Ch01 Lecture part1 video

Summary

This astronomy lecture introduces the concept of celestial cycles, explaining the ancient Greek model of a celestial sphere encompassing fixed stars. It discusses the importance of models in scientific predictions and the distinction between daily and annual motions of celestial bodies. Constellations are described as historical signposts in the sky, with proper motion and the Earth's axial tilt affecting star positions over time. The lecture emphasizes the difference between astronomy and astrology, highlighting the scientific method and the Earth's movements as the basis for understanding the sky.

Takeaways

• 🌌 The ancient Greeks envisioned the sky as a celestial sphere with stars fixed at equal distances from Earth, akin to a glass sphere surrounding the planet.
• 📐 A model in science is a conceptual tool used to understand and predict phenomena; it's a description that can be tested and refined based on observations and predictions.
• 🔮 Constellations are patterns of stars named after mythological figures or animals, serving as signposts in the sky but not necessarily indicative of the stars' actual proximity to each other.
• 🌟 Stars exhibit proper motion, moving slightly over long periods due to their orbits around the Milky Way galaxy, though this motion is barely noticeable within a human lifetime.
• 🌍 Earth's daily motion is caused by its rotation on its axis, leading to the apparent rising and setting of celestial bodies like the Sun and Moon.
• 🌞 The Earth's axial tilt, at 23.5 degrees, affects the path of the Sun in the sky, causing seasonal variations in the Sun's position at noon.
• 🌐 The celestial equator and the ecliptic are two significant imaginary lines in astronomy; the former is the projection of Earth's equator into space, and the latter is the path the Sun appears to follow in the sky.
• 📅 The zodiac constellations, historically, function as a calendar, indicating the time of year based on the Sun's position in relation to these constellations during the day.
• 🚫 Astrology, which may use zodiac signs, is distinct from astronomy and lacks scientific validity; the script emphasizes the importance of distinguishing between the two.
• 🌠 The annual motion of Earth around the Sun results in different visible constellations at night, depending on the Earth's position in its orbit, effectively using the sky as a timepiece.
• 🌙 The Moon, like the Sun, rises and sets due to Earth's rotation, showcasing the daily motion that affects all celestial bodies as observed from our planet.

Q & A

What is the celestial sphere as described by the ancient Greeks?

The celestial sphere is an ancient Greek concept where they imagined all the stars in the sky as being attached to a large, spherical surface that surrounds the Earth, with the stars all appearing to be at the same distance away.

What is a scientific model and why is it important?

A scientific model is a theoretical construct that helps us understand and predict phenomena in the universe. It is important because it allows us to make predictions about future events, and if those predictions are correct, it builds confidence in the model.

What are constellations and how have they historically been used?

Constellations are patterns of stars in the sky that have been named after mythological, historical, or imaginary figures. Historically, they have been used as signposts or markers in the sky to help navigate and locate objects in the night sky.

Why do stars in a constellation appear to be close together when they might not be?

Stars in a constellation appear close together from our perspective on Earth, but they can actually be very far apart in space. This is because their positions in the sky are a result of our viewpoint and not their actual distances from each other.

What is proper motion and how does it relate to the stars' movement?

Proper motion is the apparent change in a star's position in the sky over time due to its actual movement through space. All stars orbit the Milky Way, and because of their unique orbits, they slowly move relative to our viewpoint on Earth.

How does the Earth's rotation affect what we see in the sky?

The Earth's rotation on its axis causes the daily motion we observe, such as the sunrise and sunset. As the Earth spins, different parts of the sky come into view, creating the illusion that the stars, Sun, and Moon are moving across the sky.

What is the difference between daily motion and annual motion?

Daily motion is caused by the Earth's rotation on its axis, leading to the apparent movement of celestial bodies across the sky within a 24-hour period. Annual motion, on the other hand, is due to the Earth's orbit around the Sun, which results in different constellations being visible at different times of the year.

Why do ancient astronomers believe the Earth was stationary?

Ancient astronomers believed the Earth was stationary because they observed the stars and other celestial bodies appearing to move around the Earth. They thought the Earth was at the center of the universe, with everything else moving around it, which was a concept known as the geocentric model.

