Ch01 Lecture part2 video

Summary

This astronomy lecture explores the concept of Earth's axial tilt and its impact on the seasons. Contrary to common misconceptions, seasons are not due to Earth's proximity to the Sun but rather its 23.5-degree tilt. The tilt results in varying sunlight distribution, causing summer in the Northern Hemisphere when it's tilted towards the Sun and winter when tilted away. Equinoxes, with equal day and night, occur when Earth is perpendicular to the Sun, while solstices mark the extremes of tilt, resulting in the longest and shortest days. The celestial equator and ecliptic, the Sun's apparent path in the sky, further explain seasonal variations and the Sun's position relative to Earth's equator.

Takeaways

• 🌏 The Earth orbits the Sun in an almost perfect circle, which affects what we see in the night sky as the Earth's position changes.
• 🌞 Seasons are not due to the Earth being closer to or farther from the Sun, but rather because of the Earth's axial tilt relative to its orbit.
• 📍 The axial tilt of the Earth is 23.5 degrees, which causes different hemispheres to receive varying amounts of sunlight throughout the year.
• ❄️ In the Northern Hemisphere winter, the Earth's axis is tilted away from the Sun, resulting in less direct sunlight and colder temperatures.
• ☀️ Conversely, during the Northern Hemisphere summer, the Earth is tilted towards the Sun, receiving more direct sunlight and experiencing warmer weather.
• 🌡️ The tropics, located between 23.5 degrees north and south of the equator, can have the Sun directly overhead due to the Earth's axial tilt.
• 🌐 The Earth's equator is the center of the sphere, and the ecliptic is the path the Sun appears to take across the sky, which is tilted relative to the equator.
• 🌅 The position of the Sun at sunrise and sunset changes throughout the year, indicating the Earth's tilt and its position in its orbit.
• 🌓 Equinoxes occur when the Earth is perpendicular to the Sun, resulting in equal day and night lengths for both hemispheres.
• 🌑 Solstices happen when the Earth's tilt is at its maximum towards or away from the Sun, leading to the longest and shortest days of the year.
• 🌌 The celestial equator is the projection of the Earth's equator into space, while the ecliptic is the path of the Earth's orbit around the Sun.

Q & A

What is the primary reason for the change of seasons as explained in the script?

The primary reason for the change of seasons is the axial tilt of the Earth relative to its orbit around the Sun, not because the Earth is closer to or farther from the Sun.

What is the significance of the Earth's axial tilt in relation to the Sun?

The Earth's axial tilt of 23.5 degrees causes different hemispheres to receive varying amounts of direct sunlight at different times of the year, leading to the seasons.

Why is it not summer all year round on Earth?

It is not summer all year round because the Earth's axial tilt changes its orientation towards the Sun throughout the year, causing variations in the amount of direct sunlight received by each hemisphere.

What happens during the equinoxes in terms of sunlight distribution?

During the equinoxes, both the northern and southern hemispheres receive equal amounts of sunlight, resulting in approximately equal day and night lengths.

What is the term used to describe the position of the Earth when it is tilted neither towards nor away from the Sun?

This position is known as an equinox, which results in equal day and night lengths for both hemispheres.

What causes the Sun to appear higher or lower in the sky throughout the year?

The Earth's axial tilt and its position in the orbit around the Sun cause the Sun to appear higher in the sky during summer (more direct sunlight) and lower during winter (less direct sunlight).

What is the term for the time of year when the Earth is tilted most towards or away from the Sun?

This time of year is called a solstice, during which one hemisphere experiences the most direct sunlight (summer solstice) and the other experiences the least (winter solstice).

Why don't we see the same seasons on the equator as we do in other parts of the Earth?

The equator does not experience significant seasonal changes because the Sun is always relatively close to being directly overhead, resulting in a more consistent climate throughout the year.

How does the Earth's position in its orbit and its axial tilt affect the Sun's position in the sky?

The Earth's position in its orbit combined with its axial tilt determines the Sun's position in the sky, causing it to rise and set in different directions throughout the year.

What is the ecliptic and how does it relate to the Sun's path in the sky?

The ecliptic is the path the Sun appears to follow in the sky over the course of a year. It is defined by the Earth's orbit around the Sun and is tilted relative to the celestial equator due to the Earth's axial tilt.

