Discovering the Universe - University of North Floridan00006757/astronomylectures... · Discovering...

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

http://www.nasa.gov/externalflash/nasacity/index2.htm

• 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


• 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




• When there is something there, we sometimes identify it as something that it isn’t




• 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


It has a celestial equator, which is an extension of Earth's equator


The ecliptic is the apparent path of the Sun against the background of the stars


Celestial coordinates are analogous to latitude and longitude


Declination (Dec) ~ latitude

Dec = 0° at the celestial equator


Right ascension (RA) ~ longitude

RA = 0 h in Pisces, which is where the Sun is at the Spring equinox


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 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


•Even as it goes through phases, the Moon always presents

the same face to us



the same face to us

•That does not mean that it does not rotate…



the same face to us


• It does mean that from the Moon, the Earth hangs

motionless in the sky



the same face to us


• 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 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...





• Usually... but when there are eclipses, things are different


•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




•But that doesn't happen

•Why?


• 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



ecliptic plane

• Which is when it is in

locations called “nodes”



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

Discovering the Universe - University of North Floridan00006757/astronomylectures... · Discovering...

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