What is the significance of the Earth's axial tilt?

The Earth's axial tilt of 23.5 degrees relative to its orbit around the Sun causes seasonal changes and affects the Sun's position in the sky at noon. It also influences the location of the celestial equator and the ecliptic, which are important for understanding the changing sky throughout the year.

How can constellations be used as a calendar?

Constellations can be used as a calendar because their positions in the sky change predictably throughout the year due to the Earth's orbit around the Sun. By observing which constellations are visible at certain times, one can determine the approximate time of year.

What is the difference between the zodiac constellations and astrology?

The zodiac constellations are a set of constellations that the Sun appears to pass through in the course of a year, and they have historically been used as a calendar. Astrology, on the other hand, is a belief system that suggests these constellations influence individual personality traits and life events, which is not supported by scientific evidence.

Outlines

00:00

🌌 Introduction to Celestial Spheres and Ancient Greek Astronomy

The lecture begins with an introduction to the concept of cycles in the sky, focusing on the ancient Greek idea of a celestial sphere encompassing all stars at equal distances from Earth. The celestial sphere is likened to a glass sphere with stars affixed to it, spinning around the Earth. This model serves as a foundational framework for understanding the universe and is a key example of how scientific models work, providing predictions and insights into the nature of the cosmos. The importance of predictions in science is emphasized, as they are a means to test and validate models.

05:03

🌟 Understanding Constellations and Their Role in Astronomy

The script moves on to discuss constellations, which are described as signposts in the sky, historically used to map the heavens. Constellations like Leo and Orion are named after mythological figures, and while they appear close together to the naked eye, they can be vastly distant from one another. The paragraph also touches on the proper motion of stars, explaining that stars' positions change over centuries due to their orbits around the Milky Way, although this motion is barely perceptible within a human lifetime.

10:07

🌍 Earth's Daily and Annual Motions and Their Impact on the Night Sky

This section delves into the concepts of daily and annual motion as observed from Earth. The daily motion is attributed to Earth's rotation on its axis, causing the Sun, Moon, and stars to rise and set. Annual motion, on the other hand, is due to Earth's orbit around the Sun, leading to the visibility of different stars and constellations throughout the year. The ancient Greeks' belief in a stationary Earth with celestial bodies moving around it is contrasted with the modern understanding of Earth's active role in these motions.

15:08

📍 The Earth's Axis, Seasons, and the Celestial Equator

The Earth's tilted axis and its implications for the position of the Sun in the sky are explored. The tilt results in the Sun never being directly overhead outside the tropics and is responsible for the changing seasons. The concept of the celestial equator, an extension of Earth's equator into space, is introduced, and its tilt relative to the ecliptic—the path of the Sun—is explained. The ecliptic is identified as the apparent annual path of the Sun across the sky.

20:11

🗓 Zodiac Constellations as a Calendar in Ancient Astronomy

The lecture discusses the historical use of zodiac constellations as a calendar, with the Sun's position among these constellations indicating the time of year. The zodiac is distinguished from astrology, with the instructor emphasizing the scientific nature of astronomy. The paragraph explains how different constellations are visible at different times of the year due to Earth's orbit, and how this can be used to track the passage of time, much like a celestial calendar.

25:12

🌙 Conclusion and Preview of Upcoming Lecture Topics

The script concludes with a summary of the lecture's first part, highlighting the importance of understanding the motions observed in the sky. The instructor expresses hope for the students' enjoyment and previews upcoming topics that will further explore the movements and significance of celestial bodies.

Keywords

💡Celestial Sphere

The celestial sphere is an ancient concept that represents the sky as a hollow sphere surrounding the Earth, with all stars and celestial bodies attached to its inner surface. In the video, it is used to explain the early Greek understanding of the night sky and how it influenced their model of the universe, suggesting that stars appear equidistant because they are all the same distance from Earth on this sphere.

💡Model

In the context of the video, a model is a theoretical representation of a system or process, used to enhance understanding and make predictions. The celestial sphere is an example of a model, and the video emphasizes the importance of models in science, particularly in astronomy, as they provide a framework for understanding and predicting celestial movements.