Outlines

00:00

🌏 Earth's Orbit and Seasons

This paragraph explains the concept of Earth's orbit around the Sun and the resulting seasonal changes. Contrary to a common misconception, seasons are not due to Earth's proximity to the Sun but rather its axial tilt of 23.5 degrees. When the Earth's axis is tilted towards the Sun, it's summer in the respective hemisphere, and winter when it's tilted away. The tropics experience direct sunlight due to their location relative to the equator, leading to less pronounced seasons. The axial tilt causes variations in the angle at which sunlight hits the Earth, affecting the amount of energy received and thus the temperature.

05:06

🌞 Sunlight and Seasonal Variations

The second paragraph delves into how the directness of sunlight influences the seasons. It describes how the Earth's axial tilt affects the distribution of sunlight across the hemispheres. During summer in the northern hemisphere, the direct light is concentrated, leading to warmer temperatures, while in winter, the same light is spread over a larger area, resulting in less heat per unit area. The equator consistently receives direct sunlight, avoiding pronounced seasonal changes. The paragraph also introduces the concepts of the ecliptic—the path of the Sun in the sky—as being related to the tilt of Earth's rotational axis, which is key to understanding the seasonal variations.

10:08

🌡 Equinoxes and Solstices Explained

This paragraph clarifies the astronomical events of equinoxes and solstices, which are directly related to the Earth's axial tilt. An equinox occurs when the Earth is positioned such that it is perpendicular to the Sun, resulting in equal day and night lengths across the globe. Conversely, solstices occur when the Earth's tilt is maximized towards or away from the Sun, leading to the longest and shortest days of the year, respectively. The paragraph also encourages observing the Sun's position at sunset to discern the season, with the Sun setting directly west during equinoxes and deviating north or south of west depending on the season.

15:13

📍 Geographical Impact on Sun's Position

The final paragraph discusses how the Earth's position in its orbit, combined with its axial tilt, affects the Sun's position in the sky relative to an observer's location on Earth. It explains that the Sun's path, or ecliptic, is always overhead at the equator but shifts north or south of the zenith as one moves away from the equator. The paragraph emphasizes that the Sun's position in the sky changes throughout the year due to Earth's orbit and tilt, and these changes are consistent at different latitudes across the globe, such as in Tokyo and the Bay Area, despite the time difference.

Keywords

💡Axial Tilt

Axial tilt, also known as obliquity, is the angle between a planet's rotational axis and its orbital axis. In the video, it is explained as the reason for the seasons on Earth. The Earth's axial tilt of approximately 23.5 degrees causes different hemispheres to receive varying amounts of sunlight throughout the year, leading to seasonal changes. The script uses the concept to illustrate why summer in the northern hemisphere is characterized by the Earth being tilted towards the Sun, receiving more direct sunlight, while winter is when the Earth is tilted away, receiving less.

💡Orbit

Orbit refers to the path that an object in space takes around another object due to gravity. The video script discusses Earth's orbit around the Sun, which is nearly a perfect circle, and how this orbit, combined with Earth's axial tilt, results in the changing seasons. The concept is central to understanding the celestial mechanics that lead to the observed phenomena of summer, winter, and the variation in daylight hours.

💡Seasons

Seasons are the distinct periods characterized by particular weather patterns, temperatures, and lengths of day and night, resulting from the Earth's axial tilt and orbit around the Sun. The script clarifies that seasons are not due to the Earth's proximity to the Sun but rather the tilt of its axis. The video uses the concept to explain the differences in climate experienced at various times of the year.

💡Equinox

An equinox is one of the two points in the year when the Earth's equator passes through the center of the Sun, resulting in nearly equal day and night lengths worldwide. The video script explains that during an equinox, the Earth is not tilted towards or away from the Sun, leading to equal amounts of sunlight for both hemispheres. The term is used to describe the times when day and night are of equal duration.

💡Solstice

A solstice is an event that occurs when the Earth's axial tilt is at its maximum angle relative to the Sun, resulting in the longest and shortest days of the year. The script mentions that during a solstice, one hemisphere is tilted towards the Sun (summer solstice), receiving the most direct sunlight, while the other is tilted away (winter solstice), receiving the least. This concept is key to understanding the extremes of seasonal variation.

