Nov 23
Scorpius - Nov 23 to Nov 29
Ophiuchus - Nov 29 to Dec 17
Sagittarius - Dec 17 to Jan 20
Is your sign what you thought it was?
A. Yes
B. No
• It could be that the horoscopes are wrong, but that celestial bodies really
can influence terrestrial events
• But this is not consistent with our scientific understanding...
• For astrology to work we need a long-range effect—some sort of force field
• We know of only four physical forces in our universe:
strong weak electromagnetic gravity
• The only long-range physical force that could mediate the influence of
planets on a child at birth is gravity
• The strength of gravity coming from the obstetrician that delivered you is
comparable to the strength from a planet
Object
(planets are at closest distance)
Ratio gravity object to gravity of
100 kg human at distance of 0.5 m
Earth 367,000,000
Sun 230,000
Moon 1,410
Mercury 0.1
Venus 8.3
Mars 0.5
Jupiter 13.7
Saturn 1.0
Proxima Centauri 0.0000004
3 story 55 m x 35 m building at
distance of 120 m0.5
100-kg human at distance of 0.5 m 1.0
So the specific surroundings at your birth have similar effects to celestial bodies
Stars in the Local Sky
• You see any stars that pass through the half of the celestial sphere
centered on your zenith
• Most stars rise in the east and set in the west
• But some stars—” circumpolar stars”—never set
• And other stars never rise
• Which ones these are for you depends on where you are on the globe
Stars in the Local Sky
• Even though they traveled only limited distances, the ancient
Greeks saw that stars are different at different latitudes
• This led them to suggest that the Earth was not flat, and that
the sky was not a simple dome inverted over it
• It also vouches for their powers of observation, which we’ll
come back to later…
• The brightest object in the night sky is not a star, but…?
• It is the Moon
• And it looks different at different times
The Phases of the Moon
• Main Entry: gib·bous
• Pronunciation: 'ji-b&s, 'gi-
• Function: adjective
• Etymology: Middle English, from Middle French gibbeux, from Late Latin gibbosus humpbacked, from Latin gibbus hump Date: 14th century
–1 a : marked by convexity or swelling
– b of the moon or a planet : seen with more than half but not all of the apparent disk illuminated
–2 : having a hump : HUMPBACKED
The Phases of the Moon
•Even as it goes through phases, the Moon always presents
the same face to us
The Phases of the Moon
•Even as it goes through phases, the Moon always presents
the same face to us
•That does not mean that it does not rotate…
The Phases of the Moon
•Even as it goes through phases, the Moon always presents
the same face to us
•That does not mean that it does not rotate…
• It does mean that from the Moon, the Earth hangs
motionless in the sky
The Phases of the Moon
•Even as it goes through phases, the Moon always presents
the same face to us
•That does not mean that it does not rotate…
• It does mean that from the Moon, the Earth hangs
motionless in the sky, and goes through phases that are
opposite to the Moon phases seen from Earth
The Phases of the Moon
•The Moon, and the Sun, have been humanity's constant companions
The Phases of the Moon
•The Moon, and the Sun, have been humanity's constant companions
• The Sun rises and sets everyday, as usual
The Phases of the Moon
•The Moon, and the Sun, have been humanity's constant companions
• The Sun rises and sets everyday, as usual
• The Moon goes through its phases every month, as usual
The Phases of the Moon
•The Moon, and the Sun, have been humanity's constant companions
• The Sun rises and sets everyday, as usual
• The Moon goes through its phases every month, as usual
• Usually...
The Phases of the Moon
•The Moon, and the Sun, have been humanity's constant companions
• The Sun rises and sets everyday, as usual
• The Moon goes through its phases every month, as usual
• Usually... but when there are eclipses, things are different
The Phases of the Moon
•Eclipses occur when the Sun, Earth, and Moon are in a line
The Phases of the Moon
•Eclipses occur when the Sun, Earth, and Moon are in a line
•The figure above suggests that eclipses should occur every month at new moon and at full moon
The Phases of the Moon
•Eclipses occur when the Sun, Earth, and Moon are in a line
•The figure above suggests that eclipses should occur every month at new moon and at full moon
•But that doesn't happen
The Phases of the Moon
•Eclipses occur when the Sun, Earth, and Moon are in a line
•The figure above suggests that eclipses should occur every month at new moon and at full moon
•But that doesn't happen
•Why?
The Phases of the Moon
• The Moon’s orbit is tilted
~5° with respect to the
ecliptic plane
• Eclipses can only happen
when the Moon is in the
ecliptic plane
• Eclipses can only happen
when the Moon is in the
ecliptic plane
• Which is when it is in
locations called “nodes”
• Eclipses can only happen
when the Moon is in the
ecliptic plane
• Which is when it is in
locations called “nodes”
• And that can only happen
during “eclipse seasons”
• Eclipse seasons are due
to the Moon’s angular
momentum
• You might expect eclipse
seasons to be 6 months
apart…
• You might expect eclipse
seasons to be 6 months
apart…
• But they are slightly less
than that, because the
Moon’s orbit precesses
• After half a year – the
approximate length of
time between eclipse
seasons…
• Hold the angle of the
nodes after 6 months in
your mind
• Imagine the node line is
precessing as the Earth
orbits the Sun
• Imagine the node line is
precessing as the Earth
orbits the Sun
• Precession causes
eclipse seasons to be
separated by slightly less
than 180 days
• This is another example
of a sidereal motion—
relative to the stars—and
some other kind…
• So to summarize…
• The Moon's orbit is inclined to the ecliptic ~5°
• So most of the time Moon is above or below the ecliptic plane
• Only when the orbital nodes are aligned with Earth and Sun can
eclipses occur—about every 180 days
• Eclipses can occur only when the Moon is at a node, and when it is new or full
• When it’s new (Moon between Earth and Sun) → Solar eclipse
• When it’s full (Earth between Moon and Sun) → Lunar eclipse
• Both types of eclipses involve shadows that have two parts:
• Umbra
• Penumbra
• This is due to Sun not being a point source
• As a result of the two-part shadow, both lunar and solar
eclipses come in several types
Lunar eclipse types
• Penumbral
• Partial
• Total
Solar eclipse types
• Total
• Partial
• Annular
• These are photos of an annular eclipse on
Monday, 26 Jan 2009, taken in Bandar
Lampung, Sumatra, Indonesia
• Solar eclipses are much harder to experience than lunar eclipses
• To see a lunar eclipse, it just needs to be night (and clear) when the eclipse happens
• To see a solar eclipse, you have to be in the right place at the right time…
…because the umbra of a solar eclipse is only about 100 miles wide, and it moves at
~1700 km/h across the Earth!
• Nearly 3000 years ago the ancient Chaldeans, a Babylonian dynasty, detected
a regularity in the occurrence of lunar eclipses
• They found they recurred every 18 years, 11 1/3 days
• This is now known as the “saros cycle”
• For those who wanted to impress and advance, this would have been an
important tool
• But it didn't always work
• Can you imagine why?
• Because of the 1/3 day…
• Because of that, even though an eclipse will occur with regularity
• It will not always occur at the same place
• This is a potential problem for both lunar and solar eclipses
But it is particularly a problem for solar eclipses
Why isn’t it such a problem for lunar eclipses?
But it is particularly a problem for solar eclipses
Why isn’t it such a problem for lunar eclipses?
Because they can be seen over a much larger region