💡Constellations

Constellations are patterns of stars that have been historically named and used as signposts in the sky. The video explains that constellations are not physically close to each other but appear to form patterns due to our viewpoint on Earth. They serve as a way to map the sky and understand positions of stars, but are less important in modern astronomy.

💡Proper Motion

Proper motion refers to the observable change in position of stars relative to the solar system over time. The video script mentions that stars are so far away that their motion is barely noticeable in a human lifetime, but over centuries, their positions can be seen to change, which is due to their orbits around the Milky Way galaxy.

💡Daily Motion

Daily motion is the apparent movement of celestial bodies across the sky due to the Earth's rotation on its axis. The video describes how this motion causes the Sun, Moon, and stars to rise and set, which was historically thought to be the celestial sphere rotating around a stationary Earth.

💡Annual Motion

Annual motion is the change in the position of celestial objects over the course of a year due to the Earth's orbit around the Sun. The video explains that this motion results in different stars being visible at different times of the year, as the Earth's position relative to the Sun changes.

💡Ecliptic

The ecliptic is the path the Sun appears to follow through the sky over the course of a year, as seen from Earth. The video describes it as the apparent annual path of the Sun against the backdrop of stars, which is essentially the Earth's orbital plane projected onto the celestial sphere.

💡Zodiac

The zodiac refers to a group of constellations that lie along the ecliptic and are used to represent the positions of the Sun in the sky throughout the year. The video clarifies that the zodiac has historically been used as a calendar to indicate the time of year, rather than for astrological predictions.

💡Tilt of the Earth

The tilt of the Earth refers to the angle at which the Earth's axis is inclined relative to its orbit around the Sun, which is approximately 23.5 degrees. The video explains that this tilt is responsible for the seasonal changes and the variation in the Sun's position in the sky at noon throughout the year.

💡Orbit

Orbit, in the context of the video, is the path followed by a celestial body as it moves around another body due to gravity. It discusses how both stars and planets, including the Earth, have orbits. The Earth's orbit around the Sun is what causes the annual motion and the change of seasons.

💡Astrology vs. Astronomy

The video script distinguishes between astrology, which is considered incorrect and not a science, and astronomy, which is a scientific study of celestial objects and phenomena. It emphasizes that the zodiac's historical use was in astronomy as a calendar, not in astrology for predicting personal destinies.

Highlights

Introduction to the concept of cycles in the sky and the movement of celestial bodies.

Explanation of the ancient Greek idea of the celestial sphere encompassing all stars at equal distances from Earth.

Discussion on the uniform appearance of stars and the implications for their perceived distances.

Introduction to the scientific method and the role of models in understanding the universe.

Emphasis on the importance of predictions in the scientific process and the validation of models.

Brief mention of constellations as historical signposts in the sky.

Clarification that stars within constellations are not necessarily close to each other in space.

Description of proper motion of stars and their movement over extended periods of time.

The Milky Way galaxy's role in the motion of stars and their unique orbits.

Daily motion of celestial bodies due to Earth's rotation on its axis.

Annual motion resulting from Earth's orbit around the Sun and its effect on visible constellations.

Historical belief of Earth's stationary position and celestial spheres spinning around it.

Earth's axial tilt and its impact on the Sun's position in the sky at noon.

Differentiation between the celestial equator and the ecliptic in relation to Earth's tilt and orbit.

Use of constellations as a calendar to track time and seasons.

The zodiac constellations as a historical calendar, not a determinant of personal traits.

Conclusion of the lecture part with a teaser for further discussions on celestial movements.