💡Ecliptic

The ecliptic is the path on the celestial sphere that the Sun appears to follow over the course of a year. The video script explains the ecliptic as the Earth's orbit around the Sun, which is tilted relative to the celestial equator. This tilt is crucial for understanding the changing position of the Sun in the sky and the resulting seasons.

💡Celestial Equator

The celestial equator is an imaginary line on the celestial sphere that corresponds to the Earth's equatorial plane projected into space. The video script uses this concept to contrast with the ecliptic, illustrating the tilt of the Earth's axis and how it affects the position of the Sun in the sky throughout the year.

💡Direct Sunlight

Direct sunlight refers to the rays of the Sun that strike the Earth at a perpendicular angle, providing maximum solar energy. The script explains how the amount of direct sunlight received by each hemisphere changes with the seasons, with more direct sunlight leading to summer and less leading to winter.

💡Tropics

The tropics are the region of the Earth surrounding the equator, defined by the Tropic of Cancer and the Tropic of Capricorn (23.5 degrees latitude north and south, respectively). The video script mentions that within the tropics, the Sun can be directly overhead, contributing to a generally warm climate year-round without the pronounced seasonal changes seen in higher latitudes.

💡Northern Hemisphere

The northern hemisphere is the half of the Earth that is north of the equator. The video script uses this term to describe the experiences of winter and summer, with winter occurring when the Earth's axis is tilted away from the Sun and summer when it is tilted towards the Sun, affecting the amount of sunlight and resulting temperatures.

💡Southern Hemisphere

The southern hemisphere is the half of the Earth that is south of the equator. In the script, it is contrasted with the northern hemisphere to illustrate the opposite seasonal experiences due to the Earth's axial tilt. When it is winter in the northern hemisphere, it is summer in the southern hemisphere, and vice versa, due to the differing angles of sunlight received.

Highlights

Introduction to the second part of the first astronomy lecture.

Explanation of Earth's orbit around the Sun and its effect on the night sky.

Clarification that the Earth's proximity to the Sun is not the cause of seasons.

Introduction of Earth's axial tilt as the primary reason for seasonal changes.

Description of the Earth's axis orientation during different seasons.

Explanation of the Northern Hemisphere winter due to the Earth's axis pointing away from the Sun.

Details on the axial tilt of 23.5 degrees and its significance in seasonal variations.

Differentiation between the tropics, where the Sun can be directly overhead, and other regions.

Explanation of how the Earth's axial tilt affects the distribution and intensity of sunlight.

Discussion on the equatorial region's lack of distinct seasons due to consistent sunlight.

Introduction of the ecliptic as the path of the Sun in the sky and its relation to seasons.

Clarification of the difference between the celestial equator and the ecliptic.

Description of equinoxes as times of equal day and night due to the Earth's axis being perpendicular to the Sun.

Explanation of solstices as points in Earth's orbit where the tilt is maximized towards or away from the Sun.

Observational tips for determining the season based on the Sun's position during sunset.

Discussion on the impact of Earth's tilt on the Sun's position in the sky relative to an observer's location.

Connection between latitude and the Sun's position in the sky, affecting the observer's experience of seasons.

Conclusion of the lecture with a teaser for the third part.

Transcripts

hello everyone welcome to part two of

our first lecture of astronomy now last

time in part one we talked about how the

earth goes around the Sun and so we have

and the Sun and the earth orbits around

that Sun and that's why we see different

things in the sky at night right because

and sometimes at night we're looking in

this direction sometimes at night we're

looking in that direction now the fact

that the earth goes around the Sun has

other effects that we actually observe

every year one of those effects is the

seasons now the way I've drawn this here

is not very good okay the Sun and the

earth goes around the Sun this drawing

should be almost a perfect circle now

it's not quite a perfect circle we're

gonna talk about that in upcoming

lectures but it's almost a perfect

circle so the reason why we have seasons

well a lot of people will tell you the

reason why it's sour is because the

earth is closer to the Sun so you know

we have the Sun and the earth going

around the Sun some people will tell you

well it's summer because we are closer

to the Sun that is not the case it is

not the case that it's not why it is

summer okay

why is summer has to do with the fact

that the Earth's axis is tilted relative

to its orbit around the Sun which means

that at certain times of the year in

certain times of the year the earth is

actually tilted towards the Sun and

other times the year

from our point of view here in the earth

were tilted away from the Sun that is

the season so what causes seasons well

alright if you look at the earth as it

is right here this would be our in the

northern hemisphere if you're in the

northern hemisphere this would be winter

why well if you look at where the earth

is relative to the Sun and this by the

way is not to scale not to scale at all

the earth is much much smaller than us

honest but if you look at the Earth's

axis okay we spin around this axis and

right now where you see the earth

relative to the Sun the Earth's axis is

tilted such that it points away from the

Sun this would be Northern Hemisphere

winter because sunlight coming from the

Sun okay is going to hit the southern

hemisphere this part of the earth the

southern hemisphere more directly than

the northern hemisphere and when the

southern hemisphere gets more direct

sunlight well it gets more energy

because light is energy and you get more

energy more light in the southern

hemisphere well it's it's summer they're

in the northern hemisphere we're getting

less energy from the Sun it's gonna be

winter and the tilt is 23 and a half

degrees and so 23 and a half degrees the

Sun is going to be 23 and a half degrees

off from being directly overhead if

you're in the northern miss fear or the

southern hemisphere now if you're

actually in what we call the tropics

which is 23 and a half degrees twenty

three and a half degrees above and below

the equator is

at the tropics this little region on the

earth right in the middle of the earth

right there the Sun can be directly

overhead but only if you're in that

region that is twenty three and a half

degrees above and below where the

Equator is that's the tilt of the earth

okay but so that's why we have seasons

it's the axial tilt it's the axial tilt

of the earth so here the tilt is towards

the Sun and in the northern hemisphere

you're gonna have summer the southern

hemisphere tilted away from the Sun

you're gonna have winter whereas over

here in December the northern hemisphere

tilts it away from the Sun the northern

hemisphere winter you're getting less

direct sunlight southern hemisphere

you're gonna have summer you're getting

direct sunlight or more direct sunlight

from the Sun now technically speaking it

only on the equator you get actually

direct sunlight which again in the

equator you don't really have seasons on

the equator it's always warm okay so

it's the axial tilt is why we have

seasons right now another way to look at

this or think about this is how direct

the sunlight gets to us so if you

imagine the Sun is over here okay if the

earth is tilted this way relative to the

Sun so the Earth's orbit going around

the Sun the earth is tilted relative to

that orbit well if you're in the

northern hemisphere this would be like

June direct light it's you and that

direct light is spread out over this

area the southern hemisphere that same

light

cuz the southern hemisphere is tilted

away from the Sun and same light is

spread out over a much larger area so

the same amount of light is spread out

over a larger area which means that each

individual spot is getting less light

the southern hemisphere this would be

winter the northern hemisphere we're

getting all that light concentrated in a

small spot and that's it's summer it's

why it's summer we're getting a more

direct light from the Sun and that's it

it's Oh the reason why we have seasons a

it has to do with the ecliptic ok the

tilt the tilt of the Earth's rotational

axis relative to the eclectic ecliptic

that's why we have seasons and if you're

in the northern miss fear it has to do

with how high the Sun is above the

celestial equator Sun in the summer

sun's high up in the sky well high up in

the sky means we're getting more direct

sunlight if it's a really low in the sky

we're getting less direct sunlight you

can see this day a day right if you know

in the morning the very when the Sun

just first rises it's very low on the

horizon right it's right near the

horizon it's just rising well the Sun is

not nearly it doesn't feel as warm right

then as it does at noon when the Sun is

as high as it gets in the sky right and

at sunset the Sun seems less warm than

it does when the Sun is in the middle of

the sky well this is seasons

hey this is seasons if the Sun gets

higher in the sky and summer that means

we're getting more direct light we are

tilted towards the Sun in the winter and

the sun's not nearly as high in the sky

we're getting less direct light we're

getting less energy and it's cold