Transcripts

okay hello everyone this is gonna be

part 1 of lecture 1 our very first

lecture in astronomy this lecture is

gonna be about cycles in the sky which

is to say how things move in the sky and

why they move the way they do so let's

first of all talk about an idea that's

been going on since the ancient Greeks

the ancient Greeks thought of the sky

around us as a celestial sphere now what

that means is basically if you imagine

all the stars so these stars you imagine

all the stars in the sky as a or in a

sphere now this idea if you go outside

tonight and look at the night sky squat

and look and well you're gonna see when

you go out and look at the night sky at

the Stars is you're gonna see all the

stars in the sky looking like stars now

what does that mean well looking like

stars well

they don't appear to be any further or

closer than any other stars which is to

say that if you look at this star here

in the night sky and look at this star

here in the night sky when you look at

them they don't look that different they

look pretty much the same and if they

look the same

maybe one's brighter

maybe one's a little dimmer but they

don't look that far apart and so fine if

you imagine as the Greeks did what they

believed was that all these stars we're

basically the same distance away from

the earth they were all attached to a

celestial sphere a

imagine it as a large glass sphere that

surrounds the earth and all the Czar's

are attached to it and the celestial

sphere spun around the earth now this is

a model and this is how science works

models what does a model mean well a

model is basically a way to think about

the earth or think about the universe

overall it is a way to well I mean it

says model it is a model it is a

description of the universe around us

and it's a description which can provide

ok can provide a means to enhance our

understanding of the nature which is

what it says right there but it is a

means to understand nature that has

predictions predictions are important

predictions is what science is so

basically you come up with a model and

your model should tell us what is going

to happen in the future and if in the

future you predict something's gonna

happen

well that is science science predicts

what's gonna happen in the future and if

it turns out to be correct

if you're your model predicts what's

gonna happen then you have some

confidence in your model but

if your model predict something's gonna

happen in the future and it doesn't

happen well then your model is wrong

this is the basic foundation of science

and astronomy was the first of the

fields of science that was actually used

or and that's what's used but the first

field of science that was actually

developed and it's because in astronomy

or looking at the Stars you could

actually see the future now we will talk

more about this in detail but models and

science these are huge for this class

and we'll be talking it again in more

detail about it as we go on now

constellations I do want to talk about

constellations a little bit but not a

lot because constellations are basically

points on map constellations

historically have been used as sign

points in the sky so you pick a bunch of

stars usually bright stars and those

stars you name a name after something

that you know say a lion so in this case

is Leo the Lion or historical figures or

[Music]