okay the tilt the the earth this is what

we call the ecliptic now the ecliptic is

the path of the Sun in the sky so it's a

path the Sun seems to go through our sky

go out during the day look at the Sun

that's the ecliptic period now the

earth's equator the center or if you

imagine the earth as a sphere and the

equator is the center of that sphere

well the earth is actually tilted so if

you imagine a line that goes up and down

well the Sun we orbit around the Sun we

are tilted relative to that so is

actually 23 and 1/2 degrees tilted

that's why we have seasons that is why

we have a different ecliptic than an

equator the celestial equator is just

the earth's equator projected out into

space the ecliptic is the earth Earth's

orbit around the Sun so if you imagine

the Sun earth goes around the Sun a

spherically and the Earth's orbit around

the Sun that is the ecliptic and it's

tilted 23 and 1/2 degrees relative to

the earth's equator now there will be

points when the earth is going around

the Sun so you know the Sun you have the

earth going around the Sun and the earth

again is tilted relative to the Sun now

there will be times in summer

ok the earth is say here and we are

tilted towards the Sun so that's our

summer however what if the earth is here

and we are tilted still like this the

tilt is always the same direction

so the tilt is there well we're not

pointed towards or away from the Sun

right we are perpendicular to the Sun

not towards or away that is known as an

equinox on an equinox you have equal

days and nights both hemispheres of the

earth are getting equal amounts of

sunlight so northern hemisphere and

southern hemisphere are getting equal

amounts of sunlight that's why it's

called an equinox however if you are

here or here well the earth is either

tilted as much as it ever is towards the

Sun or tilt it as much as it ever is

away from the Sun those are called

solstices and in the northern hemisphere

let's say the northern miss fear is

tilted towards the Sun here so this

would be the North hemisphere sorry well

that is as much direct sunlight as this

the earth is going to get in the

northern hemisphere ever that is the

Solstice when we're directly torque

pointed towards the Sun is a solstice

war if the earth is here or directly

pointed away from the Sun that's also a

sources so we either get the most amount

of direct sunlight or the least amount

of direct sunlight during the year now

again the Earth's axis is always pointed

in the same direction so when the earth

is here there's access is pointed in

this direction up and down and that

would be another equinox we're not

pointed towards or away from the Sun now

from our point of view what it's going

to look like is the Sun is going to rise

and set in different directions

if you are looking encourage you to do

this go outside at night or in the early

evening and watch where the Sun sets now

if the Sun is setting

exactly west exactly west well that

means it's an equinox if it's setting a

little bit north of West well that means

we are in the summer seasons if it's

setting south of West we are in the

winter seasons if you are in the

northern hemisphere if you're in the

southern hemisphere it's the opposite

and again this is from our point of view

the Earth's point of view of where the

Sun is rising and setting and it all has

to do with the fact that the earth is

tilted relative to its orbit around the

Sun so we spin on an axis and that axis

is tilted relative to the Sun that's why

we have seasons that's it and again you

can observe this so you go outside you

know at night watch the sunset watch it

day after day after day and you will see

its position when it sets it's gonna

change day after day after day it's

gonna get more and more if this

direction is north it's gonna get more

and more north during the summer and the

winter it's gonna get more and more

south and on the equinoxes it's gonna

set exactly west all right now

it is also the case that where the Sun

is in the sky depends on where you are

on the earth that should make sense

right if you're on the side of this

earth that's facing away from the Sun

it's gonna be night if you're on the

side of the earth face in the Sun it's

gonna be day but because of the Earth's

orbit around the Sun where the Sun is in

the sky also depends on where you are in

the earth so if you're in the Omni

equator the Sun is gonna be pretty much

directly overhead if you're on the

equator the sun's would be directly

overhead but if you're north of the

equator or southeast equator the Sun is

gonna be either north or south of that

directly overhead and so you're gonna

see different paths and the Sun in the

sky but the Sun is always on the

ecliptic always on the ecliptic that is

kind of by definition the ecliptic again

is the Earth's orbit around the Sun and

from our point of view because we're on

the earth the ecliptic is always where

the Sun is how far how high it is above

the horizon is gonna depend on where we

are on the earth and what time of year

it is if you are in the Bay Area in

California

well what you're gonna see above you

that latitude that latitude on the earth

if you imagine the earth ok the equator

is latitude 0 you go up on the earth

that's positive latitude if we go down

on the earth it's negative latitude

you're in the Bay Area it's a brown

right here it's about 40 degrees plus

okay 40 plus now the Bay Area is here if

you look at say Tokyo and Japan took you

in Japan is over here in the earth it's

also at the same latitude so in Tokyo in

the Bay Area you're gonna see the same

things happen

in the sky so you're gonna see the same

stars you're gonna see the Sun rise and

set in the same places they're just

gonna be a bit different in time so you

know Tokyo is like 10 hours apart from

the bay area so it's gonna happen a

little bit later but your to see the

same stuff in this guy alright so that

is the second part of this stay tuned

for part 3