imaginary figures things like Orion

constellations are basically a map you

can use constellations to figure out

where things are in the sky but you

should note that stars like this one

here

and this one here they look like they're

close together in the sky they are not

necessarily close together this guy they

can be very very far apart

okay constellations are only they only

look the way they do because of where we

are in space and we're gonna talk a lot

about that in the future okay like you

know our position in the galaxy because

we this in the galaxy all that stuff

that's coming up but from our point of

view here on the earth looking out into

space there are stars and from our point

of view of those stars and make a

pattern and we named those stars after

that pattern and this has been going on

for probably as long as humans have been

around okay so fine constellations they

are just stars in the sky we've named

them after certain mythological beasts

and real beasts and whatnot

constellations themselves are not that

important for modern astronomy they are

really just used as signposts hey this

is where you are in the sky

so now constellations over time well

over time

technically stars do move which is to

say that if you looked at say this star

right here if you look at this star

today you'll be right there tomorrow

right there but if you were to look at

the star say 100 years into the future

let's say a hundred years from now it

might have moved slightly so it might be

right there right here and if you look

at it again a thousand years from now it

would maybe move a bit further now

this kind of motion we call it proper

motion it has to do with the fact that

all stars orbit our galaxy our galaxy

the Milky Way galaxy there are hundreds

of billions of stars in the Milky Way

galaxy they all orbit around the center

of the galaxy kind of like you know

planets orbit the Sun stars orbit the

Milky Way and because of that and

because all stars have unique orbits

actually from our point of view all the

stars are moving but because they're so

far away because they're so far away

they only appear to move a little bit

and during your lifetime they will not

move much at all in fact they will move

almost from your point of view without a

telescope which is your eyeball they

won't move at all they won't move at all

period now the fact that they move over

time is something you're gonna be able

to see only over a long periods now if

you go back in time to like the ancient

Greeks and we'll be talking a lot about

the Greeks because they kind of codified

the idea of modern astronomy they look

at the Stars and they move and because

of that they thought the earth had to be

stationary now I want you to think about

the Stars and the fact that you know the

appearance over time and you know the

appearance over time I want you to think

about motion and motion with distance so

how far away a thing is and how much it

appears to move so think about this you

stand on the side of a rope

and a car goes by you well you can see

the car go by you it goes fast it

appears to move fast but what happens if

you look up into the sky and you see a

plane an airplane flying across the sky

way up in the sky look at it

that airplane appears to be moving very

slowly

is it moving slowly no it's actually

moving very fast it's actually moving

faster than the cars on the road much

faster the airplane is moving like five

hundred miles per hour the cars well

they should be moving much slower than

that right but the car moving past you

when it's only a few feet away looks

like it's moving west faster than the

airplane that's moving across the sky

very very far away from you and the deal

is the airplane is like these stars okay

it's very far away now the stars are

much much further away so even though

the stars are moving because they're so

far away during your lifetime you will

not notice that movement so the Swan

Cygnus this constellation is Cygnus the

Swan it will look like Cygnus the Swan

during your entire lifetime but say your

great-great-grandchildren

well the positions of these stars will

be slightly different not a lot

different again they are very far away I

mean the airplane the airplane in the

sky is maybe a few miles away mean at

most like five or six miles away

which is why it looks like it's moving

so slowly these stars are trillion miles

away trillion

of miles away hugely far away which is

why they don't seem to move all right

okay now when we look at the night sky

or day a time sky there are a couple of

things we notice first of all we notice

that there is daily motion so the Sun

rises the Sun sets the moon rises the

moon sets stars rise and they set and

now this has to do with the fact that

the earth is a ball it's a spherical

structure and it's spins on it and on an

axis that axis so if you take a ball and

you spin it it spins around

a an axis that points through the ball

now it the northern hemisphere in the

southern hemisphere are defined by this

motion so you have a sphere and you have

an axis and the sphere spins around that

axis and the northern hemisphere where

the axis comes out of the ball on the

north that is the North Pole where it

comes out in the South that is the South

Pole and the whole earth spins around

this axis about once every 24 hours so a

day now ancient astronomers assumed that

what was actually happening was that

things were spinning around the earth so

the earth was stationary in the center

and everything spun around the earth

okay and that is the idea of the

celestial sphere so all the stars all

the planets and the Sun were attached to

celestial spheres these the big things

that's

around the earth now today we know that

this daily motion is actually caused by

the fact the earth itself is spinning

once about every 24 hours

now this is not the only kind of motion

we see in the sky

it's just daily motion so you see the

sunrise and sunset that's because the

earth is spinning on its axis so

sometimes we're facing the Sun sometimes

we're not but it is also the case that

you have a Sun and you have the Earth

orbiting around the Sun so it goes

around the Sun once per year now what

this means is that different times of

the year so for instance if the earth is

here at night we will see stuff that is

in this direction because during the day

we're looking towards the Sun right at

night we're looking away from the Sun

now six months later the earth is over

here which means at night we're looking

in this direction right so we're seeing

different stuff at night different stuff

in the sky this is annual motion this

has to do with the fact that the Earth

orbits around the Sun once a year daily

motion has to do with the fact they were

spins on its axis that's why the Sun

rises and sets but the fact that we see

different stars in the sky at different

times of the year that has to do the

fact that we go around the Sun okay so

say we're here we're looking in this

direction if we are here we're looking

in this direction alright that makes

sense

hopefully so there are two different

kinds of motion that are happen

but the ancient Greeks and again bad

reasons for this we'll talk about the

reasons in particular they had for this

it had to do with stars and had to do

with the position of stars in the sky

they believed the earth was stationary

so the daily and annual motion that we

observe

they believed that happened because of a

celestial sphere that surrounded the

earth and we saw different stuff based

on how that sphere spun today we know

that's not true

but again they had good reasons and

we'll talk about now the motion again

the Earth spins on an axis now this axis

that the Earth spins on is tilted

relative to the Sun which is a say if we

are here in the northern hemisphere or

if here in the southern hemisphere

you're in this other hemisphere wherever

you are if you're not in the tropics

when you look at the Sun when it gets to

the highest point and where it gets in

the sky which technically we would call

that noon when the Sun gets the highest

point in the sky is it directly overhead

well if you're not in the tropics the

answer there should be no oh the Sun is

never directly overhead if you're not in

the tropics never okay fine the sun has

ever ahead why well because the earth is

tilted relative to its orbit around the

Sun so we orbit around the Sun imagine

that orbit as a frisbee a plane okay or

the the top of a desk we go around the

Sun on that desk on the top of that desk

okay now

the earth itself when it spins again you

can think of the earth as like a little

marble and you can spin that marble the

Earth's spin is tilted relative to the

Sun it's tilted twenty three and a half

degrees that is this tilt here and

because of that because of that tilt

when the Sun well first of all will

never be directly overhead unless you're

in the tropics near the earth's equator

but the earth's equator is tilted

relative to the the Earth's orbit around

the Sun and the Earth's orbit around the

Sun we call that the ecliptic and the

earth the earth itself when it spins

because it's tilted relative to that the

earth the center of the earth we call

that the equator and if you imagine the

earth's equator projected out into space

we call that the celestial equator but

it's tilted relative to what we call the

ecliptic now the ecliptic is where or

the Earth's orbit around the Sun that is

the ecliptic and from our point of view

here on the earth you can always tell

where they clip the gives just by

looking where is the Sun because the

Earth's orbit around the Sun is the

ecliptic well from our point of view the

Sun is always going to be on the

ecliptic so basically the ecliptic is

the sun's path through the sky and at

night you can imagine where the Sun

would be in the sky and that is the

ecliptic so fine

oscillations things that we see in the

sky what happens over time because we're

moving different constellations we

visible during the day or at night

rather and it's just you know the

direction we're facing great so if

here's the Sun we orbit around the Sun

at certain times the year we're looking

in this direction we see different

constellations we're looking in this

direction six months later we're gonna

see different constellations and when

they rise and set depends on how fast

the earth is actually spinning but what

constellations are in the sky depends on

where the earth is around the Sun all

right so constellations will change

yearly based on where we are so here

we're looking in this direction we're

gonna see certain constellations here at

night we're looking in this direction

will see different constellations now

what this means is you can use

constellations as a calendar and this is

huge this is what astronomy really was

historically astronomy used to be a

calendar it's how we kept track of time

all right so this is annual motion right

the earth going around the Sun at

different times of the year at night we

can see different constellations if you

could see constellations during the day

you can't but if you could what you

would see is the Sun appears to be in

particular constellations these

constellations are the zodiac now I do

not want you to that I do not want you

guys to imagine the zodiac somehow

controls your life it doesn't if you are

interested in a different kind of class

well you might be interested in

astrology but I will tell you this right

now astrology is wrong ridiculous and

silly this is non astrology this is

astronomy this is a science ok now why

are these constellations important yeah

there are these constellations you

probably have heard of them Aries

Capricorn Sagittarius Scorpius

okay these are the constellations of the

zodiac they are historically important

because they are a calendar okay if you

are on the earth in June during the day

the Sun is up and if you could see the

constellations as Sun is in well it's

between Taurus and Gemini but at night

Scorpius is up this constellation is up

it is the constellation that at midnight

is in the center of the sky and if you

know if you are familiar with the zodiac

and you see Scorpius in the center of

the sky well you know it's June it's a

calendar okay in August center of the

sky is gonna be in this direction and if

you sees hedges Terius and Capricorn and

you know where they should be in this

guy or you know that you're looking

right in the middle of those at midnight

well that's calendar it tells you it's

August that is what the zodiac

historically has been used for not to

predict what's gonna happen to you today

that's ridiculous it can't happen but it

does tell you what time of year it is

you can use it as a calendar so fine the

ecliptic

is the path of the Sun in our sky from

our point of view what's really going on

is the Earth orbits the Sun but from our

point of view the Sun is going to be in

a particular pop spot in the sky we call

that the ecliptic and it turns out that

if we could see constellations back here

well the Sun would be in some

constellation that is the zodiac and

that is the zodiac okay now that's it

for the first part of this lecture I

hope you guys are having a enjoyable

time we will continue to talk about how

things move in the sky and what that

means in the next couple of parts of

this first lecture so enjoy